Preparation & Properties of Compounds
BORON FAMILY Elements : B, Al, Ga, In, Tl General electronic configuration : ns 2 , np1 Melting Points, Boiling Points and structures The melting points of group III elements do not show a regular trend as did the metals of group I and II, because B and Ga have unusual crystal structures. Boron has an unusual crystal structure which results in the melting point being very high. Other elements form metallic bonding, but small size and high lonisation energy make this impossible for boron. Boron exists in four different allotropic forms, all of which contains icosahedral units with boron atoms at all 12 corners. (Icosahedral contains 12 corners and 20 faces). Only 37% of the space is occupied by the atoms, compared with 74% for a close-packed arrangement. This shows that icosahedra fill up space ineffectively. The elements Al, In and TI all have close-packed metal structures Ga has an unusual structure. Each metal atom has one close neighbour at a distance of 2.43 Å and six more distant neighbours at distance between 2.7 Å and 2.79 Å. This remarkable structure tends towards discrete diatomic molecules rather than a metallic structure. This accounts for the incredibly low melting point of gallium at 30°C Melting Point (°C) Boiling Point (°C) B 2180 3650 Al 660 2467 Ga 30 2403 In 157 2080 Tl 303 1457 As is obvious from the above table, the melting point decreases in the group but irregularities occur. B has very high melting point because of its unique covalent structure. Ga has extremely low melting point again because of its unique structure. The boiling point for B is unusually high, but the values for Ga, In and Tl decrease on descending the group as expected. Note that the boiling point for Ga is in line with the others, whereas its melting point is not. The very low melting point is due to the unusual crystal structure, but the structure no longer exits in the liquid. Size of atoms and ions The metallic radii of the atoms do not increase regularly on descending the group. However, the values are not strictly comparable because of their unique structures. Metallic radius Ionic radius Paulings 3 (Å) electronegativity M Å M Å B Al Ga In Tl
(0.885) 1.43 (1.225) 1.67 1.70
(0.27) 0.535 0.620 0.800 0.885
– – 1.2 1.4 1.5
2 1.5 1.6 1.7 1.8
The ionic radii for M 3 increase down the group, though not in the regular way as observed in Group I and II. There are two reasons for this. Page No.-1 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds (i) (ii)
There is no evidence for the existence of B3 under normal condition and the value of ionic radius is an estimate. The electronic structures of the elements are different Ga and In have a d10 inner shell which is poorly screening and so have higher ionisation energies than would otherwise be expected. This contraction in size is sometimes called the d-block contraction. In a similar way Tl follows immediately after 14 f-block elements.. The size and ionisation energy of TI are affected even more by the presence of 14 f-electrons, which shield the nuclear charge even less effectively. The contraction in size from these f-block elements is called the lanthanide contraction.
Electropositive Character The electropositive or metallic nature of the elements increase from B to Al, but then decreases from Al to TI as is shown by the standard electrode potentials for the reaction: M 3 3e M M 3 | M Volts M 1 | M Volts B Al Ga In Tl
(–0.87)* –1.66 –0.56 –0.34 +1.26
0.55 –0.79f –0.18 –0.34
*For H 3BO3 3H 3e B 3H 2O f Value in acidic solution. Ga, In and TI have lesser tendency to lose electrons (and are thus electropositive), because of the poor shielding by d-electrons. The E° values show that the stability of +3 oxidation state decreases down the group whereas that of +1 increases down the group.
Ionisation Energy The ionisation energies increase as expected (first ionisation energy < second ionisation energy
1st 801 577 579 558 589
2nd 2427 1816 1979 1820 1971
3rd 3659 2744 2962 2704 2877
Sum of three 6887 5137 5520 5082 5437
Boron Page No.-2 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Preparation: (i)
Boron is obtained by the reduction of B 2O3 with magnesium or sodium B2O3 is first prepared by strongly heating H3BO3 which is obtained by the action of HCl or H2SO4 on a concentrated solution of borax: Na2B4O7 + 2HCl + 5H2O 2H3BO 3
4H3BO3 + 2NaCl
B2O3 + 3H2O
B2O3 + 3Mg
2B + 3MgO
Pure crystalline boron may be obtained in small quantities by the reduction of BBr 3 with H2 on a heated metal filament at 1275-1475 K. (ii)
Modern Method B is obtained these days by the electrolysis of a fused mixture containing boric anhydride, MgO and MgF2 at 1100°C. The electrolysis is done in a carbon crucible, which acts as anode and Fe rod is used as cathode. The Mg discharged at cathode reduces B2 O3 to B. 2MgO
(iii)
2Mg O2
B2 O3 3Mg 2B 3MgO B thus obtained is heated electrically in vacuum at 1100°C, when the impurities are volatilised off and pure boron is obtained. By thermal decomposition of BI3 over red hot tungsten filament (Van Arkel method) W 2BI3 1173K 2B s 3I2 g
Properties: Physical a) Small atomic size b) Low electronegativity and hard, absorbs neutron, steel grey in colour. Some dissimilarities of boron with other elements of this group are : i) Boron is a non-metal while all other elements of this group are metals. ii) Boron forms only covalent compounds, while other elements of this group form both covalent and ionic compounds. iii) Boron shows a maximum covalency of four, while other elements of this group show a maximum valency of six, absorbs neutron, steel grey in colour. Its density is 2.34 gm/cc. Chemical 1. Crystalline Boron is not active. Amorphous Boron reacts. 4B 3O 2 700C 2B2 O3 ; 2B N 2 700C 2BN 2. These above reaction accompanies red flame. BN 3H 2 O H 3BO3 NH 3 3. Action in alkalis and acids Page No.-3 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds 2B 2NaOH 2H 2 O 2B 3H 2 O
2NaBO 2
B2 O3
2B 3H 2SO 4 2B 6HNO 3
3H 2 2H 3BO3 3SO 2
oxidation oxidation
3H 2
H 3BO3 6NO 2
4. Boron reacts with Mg and consequent hydrolysis gives diborane. 3Mg 2B
Mg 3 B2
3Ca 2B
Ca 3 B2
Mg 3 B2 6HCl
3MgCl 2
B2 H 6 6H 2 O
B2 H 6
2H 3BO3 6H 2
5. Boron reduces silica and CO2 3SiO 2 4B
2B2O3
3Si
3CO 2 4B
2B2O3
3C
Illustration - 1
Which of the statement is true for the above sequence of reactions ? (a) Z is hydrogen
(b) X is B2H6
(c) Z and Y are F2 and B2H6 respectively
(d) Z is potassium hydroxide
Ans. (c) Note :
B F2 (Z)
BF3 ; ( X)
8BF3 6LiH
B2H6 (Y)
6 LiBF4
COMPOUNDS OF BORAN Diborane (B2H6) Preparation: Diborane can be prepared in almost quantitative yields by the reduction of boron trifluoride etherate (BF3.OEt2) with lithium aluminium hydride (LiAlH 4) or sodium borohydride (NaBH4). 4BF3 .OEt 2 3LiAlH 4
Et 2 O
4BF3 .OEt 2 3NaBH 4 diglyme
2B2 H 6 3LiAlF 4Et 2O 4 2B2 H 6 3NaBF 4Et 2O 4
Page No.-4 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds where, diglyme is diethyleneglycol dimethyl ether, (MeOCH2CH2)2O. Diborane can also be prepared by treating NaBH4 with conc. H2SO4 or H3PO4: 2NaBH 4 H 2SO 4
B2 H 6 2H 2
2NaBH 4 2H 3PO 4 Properties:
Na 2SO 4
B2 H 6 2H 2
2NaH 2 PO 4
1. Diborane is a colourless gas (b.p., 183K). It is rapidly decomposed by water with the formation of H3BO3 and H2: B2 H 6 6H 2 O
2H 3BO3 6H 2
2. Mixtures of diborane with air or oxygen inflame spontaneously produce large amount of heat. Diborane has a higher heat of combustion per unit weight of fuel than most other fuels. Therefore, it is used as a rocket fuel. B2 H 6 3O 2
B2O3 3H2 O, H
–2165 kJ mol –1
3. Pyrolysis of B2H6 in sealed vessels at temperatures above 375 K is an exceedingly complex process producing a mixture of various boranes, e.g., B4H10, B5H9, B5H11, B6H10, B6H12 and B10H14. By careful control of temperature, pressure and reaction time, the yield of various intermediate boranes can be optimised. For example, by storing B 2H6 under pressure for 10 days, B4H10 is produced in 15% yield according to the following equation: 2B2 H 6
B 4 H10
H2
4. Diborane reacts with ammonia 3B2 H 6 6 NH 3 2 B3 N 3 H 6 12 H 2
gives
inorganic
benzene
(inorganic benzene) Illustration - 2 When an inorganic compound (X) having 3C – 2e as well as 2e – 2e bonds reacts with ammonia gas at a certain temperature, gives a compound (Y), isostructural with benzene. Compound (X) with ammonia at a high temperature produces a substance (Z). (a) (X) is B2H6
(b) (Z) is known as inorganic graphite
(c) (Y) is B3N3H6
(d) (Z) is soft like graphite
Ans. (a, b, c) Note: Diborane, B2H6, is a compound consisting 3C–2e and 2e – 2e bonds. B 2H6 + 2NH3 B2H6
200ºC
Low temp.
B3N3H4 · 2NH . (Y) has structure similar to benzene. It is called inorganic 3 (Y)
benzene. B2H6 NH3
High temp.
(BN)X . (Z) is a hard substance]. (Z)
5. Diborane undergoes a facile addition reaction with alkenes and alkynes in ether solvents at room temperature to form organoboranes: 6RCH = CH2 + B2H 6
2B(CH2CH2R)3 (Hydroboration Reaction)
Page No.-5 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Illustration - 3 Which one of the following compounds does not exists ? (a) B2H4(CH3)2
(b) B2H3(CH3)3
(c) B2H2(CH3)4
(d) B2H(CH3)5
Ans. (d) Note
Not more than four hydrogen atoms can be substituted by methyl groups in the molecule of B2H6. The bridge hydrogen atoms are not to be substituted. Boric Acid: Orthoboric acid H3BO3 commonly known as boric acid and metaboric acid HBO 2, are two well-known and important oxoacids of boron. Preparation: On a large scale, H3BO3 is prepared by the action of HCl or H 2SO4 on a concentrated solution of borax: Na 2 B4 O 7 2HCl 5H 2O Properties:
4H 3BO3 2NaCl
1. Boric acid is a flaky, white crystalline solid. 2. It is moderately soluble in water. 3. Boric acid is a very weak monobasic acid (pK = 9.25), because it acts as an electron pair acceptor (Lewis acid) from OH – rather than as a proton donor (Arrhenius acid). – H 3BO3 2H 2 O ‡ˆ ˆ† ˆˆ [B(OH) 4 ] H 3O
4. On heating boric acid at 375 K, metaboric acid, HBO2 is formed. On heating above 500 K, B2O3 is formed: H 3BO3
375 K
HBO 2
H 2 O ; 2HBO 2
500 K
further
B2O3 H 2 O
In solution metaboric acid changes into orthoboric acid. Borax (Sodium tetraborate decahydrate, Na2B4O7. 10H2O) Preparation: It is obtained by extracting impure borax with water and then concentrating the solution until crystals of borax separate out. Borax can also be prepared from the mineral colemanite by boiling it with Na 2CO3 solution::
Properties:
Ca 2 B6O11 2Na 2CO Na 2 B4O 7 2NaBO 2 2CaCO 3 3 1. Borax is a white crystalline solid. It is hydrolysed by water to give an alkaline solution: Na 2 B4 O 7 7H 2 O ‡ˆ ˆ† ˆˆ 4H 3BO3 2NaOH 2. On heating, borax loses water to become anhydrous. Anhydrous borax on strong heating with NH4Cl gives boron nitride and boron trioxide: Na 2 B4O 7 2NH 4Cl 2BN B2O3 2NaCl 4H 2O 3. On heating alone, it decomposes to form NaBO2 and B2O3 Na 2 B4 O 2NaBO 2 B2 O3 7
Page No.-6 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Illustration -4 Na2B4O7 · 10H2O is correctly represented as (a) 2NaBO2 · Na2B2O3 · 10H2O
(b) Na2[B4O5(OH)4] · 8H2O
(c) Na2[B4O5(OH)4] · 8 H2O
(d) Na2[B4(H2O)4O7] · 6H2O
Ans. (b) Note Borax molecule is actually made of two tetrahedra and two triangular units joined as shown below : O H O — B N a
2
–
HO — B O — B
–
— O O — O
B — O H
O H
Illustration - 5 Amorphous boron is extracted from borax by following steps : (B) 3BO3 Heat B2O3 Boron Borax (A) H (a) H2SO4, Al
(b) HCl, carbon
(c) H2SO4, Mg
Ans. (c) Note Na2B4O7 + H2SO4 + 5H2O Na2SO4 + 4H2BO3 ; 2H2BO3 B2O3 3Mg
2B
(d) HCl, Fe B2O3
3H2O;
3MgO]
4. Borax bead Test: The formation of coloured metaborates by transition metal salts is used in borax bead test in qualitative analysis. The colour depends on the oxidising or reducing flame of the bunsen burner. A cupric salt forms blue cupric metaborate in the oxidising flame: Na2B4O7 + CuO
Cu(BO2)2 + 2NaBO2
In the reducing flame , (ie. in presence of carbon ) the coloured salt is reduced to colourless cuprous metaborate: 2Cu(BO2)2 + 2NaBO2 + C 2CuBO2 + Na2B4O7 + CO. and to metallic copper and hence bead becomes dull red and opaque. 2Cu(BO2) + 4NaBO2 + 2C 2Cu + 2Na2B4O7 + 2CO Compounds of Chromium Manganese Iron Cobalt Nickel Copper
Colour of borax bead Oxidising flame Reducing flame Green Green Amethyst Colourless Yellow(cold) Bottle green Deep blue Deep blue Brownish(cold) Grey Green(hot),blue(cold) Colourless or red
Page No.-7 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Aluminium Aluminium oxide - Al2O3(Alumina)
Preparation: In the lab, it is prepared by igniting aluminium hydroxide, aluminium sulphate or ammonium alum. 2Al(OH)3 Al2O3 + 3H2O Al2(SO4)3 Al2O3 + 3SO3 (NH4)2SO4.Al2(SO4)3.24H2O 2NH3 + Al2O3 + 4SO3 + 25H2O Properties:
White crystalline powder and it is an amphoteric oxide. Al2O3 + 6HCl 2AlCl3 + 3H2O Al2O3 + 2NaOH 2NaAlO2 + H2O
Aluminium Chloride: AlCl3.6H2O Preparation: It is prepared by the action of dil or conc. hydrochloric acid on aluminium 2Al + 6HCl
2AlCl3 + 3H2
i) Aluminium chloride exists as dimer (Al2Cl6) in inert solvent as well as in vapour state. ii) It is a white crystalline, hygroscopic solid and it fumes in moist air due to hydrolysis. AlCl3 + 3H2O
Al (OH)3 + 3HCl
Alums: Alums are the double sulphates having general formula: X2SO4.M2(SO4)3.24H2O X = monovalent cation such as Na+, K+, NH4+ etc. M = trivalent cation such as Al3+, Cr3+, Fe3+ etc. when alum contains aluminium as trivalent cation then it is named after monovalent cation. e.g.
K2SO4.Al2(SO4)3.24H2O potash alum Na2SO4.Al2(SO4)3.24H2O - Soda alum.
When trivalent cation is not aluminium then alum is named after both, monovalent as well as trivalent cation. (NH4)2SO4.Fe2(SO4)3.24H2O - ferric ammonium alum. Illustration - 6 Which of the following minerals does not contain aluminium ? (a) Cryolite
(b) Mica
(c) Feldspar
(d) Fluorspar
Ans. (d) Note :
Cryolite – Na3 AlF6; Feldspar – KAlSi3O8’ [Mica – K2O · 3Al2O3 ·6SiO2 · 2H2O; Fluorspar – CaF2]
Page No.-8 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Illustration - 7 The function of luorspar in the electrolytic reduction of alumina dissolved in fused cryolite (Na3AlF6) is (a) as a catalyst (b) to lower the temperature of melt and to make the fused mixture very conducting (c) to decrease the rate of oxidation of carbon anode (d) none of these Ans. (b) Illustration - 8 Al2O3 can be converted to anhydrous AlCl3 by heating (a) a mixture of Al2O3 and carbon in dry Cl2 gas (b) Al2O3 with Cl2 gas (c) Al2O3 with HCl gas (d) Al2O3 with NaCl in solid state Ans. (a) Hint:Al2O3 + 3C + 3Cl2 2AlCl3 + 3CO] Illustration - 9 Hydrated AlCl3 is used as (a) catalyst in cracking of petroleum
(b) catalyst in Friedel-Craft’s reaction
(c) mordant
(d) all of these
Ans (c) Illustration - 10 AlCl3 on hydrolysis gives (a) Al2O3 · H2O
(b) Al(OH)3
(c) Al2O3
(d) AlCl3 · 6H2O
Ans (b)
Page No.-9 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
CARBON – FAMILY Elements : C, Si, Ge, Sn, Pb General electronic configuration : ns 2 , np 2 1. Covalent radii The covalent radii increase down the group. The difference in size between Si and Ge is less than might be otherwise expected because Ge has a full 3d-shell, which shields the nuclear charge rather ineffectively. In a similar way the small difference in size between Sn and Pb is because of the filling of the 4f shell. Covalent Radius Ionisation Energy (kJmol–1) Melting point Boiling Point (Å) (°C) (°C) st nd rd th 1 2 3 4 C 0.77 1086 2354 4622 6223 4100 –– Si 1.17 786 1573 3232 4351 1420 3280 Ge 1.22 760 1534 3300 4409 945 2850 Sn 1.4 707 1409 2943 3821 232 2623 Pb 1.46 715 1447 3087 4081 327 1751 2.
Ionisation energy The ionization energy decrease from C to Si, but then change in an irregular way because of the effects of filling of the d and f-shells. The amount of energy required to form M 4 ions is extremely large and hence simple ionic compounds are rare.
3.
Melting points Melting involves breaking the strong covalent bonds in the lattice of C and thus has extremely high melting point. The melting points decrease on descending the group because the M-M bonds become weaker as the atoms increase in size. Sn and Pb are metallic and have much lower melting points. They do not use all four outer electrons for metallic bonding.
4.
Metallic and non-metallic character The change from non-metal to metal with increasing atomic number is well illustrated in group IV, where C and Si are non-metals, Ge has some metallic properties, Sn and Pb are metals. The increase in metallic character shows itself in the structures and appearance of the elements, in physical properties such as malleability and electrical conductivity, and in chemical properties such as the increased tendency to form M 2 ions and the acidic or basic properties of the oxides and hydroxides.
Carbon Allotropes of carbon: The phenomenon of the existence of an element in different forms, which have different physical properties but similar chemical properties, is known as allotropy. such different forms of an element are called its allotropes or allotropic forms. The various allotropic forms of carbon can be broadly classified into two classes. a) Crystalline form: Diamond and graphite are the two crystalline forms of carbon. Graphite is thermodynamically more stable than diamond, thermodynamically, diamonds should get transformed into graphite on their own. But it does not happen at least on our life time scale. This is because, this conversion is not favoured by kinetic factors, i.e., the activation energy for this reaction is very high. Page No.-10 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds b) Amorphous form: Coal, Coke, Charcoal (or wood charcoal), animal charcoal (or bone black), Lamp black, Carbon black, Gas carbon and Petroleum coke are the amorphous form of carbon. Structure of Diamond: In diamond, the carbon atoms are arranged tetrahedrally (sp3 hybridisation of C): Each C atom is linked to its neighbours by four single covalent bonds. This leads to a three-dimensional network of covalent bonds. It is because of this, that diamond is very hard and has high melting and boiling points. Since, all the valence electrons of carbon are used up in forming the covalent bonds, hence diamond does not conduct electricity. Structure of Graphite: In graphite, the carbon atoms are arranged in regular hexagons in flat parallel layers. Each carbon in these layers is bonded to three other by sp 2 covalent bonds. This gives some double bond character to graphite. Each layer is bonded to the adjacent layers by weak vander Waal’s forces. As a result, each layer can slide over the other easily. It is because of this structure that graphite is soft and slippery and can act as a lubricant. The presence of double bond character (the presence of delocalised π-electrons) makes graphite a good conductor of electricity. Oxides: Carbon burnt in air forms two oxides, carbon monoxide, CO and Carbon dioxide CO2. a) Carbon Monoxide (CO) Preparation: i) By heating carbon in limited supply of oxygen. C+
1 O2 2
CO.
ii) By heating oxides of heavy metals e.g. iron, zinc etc with carbon. Fe2O3 + 3C 2Fe + 3CO ZnO + C Zn + CO Two important industrial fuels water gas and producer gas contain carbon along with hydrogen and nitrogen, Water gas is obtained by passing steam over hot coke CO H 2 C + H2O (water gas) When air is passed over hot coke, producer gas is obtained. 2CO 4N 2 2C + O2 + 4N2 (producer gas) Properties:
i) It is powerful reducing agent and reduces many metal oxides to the corresponding metals e.g. Fe2O3 + 3CO 2Fe + 3CO2 CuO + CO Cu + CO2 ii) It burns in air to give heat and carbon dioxide 1 CO + O2 CO2 + heat. 2
Tests:
a) Burns with blue flame b) A filter paper soaked in platinum or palladium chloride is turned pink, green or black due to reduction of the chloride by carbon monoxide.
b) Carbon di-oxide (CO2): Preparation: i) In the lab., it is prepared by the action of acids on carbonates. Page No.-11 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds CaCO3 + 2HCl CaCl2 + H2O + CO2 ii) By combustion of carbon C + O2 CO2 Properties:
i) It turns lime water milky and milkiness disappears when CO 2 is passed in excess Ca(OH)2 + CO2 CaCO3 + H2O CaCO3 + H2O + CO2 Ca(HCO3)2 ii) Solid carbon dioxide or dry ice is obtained by cooling CO2 under pressure. It passes to the solid state straight from gaseous state without liquefying (hence dry ice). iii) A burning candle is put out but burning magnesium continues burning in the gas jar.
Carbides: Carbon combines with more electropositive elements than itself when heated to high temperature to form carbides. Carbides are of mainly three types. i) Salt like Carbides: These are the ionic salts containing either C22- (acetylide ion) or C4(methanide ion)e.g. CaC2, Al4C3, Be2C. ii) Covalent Carbides: These are the carbides of non-metals such as silicon and boron. In such carbides, the atoms of two elements are bonded to each other through covalent bonds. SiC is also known as Carborundum. iii) Interstitial Carbides: They are formed by transition elements and consist of metallic lattices with carbon atoms in the interstices. e.g. tungsten carbide WC, vanadium carbide VC. Illustration - 11 Graphite is not (a) a good conductor of heat
(b) an amorphous allotrope of carbon
(c) softer than diamond
(d) used for making lubricants
Ans (b) Hint :
Graphite is crystalline allotrope of carbon]
Illustration - 12 When heated with concentrated H2SO4, K4Fe(CN)6 gives (a) CO
(b) CO2
(c) (CN)2 and CO
(d) (CN)2 and CO2
Ans (a) Hint :
K4Fe(CN)6 + 6H2SO4 + 6H2O 2K2SO4 + FeSO4 + 3(NH4)2SO4 + 6CO]
Page No.-12 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Illustration - 13 The number and type of bonds between two carbon atoms in CaC2 are (a) one sigma and one pi bond (b) one sigma and two pi bonds (c) one sigma and one and a half pi bonds (d) one sigma bond Ans (b) Hint : Illustration - 14 C a C O 3(s )
H eat
(A ) (s ) + (B ) (g ) + C a rb o n heat
(C ) (s ) + (D ) (g )
( C ) ( s ) + H 2O (E )(g )
The Compound (E) (g) is (a) CO
(b) CO2
(c) CH4
(d) C2H2
Ans. (d) Hint :
CaCO3
Heat
CaO C
CaO (A )
Heat
CaC2 (C)
CaC2 2H2O
CO (B)
CO (D)
Ca(OH) 2
C2H2 ] (E)
Silicon Extraction:
Commercial form of silicon is obtained by reduction of SiO 2 with C or CaC2 in an electric furnace. High purity silicon is obtained either from SiCl4 or from SiHCl3. These volatile compounds are purified by exhaustive fractional distillation and then reduced with very pure Zn or Mg. The resulting spongy Si is melted, grown into cylindrical single crystal and then purified by zone refining.
Properties:
Silicon is obtained by the reduction of silica. It exists in two allotropic forms: (a) amorphous and (b) crystalline. The amorphous variety is obtained by heating dry powdered silica with magnesium.
Page No.-13 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds SiO 2 2Mg
Si
2MgO
1. The crystalline variety is obtained by heating a finely powdered sand or quartz with carbon and in electric furnace a small amount of iron is added to prevent the formation of carborundum (SiC). SiO 2 2C Si 2CO 2. Amorphous silicon is chemically more reactive than crystalline silicon. Amorphous silicon is brownish powder. It burns brilliantly in oxygen and ignites spontaneously in fluorine. Si O SiO 2 2 Si 2F 2
SiF4
3. It decomposes steam at red heat. It dissolves in the mixture of HNO 3 and HCl. However, it dissolves readily in alkaline medium. Si 2H 2 O SiO 2 +2H2 Si 2F 2
SiF4
4. It combines with certain metals forming silicides 2Mg Si Mg 2Si 5. When amorphous silicon is strongly heated, it fuses and on cooling solidifies to the crystalline form. It is very hard crystalline silicon, does not burn in oxygen but it readily combines with fluorine. It dissolves in mixture of HNO 3 and HF. When fused with alkali, it gives a silicate. Na 2 CO3 Si
Na 2SiO 3 C
Silicones: Silicones are a group of organo silicon polymers. Unlike SiCl 4 which on complete hydrolysis gives SiO2, alkyl substituted chlorosilanes on hydrolysis do not give the expected silicon compound analogous to ketone but get hydrolysed to long chain polymers or silicones. While the hydrolysis of trialkylmonochlorosilane yields hexalkyldisiloxane, the alkyldichlorosilane gives straight chain polymers with active hydroxyl groups at each end of the chain and trichlorosilane gives complex cross-linked polymers. The chain size is limited by the size of alkyl group and the amount of cross-linking is regulated by the relative amounts of diand tri-methylchlorosilanes R R Si R
R O
Si R
H3C
R O
Si
O
R O
Si
R O
O
Si
O
CH3 O
O
R
R
Linear silicone polymer
R
H3C
O R
O
O R
R
CH3
Si O R
R
O
Si O R Si R
Cyclic silicone
Cross linked silicone polymer
Silicates: Silicates are regarded as the salts of silicic acid, H 4SiO4. All the silicates are comprised of SiO4 units. These units have a tetrahedral structure formed as a result of sp 3 hybridistion. Page No.-14 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Silicon atom has its complete octet but each oxygen atom is still short of one electron to complete its octet. They can complete their octet by taking up 4 electrons from a metal, getting converted to an anion [SiO4]4–.
O O
Si
O
O 4–
The [SiO4] tetrahedral can be represented in three ways O O
O
O
Si O
O
O O
O
Representations of SiO
O 4– 4
tetrahedra
In some silicates, the oxygen atoms of SiO 4 units tend to complete their octet by sharing electrons with other silicon atoms, the oxygen atoms, thus, form bridges of the type Si—O—Si to other silicon atoms. The number of such bridges can vary from one to four. This leads to the formation of complex silicates. Any oxygen which fails to pick up electrons from the other silicon atom is not able to complete its octet. The resulting silicate chains are, therefore, negatively charged anions. The metal cations generally present in silicate minerals are Li +, Na+, K+, Ca2+, Al3+, etc. Depending upon the way these SiO 4 units are linked, silicates of different structure and complexity are obtained. Some representative types are:
(a) pyrosillicate
(b) cyclic silicate
(c) chain silicate
(d) sheet silicate
Silicon Carbide or (Carborundum) Preparation: SiC is made commercially by reducing silicon with carbon in an electric resistance furnace. SiO2 + 3C Properties:
SiC + 2CO
It is extremely hard and is very difficultly fusible (does not decompose below 2200o C)
Page No.-15 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds It resists most chemical reagents but is oxidised by fused NaOH in contact with air. SiC + 4NaOH + 2O2
Na2CO3 + Na2SiO3 + 2H2O
In SiC, carbon and silicon atoms are alternate and are each surrounded tetrahedrally. It is widely used as an abrasive for grinding, cutting and polishing.
Illustration - 15 Which of the following is correct ? (a) Silicones are organosilico polymers containing Si — O — Si linkage (b) R3SiCl on hydrolysis gives R3Si — O — SiR3 (c) Both of these (d) Both (b) and (c) Ans. (c)
Illustration - 16 In silicon dioxide (a) each silicon atom is surrounded by four oxygen atoms and each oxygen atom is bonded to two silicon atoms (b) each silicon atom is surrounded by two oxygen atoms and each oxygen atom is bonded to two silicon atoms (c) silicon atom is bonded to two oxygen atoms (d) there are double bonds between silicon and oxygen atoms Ans. (a) Hint : [ Each silicon atom is surrounded tetrahedrally by four oxygen atoms | O
| O
|
|
—O — Si — O — Si — O —] |
|
O |
O |
Illustration - 17 The compound (C) is (a) SiO2
(b) Si
(c) SiC
(d) Na2SiO3
Ans. (d) Page No.-16 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Hint: [ SiCl4 H2O
Heat
Si(OH) 4
(A)
SiO2
Na2CO3 Heat
Na2 SiO3 ] (C)
Illustration - 18 When a mixture of air and steam is passed over red hot coke, the outgoing gas contains (a) producer gas
(b) water gas
(c) coal gas
(d) mixture of (a) and (b)
Ans. (d) Hint: Coke air
CO N2 ; Pr oducer gas
Coke
H 2O
Steam
CO
H2 ] Water gas
Oxides and Chlorides of Tin Stannous Oxide (SnO) SnCl 2 2NaOH Sn OH 2 2NaCl Stan nous chloride
Sn(OH) 2
Heat in CO 2Atmosphere
SnCl2 Na 2CO 3
SnO
H 2O
SnO CO 2 2NaCl
Heat in atm of CO2
Stan nous chloride
Properties (i) It is a grey or black power. It is insoluble in water. (ii) It is brns in air with incandescence forming stannic oxide, SnO2 . 2SnO O 2SnO 2 2 (iii) It is an amphoteric oxide. It dissolves both in acids and alkalies. SnO 2HCl SnCl 2 H 2O Stan nous chloride
SnO 2NaOH Na 2SnO 2 H 2 O sodium stannite
Stannites are known only in aqueous solutions. Stannites absorb oxygen from air and are oxidized to stannates which are stable in nature. 2Na 2SnO 2 O 2 2Na 2SnO 3 Sodium stannate
STANNIC OXIDE SnO 2 It is found in nature as Cassiterite or tin stone. Preparation Sn O2 SnO2 Sn 4HNO3 H 2SnO3 4NO 2 H 2O H 2SnO3
Metastannic acid
SnO 2 H 2O
Page No.-17 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Properties It is a white powder, insoluble in water. It is somewhat unreactive. Howaver, it dissolves in concentrated H 2SO 4 forming stannic sulphate. SnO 2 2H 2SO 4 Sn(SO 4 ) 2 2H 2O When the solution is diluted, stannic oxide is reprecipitated. Sn SO 4 2 2H 2O SnO 2 2H 2SO 4 It readily dissolves in alkalies forming stagnates. SnO 2 2KOH K 2SnO3 H 2O
Stannous chloride ( SnCl2 Preparation (i) Hydrated stannous chloride SnCl2 .2H 2O is prepared by dissolving tin in hot concentrated hydrochloric acid and subjecting the solution to crystallization. Sn 2HCl SnCl 2 H2 Hydrated stannous chloride consists of two molecules of water as water of crystallization (SnCl2 .2H 2O). Anhydrous salt cannot be obtained by heating the hydrated salt as it undergoes hydrolysis and a white solid of tin hydroxyl chloride is formed. SnCl2 .2H 2O Sn(OH)Cl HCl H 2O It can also be obtained when a mixture of Sn and calculated quantity of mercuric chloride is heated. Sn HgCl 2 SnCl 2 Hg Properties (i) It is a white crystalline solid. It is soluble in water, alcohol and ether. (ii) In water, it is soon hydrolysed. However in presence of HCl (acid), hydrolysis is revered. (iii) It forms a white precipitate with alkalies. The precipitate of stannous hydroxide, however, dissolves in excess of alkali. SnCl2 2NaOH Sn(OH) 2 2NaCl Sn(OH) 2 2NaOH Na 2SnO 2 2H 2O (iv) It forms a dark brown precipitate of stannous sulphide on passing H 2S through its solution. The precipitate dissolves in yellow ammonium sulphide. SnCl2 H 2S SnS 2HCl SnS (NH 4 ) 2 S2 (NH 4 ) 2 SnS3 Yellow ammonium Sulphide
(v) (a)
Ammoniumthios tan nate
It is a strong reducing agent. Few examples are given below: It reduces mercuric chloride to mercurous chloride (white ppt) and finally to metallic mercury (dark grey or black) 2HgCl 2 SnCl 2 Hg 2Cl 2 SnCl 4 Mercurouschloride
(b)
(c)
Hg 2Cl 2 SnCl2 2Hg SnCl 4 It reduces ferric salts to ferrous salts and cupric salts into cuprous salts. 2FeCl3 SnCl2 2FeCl2 SnCl 4 2CuCl 2 SnCl2 2CuCl SnCl 4 It decolourises iodine and thus can be titrated with it.
Page No.-18 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
(d)
SnCl2 2HCl I 2 SnCl 4 2HI Organic nitro compounds are reduced to amino compounds. C6 H5 NO 2 6HCl 3SnCl 2 C6H 5 NH 2 3SnCl 4 2H 2O Nitrobenzene
(e)
Aniline
Its reduces gold chloride to metallic gold. 2AuCl3 3SnCl 2 2Au 3SnCl 4 Colloidalgold
SnCl4 undergoes hydrolysis forming stannic acid which absorbs colloidal particle of gold and thus forms purple of cassius. Stannic chloride (SnCl4 ) Preparation Sn 2Cl 2 SnCl 4 Properties (i) It is a colourless fuming liquid having disagreeable smell. (ii) It is hygroscopic and forms crystalline hydrates containing 3, 5, 6 and 8 molecules of water as water of crystalllisation. The pentahydrate SnCl4 .5H 2O, is known as “butter of tin” or “oxymuriate of tin”. (iii) It is soluble in water in which it undergoes hydrolysis. SnCl4 4H2O Sn (OH)4 + 4HCl It is also soluble in organic solvents showing that it is a covalent compound. (iv) It dissolves in concentrated HCl forming chlorostannic acid. In presence of ammonium chloride, it forms ammonium salt of this acid. SnCl4 + 2HCl H2SnCl6 Chlorostannic acid
SnCl4 + 2NH4Cl
(NH4)2SnCl6
Ammounium chlorostannate
Oxides and Chlorides of Lead Lead Monoxide (PbO) It is known in two forms: (i) a yellow powder commonly known as massicot and (ii) a buff coloured crystalline form known as litharge. Preparation It is obtained by heating lead or lead sulphide in air at 300°C (massicot form). When the temperature of oxidation is kept at 900°C, litharge is formed. 2Pb O 2PbO 2 2PbS 3O 2PbO 2SO 2 2 It is also formed by heating lead nitrate or lead carbonate. 2Pb NO3 2PbO 4NO 2 O2 2 PbCO3 PbO CO 2 Properties It is insoluble in water. It is an amphoteric oxide. It dissolves both in acids and alkalies. PbO 2HNO Pb NO 3 2 H 2 O 3 PbO 2NaOH
Na 2 PbO 2 H 2O
Page No.-19 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds On heating in air at 470°C, it forms red lead. 6PbO O 2Pb3O 4 (red lead) 2
RED LEAD Pb3O 4 Preparation It is obtained by heating litharge at 470°C in air. o
6PbO O 2 470 C 2Pb3O4 Properties It is a red powder, insoluble in water. When heated, it becomes almost black, but it again becomes red on cooling. On heating above 470°C, it decomposes into PbO and O 2 . 2Pb3O 4
6PbO O2
When treated with concentrated HNO3 , lead nitrate and brownish black insoluble oxide, PbO2 , are formed. This indicates that Pb3O 4 is a compound oxide containing both PbO2 and PbO in the ratio of 1 : 2 Pb3O 4 4HNO 2Pb NO 3 2 PbO 2 2H 2 O 3 With H 2SO 4 , it evolves oxygen, 2Pb3O 4 6H 2SO 6PbSO 4 6H 2O O 2 4 It acts as an oxidising agent. Pb3O 4 8HCl 3PbCl 2 4H 2O Cl2 Pb3O4 4C Pb3O4 4CO
3Pb 4CO 3Pb 4CO 2
LEAD CHLORIDE PbCl2 It is made by adding hydrochloric acid to a cold solution of lead salt. Pb NO3 2 2HCl PbCl 2 2HNO3 It is only slightly soluble in cold water, but appreciably soluble in hot water. It dissolves in concentrated HCl forming a complex ion. PbCl 2 2HCl ‡ˆ ˆ† H 2 PbCl 4 ˆˆ Chloroplumbous
acid
Note Most of the plumbic compound are unimportant because they decompose readily on heating and are hydrolysed to PbO2 by even cold water. An exception is tetraethyl lead, Pb C2 H 5 4 , a colourless liquid which is used as an anti-knock agent in petrol.
Page No.-20 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
NITROGEN FAMILY THE NITROGEN FAMILY Group 15 includes nitrogen phosphorus, arsenic, antimony and bismuth. As we go down the group, there is a shift from non-metallic to metallic through metalloidic character. Nitrogen and phosphorus are nonmetal, arsenic and antimony metalloid and bismuth is a typical metal. Electronic Configuration: The valence shell electronic configuration of these element is ns2 np3 the s orbital in these element is completely filled and p orbitals are half- filled, making their electronic configuration extra stable. Atomic and Ionic Radii: Covalent and ionic (in a particular state) radii increase in size down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and / or f orbitals in heavier members. Ionisation Enthalpy: Ionisation enthalpy decreases down the group due to gradual increase in atomic size. Because of the extra stable half- filled p-orbital electronic configuration and smaller size, the ionisation enthaply of the group 15 element is much greater than of group 14 elements in the corresponding periods. The order of successive ionisation enthalpies, as expected is H1 H 2 H3 Electronegativity: The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that much pronounced. Physical Properties: All the elements of this group are polyatomic. Dinitrogen is a diatomic gas while all others are solids. Metallic character increases down the group. Nitrogen and phosphours are non—metals, arsenic and antimony metalloids and bismuth is a metal. This is due to decrease in ionisation enthalpy and increase in atomic size. The boiling points, in general increase from top to bottom in the group but the melting point increases upto arsenic and then decreases upto bismuth. Except nitrogen, all the elements show allotropy. Element Atomic Number Atomic Mass
N
P
As
Sb
Bi
7
15
33
51
83
14.01
309.7
74.92
121.76
208.98
He 2s2 2p3 Ne 3s2 3p3 Ar 3d10 4s2 4p3 Kr 4d10 5s2 5p3 Xe 4d10 5s 2 5p3
Electronic configuration Covalent Radius/pm
70
110
120
140
150
171a
212a
222a
76b
103b
I
1402
1012
947
834
703
II
2856
1903
1798
1595
1610
III
4577
2910
2736
2443
2466
3.0
2.1
2.0
1.9
1.9
Ionic Radius /pm
a M 3 , b M 3 Ionization enthalpy \
kJ mol 1
Electronegativity
Page No.-21 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Chemical Properties : Oxidation States and trends in a chemical reactivity: The common oxidation states of these elements are –3, +3 and +5. The tendency to exhibit –3 oxidation state decreases down the group, bismuth hardly forms any compound in –3 oxidation state. The stability of +5 oxidation state decreases down the group. The only well characterized Bi(V) compound is BiF5 . The stability of +5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group. Bi3 Sb3 As3 ; Bi5 Sb5 As5 Nitrogen exhibits +1, +2, +4, +5 oxidation states also when it reacts with oxygen. Phosphours also shows +1 and +4 oxidation states in some oxoacids. In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example, 3HNO HNO 3 H 2O 2NO 2 Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate into +5 and –3 both in alkali and acid. However +3 oxidation state in case of arsenic, antimony and bismuth become increasingly stable with respect to disproportionation. Nitrogen is restricted to a maximum covalency of 4 since only four (one s and three p) orbitals are available for bonding. The heavier elements have vacant d orbitals in the outermost shell which can be used for bonding (covalency) and hence, expand their covalence as in PF6 . Reactivity towards hydrogen : All the elements of Group 15 form hydrides of the type EH3 where E = N, P, As, Sb or Bi. Some of the properties of these hydrides are shown in Table. The hydrides show regular gradation in their properties. The stability of hydrides decreases from NH3 to BiH3 which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides. Basicity also creases in the order NH3 PH3 AsH3 SbH3 BiH3 PROPERTIES OF HYDRIDES OF GROUP 15 ELEMENTS PROPERTY
NH3
PH3
AsH3
SbH3
BiH3
Melting point / K
195.2
139.5
156.7
185
–
Boiling point / K
238.5
185.5
210.6
254.6
290
(E–H) Distance / pm
101.7
141.9
151.9
170.7
–
HEH angle (°)
107.8
93.6
91.8
91.3
–
f H / kJ mol1
– 46.1
13.4
66.4
145.1
278
diss H E H / kJ mol 1
389
322
297
255
–
Strength of oxoacids of group VB elements For an oxidation state of +5, the strengths of various acids of group 15 elements follow the order, HNO3 H 3PO 4 H3AsO 4 H 3SbO 4 For the oxyacids involving the same element in different oxidation states, the strength of an acid depends upon the number of unhydrogenated oxygen atoms attached to the central atom. For Page No.-22 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
example, for the oxyacids of the type OH m ZOn , the acid strength varies directly with the value of n. Thus, nitric acid HNO3 is stronger than nitrous acid HNO2 . The acids H3PO 2 , H3PO3 and H 3PO 4 are approximately of equal strength, because all these acids contain only one unhydrogenated oxygen atom each. The order of acid strength is H3PO 2 H 3PO3 H 3PO 4
Nitrogen Preparation: 1. NH 4 NO 2
N2
2H 2O
Since ammonium nitrite is very unstable, it cannot be kept as such. Hence nitrogen is usually prepared by heating a mixture of ammonium chloride and sodium nitrite. NH 4 Cl NaNO 2 NH 4 NO 2
N2
NH 4 NO 2
NaCl
2H 2O
2. By heating ammonium dichromate: Ammonium dichromate on heating decomposes to give nitrogen gas. (NH 4 ) 2 Cr2 O 7 2NH 3 3CuO
N2
4H 2O
Cr2O3
N2 (g) 3Cu
NH 2 CONH 2 2HNO 2
2N 2
NaNO 2 HCl
HNO 2
NaCl
NH 2 CONH 2 2HNO 2
3H 2O CO 2
3H 2O
CO 2
3H 2O 2N 2
Fixation of atmospheric nitrogen in cyanamide fertilizer (The cyanamide process) Nitrogen is also fixed as calcium cyanamide on heating it with calcium carbide at 1000°C in an electric furnace. CaC2
calcium carbide
N 2
electric furnace
CaCN 2
C
calcium cyanamide
The mixture of calcium cyanamide and carbon (trade name nitrolim) is an important fertilizer. Calcium cyanamide may also be used as a source of ammonia. The ammonia so produced can be converted into useful fertilizers. Calcium cyanamide is decomposed by water to give ammonia. CaCN 2 3H 2O
CaCO 3
2NH 3 H 2SO 4
(NH 4 ) 2 SO 4
2NH 3 H 2SO 4
(NH 4 ) 2 SO 4
P2 O5 6NH3 3H 2O CaCl 2 8NH 2
2NH 3
2(NH 4 )3 PO 4
CaCl2 .8NH 3
Page No.-23 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Illustration - 19 Ammonia will be obtained in (a) CaCN2 + H2O
(b) NH4 H2 PO4
(c) Both of these
(d) None of these
Heat
Ans (c) Hint: [ CaCN2 3H2O
CaCO 3
2NH3 ; NH4 H2PO4
NH 3
HPO 3
H2O]
Illustration - 20 Which one of the following compounds on strong heating evolves ammonia gas ? (a) (NH4)2 Cr2O7 (b) NH4NO3
(c) NH4NO2
(d) (NH4)2SO4
Ans (d) Hint: [ (NH4 )2 Cr2O7
N2 ; NH3 NO 3
N2O; NH3NO2
N2 ; (NH4 )2 SO 4
NH3 ]
Oxides of Nitrogen Formula
Name
Colour
Remarks
N2O
Nitrous oxides
Colourless
Rather unreactive
NO
Nitric oxide
Colourless
Moderately reactive
N2O3
Dinitrogen trioxide
Dark blue
Extensively dissociated as gas
NO2
Nitrogen dioxide
Brown
Moderately reactive
N2O4
Dinitrogen tetroxide
Colourless
Extensively dissociated to NO2 as gas and partly as liquid
N2O5
Dinitrogen pentoxide
Colourless
unstable as gas; ionic solid
NO3, N2O6
Not well characterized and quite unstable
Preparation: 1. N2O is obtained generally by heating NH4NO3: NH 4 NO 3
N 2O
2H 2O
N2O is also known as laughing gas because it induces laughter mixed with N2. It is used as an anesthetic by dentists 2. NO is best prepared by the reduction of HNO 3 with reducing agents like Cu or by reduction of nitrous acid or nitrites by Fe 2+ of I– ions: 3Cu 8HNO 3
2Cu(NO 3 ) 2 2NO 4H 2O
2NaNO 2 2FeSO 4 3H 2SO 4 2NaHSO 4 Fe2 (SO 4 )3 2NO 2H 2O 2NaNO 2 2NaI 4H 2SO 4
4NaHSO 4
2NO I2
2H 2O
Page No.-24 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds NO is formed as an intermediate in the manufacture of nitric acid by oxidation of NH3. 3. N2O3 is obtained as an intense blue liquid or a pale blue solid on cooling an equimolar mixture of NO and NO2: NO NO 2
N 2O3
On warming, its colour fades due to its dissociation into these two oxides. 4. NO2 can be prepared by reduction of conc. HNO3 with Cu or by heating heavy metal nitrates: Cu 4HNO 3 2Pb(NO3 ) 2
Cu(NO 3 ) 2 2NO 2 2H 2O 2PbO 4NO 2 O2
5. N2O5 is an anhydride of HNO 3. It is best prepared by dehydrating HNO 3 with P4O10 at low temperatures: 4HNO3 P4 O10 Properties:
250 K
2N 2 O5
4HPO3
Oxides of nitrogen are all oxidizing agents. N2O even support the combustion of S and P. NO which is thermally more stable, supports the combustion of Mg and P but not of S. Sulphur flame is not hot enough to decompose it N 2O and NO are neutral, while the other oxides are acidic. – Liquid N2O4 undergoes self-ionisation to form NO+ and NO3 ions and therefore, it has been extensively studied as a non-aqueous solvent. Solid N 2O5 exists in the – ionic form, NO 2 NO3 . In the gaseous form, the discrete N2O5 molecules have a N —O—N bond angle close to 180°.
Oxyacids: The most important oxo-acid of nitrogen is nitric acid HNO3 Preparation: i) In the lab, it is prepared by heating NaNO 3 or KNO3 with conc. sulphuric acid in a glass retort. NaNO3 + H2SO4 NaHSO4 + HNO3 ii) It is manufactured by the catalytic oxidation of ammonia and the process is known as Ostwald process 4NH3 + 5O2 Pt / Rh 4NO + 6H2O 2NO + O2 1120K 2NO2 3NO2 + H2O 2HNO3 + NO. Properties:
i) In aqueous solution, nitric acid is a strong acid and dissociates to give hydronium and nitrate ions. HNO3 + H2O
H3O+ + NO3-
ii) Action on metals: Conc. nitric acid is a strong oxidising agent and attacks most metals except noble metals such as gold and platinum. The product of reduction depend upon the concentration of the acid, temp and the nature of Page No.-25 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds the material undergoing oxidation. With dilute nitric acid the principle product is nitric oxide NO and with conc. nitric acid, the principle product is N(IV) oxides 3Cu + 8HNO3 (dilute)
3Cu(NO3)2 + 2NO + 4H2O
Cu + 4HNO3
Cu(NO3)2 + 2NO2 + 2H2O
Zinc, which is a more powerful reducing agent than copper, reacts with dilute nitric acid to give ammonium nitrate. 4Zn + 10HNO3 4Zn(NO3)2 + NH4NO3 + 3H2O Nitric acid also oxidises non - metals and their compounds. Iodine is oxidized to iodic acid, carbon to carbon dioxide, sulphur to H 2SO3 and H2SO4 and phosphorus to phosphoric acid. Nitric acid is reduced to nitrogen dioxide (NO2) I2 + 10HNO3 2HIO3 + 10NO2 + 4H2O C + 4HNO3 CO2 + 2H2O + 4NO2 1/8 S8 + 6HNO3 H2SO4 + 6NO2 + 2H2O P4 + 20HNO3 4H3PO4 + 20NO2 + 4H2O
Illustration - 21 Copper metal on treatment with dilute HNO3 produces a gas (X). (X) when passed through acidic solution of stannous chloride, a nitrogen containing compound (Y) is obtained (Y) on reaction with nitrous acid produces a gas (Z). Gas (Z) IS (a) NO (B) N2 (C) NO2 (D) N2O Ans. (d)
Page No.-26 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Hint :
[Cu dil. HNO3 NO
SnCl2 / HCl
NH 2OH · HCl
(X)
HNO2
(Y)
N2O] (Z)
Illustration - 22 Nitrogen forms N2 but phosphorus is converted into P4 from P2. The reason for this is (a) triple bond is present between phosphorus atoms (b) p-p bonding is weak (c) p-p bonding is strong (d) multiple bond is formed easily Ans. (b) Illustration - 23 When AgNO3 is heated strongly, the products formed are (a) NO and NO2
(b) NO2 and N2O
(c) NO2 and O2
(d) NO and O2
Ans. (c)
Phosphorous Extraction:
It is a very reactive element, so it does not occur free in nature. Different ores are: Phosphorite : Ca3 (PO4)2 Fluorapatite : 3Ca3(PO4)2 CaF2 Chlorapatite: 3Ca3 (PO4)2 . CaCl2 Phosphorous is obtained by heating bone ash or phosphorite rock Ca 3(PO4)2 with sand (SiO2) and coke (c) in an electric furnance at about 1770 K. The reactions are as the following: 2Ca 3 (PO 4 ) 2 6SiO 2 P4 O10 10C
Properties:
P4 O10
6CaSiO3
P4 (v) 10CO
i) Reaction with oxygen: Yellow phosphorus readily catches fire in air giving dense white fumes of phosphorus pentaoxide. Red phosphorus combines with oxygen only on heating. Both of them form either phosphorus trioxide or phosphorus pentaoxide. P4
3O 2 heat
P4
5O 2 heat
red phosphorous
red phosphorus
2P O
2 3 phosphorus trioxide
2P O
2 5 phosphorus pentoxide
ii) Reaction with chlorine: Phosphorus reacts with chlorine gas to form tri and pentachlorides. Yellow phosphorus reacts violently at room temperature, whereas red phosphorus reacts on heating only. Page No.-27 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds P4 6Cl 2 P4 10Cl 2
4PCl
3 phosphorus trichloride
4PCl
5 phosphorus pentachloride
iii) Reaction with alkalies (sodium/potassium hydroxide): Yellow phosphorus dissolves in caustic soda on boiling under an inert atmosphere liberating phosphine. 4P 3NaOH 3H 2 O boil PH3 (g) 3NaH 2 PO 2 yellow phosphorus
phosphine
sodium hypophosphite
iv) Reaction with nitric acid Phosphorus gets oxidized by nitric acid to phosphoric acid. P4 20HNO 4H 3 PO 4 20NO 2 4H 2O 3 phosphoric acid
v) With metals Phosphorus reacts with metals forming phosphides. For example 6Mg P 4 Allotropes:
2Mg P
3 2 magnesium phosphide
Phosphorus exists in the following five different allotropic forms. i) White (yellow) phosphorus is extremely reactive. ii) Below 800°C, its vapor density corresponds to the formula P4. Above 1700°C, it exists as P2. P iii) Due to is low ignition temperature (~30°C), it undergoes oxidation in the presence of air which slowly raises its temperature and after a few moments it catches fire P P spontaneously. Due to this reason, it is stored under water. P
Red Phosphorus: Red phosphorus is stable allotrope at room temperature. Red phosphorus is formed by heating white phosphorus in the absence of air at about 250°C. It is not poisonous. It is safe to handle because it does not burn spontaneously at room temperature. P
P
P
P
P
P
P
P
Structure of red phosphorus
1.
Oxides: Two important oxides of phosphorus are : 1. Phosphorus trioxide - P4O6, also called phosphorous oxide or phosphorus (III) oxide 2. Phosphorus pentoxide - P4O10, also called as phosphoric oxide or phosphorus (V) oxide
Page No.-28 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds a) Phosphorous trioxide (P4O6): Preparation: Prepared by burning white phosphorus in limited supply of air P4 + 3O2 Properties:
P4O6
i) On heating in air, it forms phosphorus (V) oxide P4O6 + 2O2
P4O10
ii) Reacts with water as follows : P4O6 + 6H2O (cold) Phosphorous acid P4O6 + 6H2O (hot)
4H3PO3 3H3PO4 + PH3
b) Phosphorous Pentoxide (P4O10) Preparation: Prepared by burning white phosphorus in excess of air or oxygen P4 + 5O2 Properties:
P4O10
i) Because of its great affinity for water, it acts as a dehydrating agent. 2HNO3 + P4O10
2N2O5 + 4HPO3
2H2SO4 + P4O10
2SO3 + 4HPO3
ii) Reacts with water as follows: P4O10 + 2H2O (cold) P4O10 + 6H2O (hot)
4HPO3 4H3PO4
Structure of P4O6 & P4O10 P O
O P
O
O
127° O
O
143 pm P O
P 100°
P 166 pm O
O P
O O
102° O 123° O
P
160 pm
P O O
O
2.
Oxoacids of Phosphours: Phosphorus forms two series of oxoacids, the phosphoric and the phosphorous acids. The oxidation state of phosphorus is +5 in phosphoric acids whereas it is +3 in phosphorous acids.
Page No.-29 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Oxoacids of phosphorus and their properties Acid
Nature
Preparation
Anion
Remarks
H 3 PO 2 or H 2 P(OH)O Hypophosphorous
crystalline white solid
white P4 + alkali
H 2 PO 2– hypophosphite
strongly reducing, monobasic pK ~ 2
H 3 PO3 or HPO(OH) 2 Orthophosphorous
deliquescent colourless solid
P4O6 or PCl3 + H2O
H 2 PO3– , HPO32– phosphite
reducing, dibasic pK 2 ~ 6
H4P2O3 Pyrophosphorous
white solid
PCl3 + H3PO3
H 2 P2 O52– pyrophosphite
reducing, dibasic
H4P2O6 Hypophosphoric
white solid
red P + alkali
P2 O64– hypophosphate
not reducing or oxidizing, tetrabasic pK1 ~ 2
H3PO4 Orthophosphoric
white solid
P4O10 + H2O
H 2 PO 4– , HPO 42– ,
not oxidizing, tribasic
H4P2O7 Pyrophosphoric
colourless solid
heat phosphates or phosphoric acid
HPO3 Metaphosphoric
deliquescent solid
heat H3PO4 to 600 K
but slow, pK1 ~ 2
PO3– 4 , phosphate P2 O74– pyrophosphate
tetrabasic pK1 ~ 2
A large number of condensed phosphoric acids or their salts are known which have rings or chains of PO4 tetrahedrally linked through P-O-P linkages, e.g., di or pyrophosphoric acid, H 4P2O7 and triphosphoric acid, H5P3O10. O HO
P OH
O O
P OH
Diphosphoric acid
O OH
HO
O
P
O
OH
P
O O
OH
P
OH
OH
Triphosphoric acid
Sodium salt of triphosphoric acid, Na5P3O10, forms stable chelate complexes with alkaline earth metal cations. It is, therefore, used in water softening. What is known as metaphosphoric acid and given the empirical formula HPO3 is in fact a mixture of cyclo-polyphosphoric acids containing —P—O—P—O — linkages. Two important cyclo-polyphosphoric acids are cyclotriphosphoric acid, H 3P3O9 and cyclotetraphosphoric acid, H4P4O12.
Page No.-30 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds O HO
O
HO
P O
O
O
P
HO
HO
OH
cyclo-triphosphoric acid
3.
O
O
O
P O
P
O
P
P
OH
O O
P
OH
O O Cyclo-tetraphosphoric acid
Phosphine, PH3: Phosphine, PH3, is the most stable hydride of phosphorus. It is intermediate in thermal stability between ammonia and arsine. Preparation 1. Hydrolysis of metal phosphides such as AlP or Ca3P2: Ca 3P2 6H 2 O 2PH 3 3Ca(OH) 2 2. Pyrolysis of phosphorous acid at 480 – 485 K: 4H 3PO PH3 3H 3PO 4 3 3. Alkaline hydrolysis of phosphonium iodide: PH 4 I KOH PH 3 KI H 2O 4. Alkaline hydrolysis of white phosphorus (industrial process): P4 3KOH 3H 2 O PH 3 3KH 2 PO 2 Phosphine is a colourless, extremely poisonous gas having a faint garlic odour. As the P–H bond is not polar enough to form P–H----P or P-H----O bonds, unlike ammonia, phosphine is not associated in the liquid state and is much less soluble in water. In contrast to the basic nature of solutions of ammonia in water, aqueous solutions of phosphine are neutral, which is due to the much weaker tendency of PH 3 to protonate in water. However, it does react with HI to form phosphonium iodide: PH 3 HI
PH 4 I
Pure phosphine ignites in air at about 435 K, but when contaminated with traces of P2H4 it is spontaneously inflammable: PH3 2O 2
H 3 PO 4
Illustration - 24 H 3P O
2 5 0 ºC 4
(X ) 6 0 0 ºC
(Y )
s tr o n g ly h e a te d
(Z )
(X), (Y) and (Z) are : (a) H4P2O7, HPO3 and P4O10
(b) HPO3, H4P2O7 and P4O10
(c) H4P2O6, H3PO3 and P4P6
(d) H4P2O6, HPO3 and P4O6
Page No.-31 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Ans.(a) Hint :
[ H3PO4
250ºC
H4P2O7
600ºC
strongly
HPO3
heated
P4O10 ]
Illustration - 25 How many P – O bonds and how many lone pairs respectively are present in P4O6 molecule? (a) 12, 4
(b) 8, 8
(c) 12, 16
(d) 12, 12
Ans. (c) The structure of P4O6 is the number of P – O bonds and lone pairs are shown. These are 12 and 16 respectively]. :
Hint :
P
:P
:O : O
P :
:
P
Illustration - 26 Which of the following halides is most acidic ? (a) PCl3
(b) SbCl3
(c) BiCl3
(d) CCl4
Ans. (a) Hint :
In CCl4 carbon atom does not have d-orbitals to accommodate a lone pair of electrons and hence is not a Lewis acid. In PCl3, SbCl3 and BiCl3 central atom has empty d-orbitals in each case but electronegativity of P is maximum, hence PCl3 is strongest acid].
Illustration - 27 Among the following ions, p– d overlap is present in (a) NO3–
(b) PO43–
(c) CO32–
(d) NO2–
Ans. (b)
Illustration - 28 The compound molecular in nature in gas phase but ionic in solid state is (a) PCl3
(b) CCl4
(d) PCl5
(d) POCl3
Ans. (c) Hint : [PCl5 exists as [PCl4]+ [PCl6]– in solid state].
Page No.-32 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
ELEMENTS (OXYGEN FAMILY) General physical properties of oxygen family elements : Group VI(B) of the periodic table contains five elements namely oxygen, sulphur, selenium, tellurium and polonium. They are collectively called chalcogens or ore forming elements because many metal ores occur as oxides or sulphides. The first four members are non-metals, white the last member polonium is a radioactive metal having very short half-life. Atoms of the 2 2 1 1 elements have the outermost electronic configuration as ns np x np y np z . Thus, there are two half filled p-orbitals which are used for bonding with other elements. Some properties of these elements are given in the following table.
(i)
(ii)
Atomic and ionic radii Atomic (covalent) radius increases as we go down the group. O < S < Se < Te < Po Ionic Radius of dinegative ions (M 2 ) also increase from oxygen to polonium. The order is, O 2 S2 Se2 Te2 Catenation The self-linking property of atoms with identical atoms is called catenation. Oxygen shows a little tendency towards catenation, e.g., in peroxides, [o o ]2 Sulphur shows a strong tendency towards catenation, e.g., in polysulphides S2n sulphanes (H Sn H), polysulphuric acid HO3S.Sn .SO3H , and in various allotropes. The S - S bond is very important in the biological systems. The S - S bonding is found in compounds such as, cysteine, some proteins and enzymes. The catenation tendency decreases markedly as we go down the group
Page No.-33 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds (iii)
Density Density of group VIB elements increases in going from oxygen to polonium, i.e., the density of group 16 elements follows the order, O S Se Te Po
(iv)
Melting points and Boiling points The melting and boiling points show regular increase with the increase in atomic number. As we go down the group, the molecule size increases. As a result Van der-Waals’ forces increase and hence the meling and boiling points also increase.
(v)
Electron Affinity vs Electron Gain Enthalpy It has been recommended by IUPAC to replace the term Electron Affinity E ae by a new
term, Electron Gain Enthalpy eg H . The electron gain enthalpy is the molar enthalpy change when an isolated gaseous atom or ion in its ground state, adds an electron to form the corresponding anion. Thus, the enthalpy change for the reaction, X(g) e X (g)
is called the electron gain enthalpy eg H of the element X. The eg H may be positive or negative (like the electron affinity). Electron affinity E ae , or Electron gain enthalpy
eg H
of oxygen and sulphur are
given below O(g) e O (g)
E ae (O O ) 142kJ mol 1
S(g) e S (g)
E ae (S S ) 200kJ mol1
O (g) e O 2 (g) E ae (O O 2 ) 780kJ mol S (g) e S2 (g) E ae (S S2 ) 590 mol1
Thus, the electron affinities for O O 2 and S S2 are positive. The first electron affinity of oxygen is much smaller than those of the other elements. Electron affinity becomes smaller as we go down the group, i.e., from sulphur to tellurium. O S Se Te Po
Page No.-34 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
(vi)
Ionization energy or ionization enthalpy, H
The ionisation energies of group VIB elements are quite high. Due to their high ionization energies, it is extremely difficult to remove electrons from the atoms of these elements. The ionisation energy decreases as we go from oxygen to polonium. The first ionisation energies of lighter elements of group VIB (oxygen family) are lower than those of group VB, (nitrogen family) (vii)
Oxidation states. The outer electronic configuration of group VIB elements can be described as ns 2 np4 . Being strongly electronegativity, these elements complete their shells by gaining two electrons. Thus, all the elements of group 16 shows an oxidation state of -2. However, these elements also show other oxidation states as follows. Oxygen shows an oxidation state of + 2 F2 O in and -1 in peroxides O 22 . Other elements of group VIB exhibit oxidation states of + 2, + 4 and + 6 also. The oxidations state of + 4 and + 6 being more stable. For sulphur, selenium and tellurium, the oxidation states of + 4 and + 6 are important. The + 4 state is more stable for Se, Te and Po, then + 6 state. This is due to the availability of d-orbitals is the valence shells of the atoms of these elements.
(viii) Molecular structure (or atomicity) Oxygen forms stable diatomic O 2 molecules, while sulphur, selenium, tellurium and polonium are octa atomic molecules, viz, S8 ,Se8 , Te8 and Po8 with puckered-ring structures. The puckered ring structure of sulphur is shown in figure. Under ordinary conditions, oxygen exists as a gas, while all other elements of this group are solids.
Explanation This is because oxygen has tendency to from p p multiple bonds. So, oxygen forms a diatomic (O = O) molecule. Due to weak van der Waals’ forces between the oxygen molecules, oxygen exists as a gas. Because of their larger atomic size, sulphur and other heavier elements of this group do not form stable p p bonds. So, these elements fo not occur as diatomic molecules. Instead, S and other heavier elements of this group form M - M single bonds giving rise to polyatomic molecule. For example, sulphur froms S8 octatomic molecules. Due to stronger van der Waal’s forces between these polyatomic molecules, these elements (sulphur and other) exists as solid.
Page No.-35 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-36 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-37 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-38 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Illustration - 29 In which of the following reactions O2 is not formed as one of the product ? (a) KClO 3
MnO2
(b) SnCl2 HCl O3
Heat
(c) FeSO4 H2SO 4 O3
(d) PbS O3
Ans (c) Hint :
[ 3SnCl2 6HCl O3
3SnCl 4
3H2O ]
Illustration - 30 Acidified KMnO4 is dropped over sodium peroxide taken in a flask at room temperature, vigorous reaction takes place to produce (a) hydrogen peroxide
(b) a mixture of hydrogen and oxygen
(c) a colourless gas hydrogen
(d) a colourless gas dioxygen
Ans. (d) Page No.-39 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds [Na2O2 H2SO 4
Hint:
Na2SO 4 H2O2 ]
2KMnO4 3H2SO 4 [H2O2 O
K 2SO 4
H 2O O2 ]
5
2MnSO 4
3H 2O
50
5
Illustration - 31 The structure of O3 and N3– are (a) linear and bent respectively (b) both linear (c) both bent (d) bent and linear respectively And (d) Hint :
Page No.-40 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Potassium iodide and starch produces deeper blue colour with acidified H2O2 H2O2 2H 2I I3
2H2O
Page No.-41 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-42 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
USES 1. In bleaching of delicate materials such as silk, wool, cotton, ivory etc. 2. As a valuable antiseptic and germicide for washing wounds, teeth and ears under the name perhydrol. 3. As ‘antichlor’ to remove traces of chlorine and hypochlorite. 4. As oxidising agent in rocket fuels. Illustration - 32 An inorganic substante liberates oxygen on heating and turns acidic solution of KI brown and reduces acidified KMnO4 solution. The substance is (a) HgO
(b) H2O2
(c) KNO3
(d) Pb(NO3)2
And (b)
2H2O2
Hint :
Heat
2KI H2O 2
2H2O Acidic
O2 '
2KOH
I2 Brown
Acidified KMnO4 acts as an oxidising agent when it is decolourised, H2O2 + O H2O + O2] Illustration - 33 When H2O2 is added to ice cold solution of acidified potassium dichromate in ether and the contents are shaken and allowed to stand (a) a blue colour is obtained in ether due to formation of Cr2(SO4)3. (b) a blue colour is obtained in ether due to formation of CrO5 (c) a blue colour is obtained in ether due to formation of CrO3 (d) chromyl chloride is formed And (b) Hint :
Cr2O72 2H 4H2O2
2CrO 5 Blue colour in ether
5H2O
Illustration - 34 What is false about H2O2 ? (a) acts both as oxidising and reducing agent (b) two OH bonds lie in the same plane (c) plane blud liquid (d) can be oxidised by O3 And (a) Page No.-43 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Hint : [Two OH bonds lie in the different planes] Illustration - 35 Two liquids (A) and (B) are made of same elements and are diamagnetic. Liquid (A) on treatment with KI and starch gives blue coloured solution, however, liquid (B) is neutral to litmus and does not give any response to starch iodide paper. Ans. [ (A) H2O2; (B) H2O ]
Sulphur Extraction:
The most important method for the extraction of sulphur from native deposits is the Frasch Process. It consists of boring a hole from the ground surface to the sulphur bearing calcite deposit and lowering three concentric pipes to the ore bed. Superheated water, 438 K, is forced down the outer pipe into the ore bed where it melts the sulphur. Compressed hot air is pumped down through the innermost pipe when a frothy mixture of molten sulphur, water and air is forced to the surface through the middle pipe. As it comes out from the well, sulphur has a purity of 99.5–99.9% and virtually does not contains As, Se or Te.
Properties:
Sulphur displays allotropy to a remarkable degree, existing both in a variety of different molecular and physical forms. The molecular species, viz., S 2, S4, S6 and S8 are in equilibrium in gaseous sulphur, their proportions varying with the temperature. The common and the most stable allotrope of sulphur at room temperature is known as rhombic sulphur or α-sulphur, S . In rhombic sulpur, S8 rings are arranged in a way that gives a rhombic crystal structure. At 369 K, rhombic sulphur gets converted into monoclinic sulphur or β-sulphur, S . In monoclinic sulphur, S8 rings are arranged in a monoclinic structure. It is stable between 369 and 392 K. At 392 K it melts to produce a liquid containing S8 molecules, S. At about 433 K the S8 rings open up and join together into long spiral-chain molecules resulting in a thick viscous liquid -sulphur, S. Liquid sulphur boils at 718 K to give gaseous sulphur containing S8 molecules, which dissociate to S6, S4, S2 and finally to sulphur atoms at 2273 K. If liquid sulphur at 463 K is poured into cold water, plastic sulphur or -sulphur is formed. The allotropy of sulphur as a function of temperature is summarized as follows: Cold water S S 369 K S 392 K S
S4 1273 K
S2
2273 K
433 K
S
718 K
S8 (g)
S6
S
Page No.-44 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
COMPOUNDS OF SULPHUR
Page No.-45 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-46 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
OXOACIDS OF SULPHUR The oxoacids of sulphur are more numerous and more important than those of Se and Te. Many of the oxoacids of sulphur do not exist as free acids, but are known an anions and salts. Acids ending in -ous have S in the oxidation state (+IV) and form salts ending in –ite. Acids ending in have S in the oxidation (+IV) state and form salts ending in –ate As discussed previously under bond lengths and p – d bonding, the oxoanions have strong bonds and so they have little tendency to polymerise compared with the phosphate and silicates. To emphasize structural similarities the acids are listed in four series 1. Sulphurous acid series, 2. Sulphuric acid series, 3. Thionic acid series 4. Peroxoacid series.
Page No.-47 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-48 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Bond lengths and p – d bonding The bonds between S and O, or Se and O, are much shorter than might be expected fro a single bond. In some cases they may be formulated as localized double bonds. A –bond is formed in the usual way. In addition a -bond is formed by the sideways overlap of a p-orbital on the oxygen with a dorbital on the sulphur, giving a p–d interaction. This p–d bonding is similar to that found in the oxides and oxoacids of phosphorus and is in contrast to the more common p–p type of double bond found in ethane. To obtain effective p - d overlap, the size of the d-orbital must be similar to the size of the porbital. Thus sulphur forms stronger -bonds than the larger elements in the group. On crossing a period in the periodic table, the nuclear charge is increased and more s and p-electrons are added. Since these s and p-electrons shield the nuclear charge incompletely, the size of the atoms and the size of the d-orbitals decreases from Si to P to S to Cl. The decrease in the size of the 3d-orbitals in this series of elements leads to progressively stronger p-d bonds. Thus, in the silicates there is hardly any p-d bonding. Thus, SiO4 units polymerise into an enormous variety of structure linked by Si–O–Si –bonds. In the phosphates, –bonding is stronger, but a large number of polymeric phosphates exist. In the oxoacids of sulphur, –bonding is even stronger and has become a dominant factor. Thus, only a small amount of polymerization occurs and only a few polymeric compounds are known with S–O–S linkages. For chlorine, p–d bonding is so strong that no polymerization of oxoanions occurs. In cases where these is more than one -bond in the molecule it may be more appropriate to explain the -bonding in terms of delocalized molecular orbitals covering several atoms. Sulphuric acid, (H2SO4) Manufacture of Sulphuric Acid by Contact Process (i) Production of SO2 Sulphur dioxide (SO2) is obtained by burning sulphur or iron pyrites S O 2 SO 2 (g) Sulphur 4FeS2 11O 2 2Fe 2O3 8SO 2 (g) Iron pyrites (ii) Catalytic oxidation of SO2 + SO3 Sulphur dioxide is oxidized by air in the presence of catalyst to give sulphur trioxide. catalyst 2SO 2 (g) O2 (g) ‡ˆ ˆˆ ˆˆ ˆˆ ˆˆ†ˆ 2SO3 (g) H 196.6kJ (a) Air or oxygen required for the oxidation of sulphur dioxide must be in excess. (b) The temperature must be low. A temperature between 350-450 0C gives the maximum yield of the product. (c) The pressure of about 2 atmospheres is used to carry out the above reaction.
Page No.-49 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-50 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Illustration - 36 Sulphur on reaction with concentrated HNO3 gives (A) which reacts with NaOH gives (B). (A) and (B) are (a) H2SO3, Na2S2O3
(b) NO2, Na2S
(c) H2SO4, Na2SO4
(d) H2S2O3, Na2S2O3
Page No.-51 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Ans (c) Hint :
6HNO3 S
H2SO 4
6NO 2
(A )
H2SO4 2NaOH
Na2SO 4
2H2O; 2H2O]
(B)
Illustration - 37 Which of the following elements forms p-d bonding in its oxide ? (a) Lithium
(b) Boron
(c) Sulphur
(d) Nitrogen
Ans. (c) Hint : [Among the given elements, sulphur forms d-p bonding in its oxides such as SO2 and SO3] Illustration - 38 Compounds A and B are treated with dilute HCl respectively. The gases liberated are Y and Z respectively. Y turns acidified K2Cr2O7 paper green while Z turns lead acetate paper black. The compounds A and B are respectively. (a) Na2S and Na2SO3
(b) Na2SO3 and Na2S
(c) NaCl and Na2CO3
(d) Na2SO3 and Na2SO4
Ans. (b) Hint : [SO2 turns acidified K2Cr2O7 paper green. Na2SO3 H2SO 4
Na2SO 4
K 2Cr2O7 H2SO 4 3SO2
SO 2 (Y)
H2O
Cr2 (SO4 )3 Green
K 2SO 4
H2O
H2S turnes lead acetate paper black Na2S H2SO4
Na2SO 4
H2S Pb (CH3COO)2
H2S
PbS
(Z)
2CH3 COOH]
Page No.-52 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
ELEMENTS (HALOGEN FAMILY) Fluorine (F), chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At) are grouped together to form group 17 of the periodic table. Their salts are present in seawater and hence they are also known as halogens (Halos, in Greek, means sea salt producers). The group collectively is called halogen family. The general electronic configuration of halogens is ns 2 np5 . They are just one electron short of stable electronic configuration of inert gas elements. They have a very strong tendency to acquire stable configuration by accepting one electron. For this reason they exhibit non-metallic behaviour. Astatine, the last member of this group is a radioactive element with very short halflife. GENERAL TRENDS IN PHYSICAL PROPERTIES Some of the properties, of the elements of this group are given in the following table:
(i)
(ii)
(iii)
(iv)
Melting and boiling points Melting and boiling points increase with the increase in atomic number. The enthalpy of fusion as well as enthalpy of vaporisation also increases as we go down the group. This indicates that the strength of intermolecular forces of attraction between the molecules increases with increase in atomic number. Atomic and ionic radii Atomic radii of the elements of this group are the smallest in their respective periods. Both the atomic radii and ionic radii for the anion X increase, regularly down the group because the electrons are added to higher and higher shells. Ionisation energies Ionisation energies of the all the halogens are very high. Therefore, they have very little tendency to loose electron. However, this tendency increases down the group because the nuclear force of attraction on valence electrons decreases. Iodine is capable of forming stable compounds in which it exists as I ion. Physical state
Page No.-53 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds All the halogens are diatomic and exist as F2 , Cl2 , Br2 and I 2 . The intermolecular forces are very weak and their magnitude increases down the group. Thus, F2 , Cl 2 are gases, bromine is a volatile liquid and iodine is a volatile solid. (v)
(vi)
(vii)
(viii)
Colour Halogens are coloured. The colour of the halogen is due to absorption of certain wavelengths of visible light by their molecules resulting is the excitation of outer electron to higher energy orbitals. Fluorine being smaller in size absorbs shorter wavelengths corresponding to violet colour for excitation and appears pale yellow. Iodine on the other hand absorbs longer wavelengths corresponding to yellow colour for excitation and therefore appears violet. In between fluorine and iodine, the colour of chlorine is greenish yellow and of bromine is reddish brown. Thus, the colour deepens down the group. Non-metallic character All the halogens are non-metals because of their very high ionisation energies. The non-metallic character, however, decreases with the increase in atomic number. Iodine shows some of the distinct metallic properties, e.g., it possesses metallic lustre and forms positive ions like I , I3 etc. Electron affinities The halogens have strong tendency to accept electrons. Their electron affinities are highest in their respective periods. On moving down the group the electron affinity values generally decrease with the increase in size of the atom. The exception to this general rule is fluorine which has lower electron affinity than chlorine. It is due to the small size of fluorine atom, the incoming electron experiences repulsion due to existing electrons in the 2p subshell resulting in low value of electron affinity. Oxidation states The most common oxidation state of all the halogens is –1, as they attain stable configuration by accepting one electron. In fact this is the only oxidation state shown by fluorine because it is the most electronegative element known. Other elements of this group also show oxidation states of +1, +3, +5 and +7. Higher oxidation states of these elements are due to the presence of vacant orbitals.
Element Atomic Number Atomic Mass Electronic configuration
F 9 19 He 2s 2 2p5
Cl 17 35.45 Ne 3s2 3p5
Br 35 79.90 Ar 3d10 4s 2 4p5
I 53 126.90 Kr 4d10 5s2 5p5
Covalent Radius / pm Ionic Radius X /pm
64 133 1680
99 184 1256
114 196 1142
133 220 1008
–333
–349
–325
–296
Distance X-X/pm Enthalpy of dissociation X 2 / kJ mol1
143 158.8
199 242.6
229 192.8
266 151.1
Electronegativity Melting point / K Boiling point / K
4 54.4 84.9
3.2 172 239.0
3.0 265.8 332.5
2.7 386.6 458.2
1 Ionization enthalpy/ kJ mol
1 Electron gain enthalpy/ kJ mol
Page No.-54 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Standard Reduction potential (SRP) X 2 2e 2X F2 2e
2F o
2.87V ;Cl 2 2e
2Cl
o
1.36V
o Br2 2e 2Br o 1.09V; I 2 2e 2I 0.54V More the value of the SRP, more powerful is the oxidising agent. Hence the order of oxidising power is F2 Cl2 Br2 I 2 Since SRP is the highest for F2 (among all elements of periodic table), it is a strongest oxidising agent.
Hydration energy of X Smaller the ion, higher is the hydration energy Cl F Br I 515 381 347 305 in kJ/mol Anomalous behaviour of fluorine The anomalous behviour of fluorine is due to its small size, highest electronegativity, low F – F bond dissociation enthalpy, and non availability of d orbitals in valence shell. Most of the reactions of fluorine are exothermic (due to the small and strong bond formed by it with other elements). It forms only one oxoacid while other halogens form a number of oxoacids. Hydrogen fluoride is liquid (b.p. 293 K) due to strong hydrogen bonding. Other hydrogen halides are gases. (i) Reactivity towards hydrogen : They all react with hydrogen to give hydrogen halides but affinity for hydrogen decreases from fluorine to iodine with increasing atomic number. They dissolve in water to form hydrohalic acids. The acidic strength of these acids increases in the order : HF < HCl < HBr < HI. The stability of these halides decreases down the group due to decrease in bond (H – X) dissociation enthalpy in the order : H – F > H – Cl > H – Br > H – I. (ii) Reactivity towards oxygen : Halogens form many oxides with oxygen but most of them are unstable. Fluorine forms two oxides OF2 and O 2 F2 . However, only OF2 is the thermally stable at 298K. These oxide are essentially oxygen fluorides because of the higher electronegativity of flurorine than oxygen. Both are strong fluorinating agents. O 2 F2 oxidizes plutonium to PuF6 and the reaction is used in removing plutonium as PuF6 from spent nuclear fuel. Chlorine, bromine and iodine form oxides in which the oxidation states of these halogen vary from +1 to +7. A combination of kinetic and thermodynamic factors lead to the generally decreasing order of stability of oxides formed by halogens, I > Cl > Br. The higher oxides of halogens tend to be more stable than the lower ones.
Halogens a)
Fluorine Extraction:
Fluorine the only practicable method of preparing fluorine gas is Moissan’s original procedure based on the electrolysis of KF dissolved in anhydrous HF. He electrolysed a cooled solution of KF in anhydrous liquid HF at 250 K using platinum-iridium electrodes sealed with fluorspar caps in a platinum U-tube. In this reaction, the actual electrolyte is KF while HF acts as an ionizing solvent, F 2 is evolved at the anode and H2 at the cathode as indicated below: – KF ‡ˆ ˆ† ˆˆ K F At the anode – F F e
Page No.-55 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds F F
F2
At the cathode K e K 2K 2HF 2KF H 2 Moisson’s original method has been modified. In place of the expensive Pt/Ir alloy, cells made of copper, steel or Monel metal, which is a nickel-copper alloy, have been used. Anode is a carbon rod impregnated with copper to render it inert and cathode is made of steel or copper. A mixture of KF and HF in the molar ratio of 1 : 1 or 1 : 2 is used as electrolyte. Properties of Fluorine: Pale greenish yellow gas, which is highly poisonous and having pungent odour. It is heavier than air. Electronegativity value is 4.0 (Pauling’s scale). Chemical:
3H 2 O 3F 2
6HF O3
2H 2 O 2F 2
4HF O2
2Al 3F 2
2AlF3
Copper does not appreciably react with F 2 at this temperature because of CuF 2 layer formation on it. NaOH 2F 2
NaF O 2
2NaOH 2F 2 I 2 5F 2
F2O
2H 2 O
2NaF
H 2O
2IF5
3F2 4HBr
4HF Br2
3F2 4HI
4HF I 2
SiO 2 2F 2
SiF4
2BrF
2IF
O2
Strongest oxidising behaviour of F2 KClO3 F2 H 2O
2HF
K 2CO 3 F2 H 2O
2HF K 2CO 4
K 2SO 4 F 2
KClO 4
K 2S2 O8 2KF
CHLORINE Cl2 PREPARATION (i) By heating chloride with concentrated H 2SO 4 in presence of MnO 2 4H MnO 2 2X
(ii)
X 2 Mn 2 2H 2O Bromides and iodides also liberate Br2 and I 2 respectively with concentrated H 2SO4 and MnO 2 . (a) NaCl HNO NaNO 3 HCl 3 3 HNO3 3HCl 3NaCl 4HNO 3
NOCl Cl 2 2H 2O 3NaNO 3
NOCl
(nitrosyl chloride) + Cl 2 2H 2 O
(b) When Cl2 is used for the chlorination of hydrocarbon the byproduct is HCl. The HCl is catalytically oxidized into H 2O & Cl2 using copper powder mixed with rare earth chlorides. Page No.-56 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds 4HCl O 2
Cu powder rare earth chloride
2H 2O 2Cl 2
These methods are exclusively used only for chlorine
Properties : (i) It is a greenish—yellow gas with pungent and suffocating odour. It is about 2—5 times heavier than air. It can be liquefied into greenish—yellow liquid which boils at 239 K. It is soluble in water. (ii) At low temperature it forms a hydrate with water having formula Cl2 .8H 2 which is infact a clathrate compound. (iii) Oxidising & bleaching properties : Chlorine dissolves in water giving HCI and HOd. Hypochlorous acid (HOCI) so formed, gives nascent oxygen which is responsible for oxidising and bleaching properties of chlorine. (a) Itoxidises ferrous to ferric, sulphite to sulphate, sulphur dioxide to sulphuric acid and iodine to iodic acid.
(b)
Chlorine oxidizes both Br and I to Br2 and I 2 respectively.
X Cl X 2 2Cl 2 (c) It is a powerful bleaching agent ; Bleaching action is due to oxidation. Cl2 H 2O 2HCl O Coloured substance O colourless substance It bleaches vegetable or organic mather in the presence of moisture. Bleaching effect of chlome is permanent Note : The bleaching action of SO 2 is temporary because it takes place through reduction.
Reduced colourless material O2 of (iv) Reaction with NaOH : (a) 2NaOH cold & dilute Cl 2
air
coloured material. NaCl NaClO H 2O
Page No.-57 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
(b) 6NaOH hot & concentrated 3Cl 2
Preparation & Properties of Compounds 5NaCl NaClO 3 3H 2O
These reactions are also given by Br2 and I 2 (v) Reaction with dry slaked lime, Ca OH 2 : To give bleaching powder. 2Ca OH 2 2Cl 2
Ca OCl 2 CaCl 2 2H 2O
Uses : Cl2 is used 1. for bleaching woodpulp (required for the manufacture of paper and rayon). bleaching cotton and textiles, 2. in the manufacture of dyes, drugs and organic compounds such as CCl4 , CHCl3 , DDT, refrigerants, etc. 3. in the extraction of gold and platinum. 4. in sterilising drinking water and 5. preparation of poisonous gases such as phosgene COCl 2 tear gas CCl3 NO 2 mustard gas ClCH 2CH 2SCH 2CH 2Cl .
BROMINE Br2 : PREPARATION: Br2 MnSO 4 2NaHSO 4 2H 2O (i) Common method : 2NaBr 3H 2SO4 conc. MnO 2 (ii) From Sea-water NaCl is main component but NaBr is also present in some quantity in sea water. Cl2 gas is passed through sea water when vapours of bromine are evolved. 2Br aq Cl 2Cl aq. Br2 2 The Br2 is removed by a stream of air since Br2 is quite volatile. The gas is passed through a solution of Na 2CO3 when the Br2 is absorbed forming a mixture of NaBr and NaBrO 3. The solution is then acidified and distilled to give pure bromine. 3Br2 3Na 2CO 5NaBr NaBrO 3 3CO 2 3 5NaBr NaBrO3 3H 2SO 4 5HBr HBrO 3
5HBr HBrO 3 3Na 2SO 4
3Br 2 3H 2 O
Properties : (i) Reddish brown liquid, fairly soluble in water. It also forms hydrate like Cl2
Br2 .8H 2O
Clathrate compound
(ii) Rest reactions are same as with Cl2 IODINE I 2 : PREPARATION: I2 MnSO 4 2NaHSO 4 2H 2 O (i) Common method : 2NaI 3H 2SO4 conc. MnO2 (ii) From caliche or Crude chile salt petre: The main source of iodine is NaIO3 (sodium iodate) which is found in nature with NaNO3 (chile saltpeter). NaIO3 is present in small amount. After crystallization of NaNO3 , the mother liquor left contains NaIO3 (soluble). To this solution NaHSO3 is added where upon I 2 is precipitated. Page No.-58 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds 2IO3
6HSO 3
2I
6SO 42
6H
5I IO3 6H 3I 2 3H 2O (iii) From sea-weeds: Certain marine plants absorb and concentrate I-selectively in presence of Cl and Br . Sea-weeds are dried and burnt in shallow pits, ash left is called kelp. Ash on extraction with hot water dissolves out chlorides, carbonates, suiphates and iodides of sodium and potassium. The solution on concentration seperates out all leaving behind iodide in the solution. Solution is mixed with MnO 2 and concentrated H 2SO4 in iron retorts. Liberated iodine is condensed in series of earthenware known as aludels. 2NaI MnO 2 3H 2SO 2NaHSO 4 MnSO 4 I 2 2H 2O 4
(iv) CuSO 4 2KI
K 2SO 4 CuI 2 ; 2CuI 2
Cu 2I 2 I 2
This I 2 gets dissolved into KI forming KI3 , since I3 ions are yellow, therefore solution develops yellow colour. PROPERTIES: (i) It is a dark violet solid, undergoes sublimation, least soluble (among halogens) in water but much more soluble in K1(aq.) due to formation of KI3 . It is soluble in organic solvents like CHCl3 , CCl4 etc. to get violet solutions. (ii) Reaction with hypo : S2O32 thiosulphate I S4O 62 tetrathionate ions 2I 2 (iii) Reaction with NH3 :
8NI3 .NH 5N 2 9I 2 6NH 4I 3 (iv) Reaction with KClO3 or KBrO3 : 2KClO3 I 2
2KIO 3 Cl2 ; 2KBrO 3 I 2
2KIO3 Br2
HALOGEN ACIDS ACID STRENGTH It is at first paradoxical that HF is the weakest acid in water, since HF has a greater electronegativity difference than the other hybrids and therefore has more ionic character. However, acidic strength is the tendency of hydrated molecules to form hydrogen ions. HX (hydrated) H (hydrated) X (hydrated) This may be represented in stages: dissociation, ionization and hydration in an energy cycle. The acid strength is equal to the sum of all the energy terms round the energy cycle Acid strength = Free energy of dehydration + Free energy of dissociation + Ionization energy of H+ + Electron affinity X– + Free energy of hydration of H+ and X– Page No.-59 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds The factor, which make HF the weakest halogen acid in water, becomes apparent if the various thermodynamic terms are examined in more detail. The dissociation constant k for the change HX (hydrated) ƒ H (hydrated) X (hydrated) is given by the equation: G o RT ln k (where G 0 is the Gibbs standard free energy, R the gas constant and T is absolute temperature). However, G depends of the change in enthalpy H and the change in entropy S G = H – TS Consider first the total enthalpy change H for the dissociation of HX (hydrated) into H (hydrated) and X (hydrated) . The H values for the various halogen acids are all negative, which means that energy,
is evolved in the process, so the change in thermodynamically possible. However, the value for HF is small compared with the values for HCl, HBr and HI (which are all similar in magnitude). Thus, HF is only slightly exothermic in aqueous solution whereas the other evolve a considerable amount of heat. The low total H value for HF is the result of several factors. 1. The enthalpies of dissociation show that the H–F bond is much stronger than the H–Cl, H–Br or H–I bonds. Thus, the dissociation energy of HF is nearly twice that required to dissociate HI. (The strength of the HF bond is also shown by the short bond length of 1.0 Å. 2. The heat of dehydration for the step HX (hydrated) HX (gas) is much higher for HF than for the others. This is because of the strong hydrogen which occurs in aqueous HF solutions. 3. The unexpectedly low value for the electron affinity of F – also contributes and though the enthalpy of hydration of F– is very high, it is not enough to offset these other terms. HYDROFLUORIC ACID (HF) Chemical Properties: CaF2 + H2SO4 CaSO4 + 2HF HF etches glass. The reaction is SiF4 + 2HF H2SiF6 Hence HF is stored in polythene coated bottle. HF ƒ H F F HF ƒ HF2 Conjugate base F– bonds with HF by H–bonding thus statistically the effective number of molecules of H drops and registers acidity Ka = 7.2 10–4 H2F2 + KOH KHF2 + H2O H3BO3 + 4HF HBF4 + 3H2O SiF4 + 2HF H2SiF6 BF3 + HF HBF4 BaCl2 + 2HF BaF2 + 2HCl HYDROCHLORIC ACID Preparation of HCl Starting materials: NaCl, concentrated H2SO4 ; temperature ; 1500C Reaction: NaCl + H2SO4 NaHSO4 + HCl NaCl + NaHSO4 Na2SO4 + HCl Purification of HCl is done by conc. H2SO4 not by CaO or P2O5 because both of them react with HCl. Page No.-60 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Properties Physical properties; Colourless gas with pungent smell. It is heavier than air, highly soluble in water. Chemical properties MnO 2 4HCl MnCl2 Cl2 2H 2O PbO2 4HCl PbCl 2 Cl2 2H 2O K 2Cr2O7 14HCl
ordinary test
2KCl 2CrCl 3 3Cl 2
2KMnO 4 16HCl
ordinary test
2KCl 2MnCl 2 5l 2 8H 2O
7H 2O
AgNO3 HCl AgCl HNO3 curdy white
Hg 2 (NO3 ) 2 2HCl Hg 2Cl 2 2HNO3 curdywhite
What happens when urea is treated with alkaline sodium hypochlorite and heated to 1000C? HYDROBROMIC ACID Preparation of HBr (Red) 2P + 3Br2 2PBr3 PBr3 3H 2O 3HBr H 3PO3 Reaction of NaBr with H2SO4 does not give HBr because of the following reaction. NaBr H 2SO 4 NaHSO 4 HBr H 2SO4 H 2O SO2 O 2HBr + O H2O + Br2 Purification of HBr is done by CaCl2 from moisture. Properties: HBr is colourless and having pungent odour shows the following reactions. Zn + 2HBr ZnBr2 + H2 Na2CO3 + 2HBr 2NaBr + CO2 + H2O NaHCO3 + HBr NaBr + CO2 + H2O Reduction action 16HBr + 2KMnO4 2KBr + 2MnBr2 + 5Br2 + 8H2O 2HBr + H2SO4 Br2 + SO2 + 2H2O K2Cr2O7 + 14HBr 2KBr + 2CrBr3 + 3Br2 + 7H2O 4HBr + O2 2Br2 2H2O HYDROIODIC ACID Preparation of HI 1. (Red) 2P + 3I2 + 6H2O 6HI + 2H3PO3 2. NaI + H2SO4 NaHSO4 + HI 2HI + H2SO4 I2 + SO2 + 2H2O 3. SO2 + 2H2O + I2 H2SO4 + 2HI H2SO4 + BaI2 BaSO4 + 2HI Chemical properties 4HI + O2 2H2O + I2 Reducing Properties 1. 2KMnO 4 3H 2SO 4 10HI K 2SO 4 2MnSO 4 5I 2 8H 2O Pink
colourless
Page No.-61 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
K 2Cr2O7 6HI 4H 2SO 4 K 2SO 4 Cr2 (SO 4 )3 3I 2 7H 2O H 2O 2 2HI 2H 2O I 2
2. 3. 4. 5. 6.
2HNO3 + 2HI I2 + 2H2O + 2NO2 2FeCl3 2HI 2FeCl 2 2HCl I 2 2CuSO 4 4HI Cu 2I 2 2H 2SO 4 I 2
7. Pb(CH 3COO) 2 2HI PbI 2 2CH 3COOH Reaction with KI in acid medium gives the detection test I–. 2KI + 2H2SO4 K2SO4 + SO2 + I2 + 2H2O Reduction Property of hydrogen halides The stability of hydrogen halide decreased as we move down the group. The reducing property is in the order HF < HCll < HBr < HI. The ease of oxidation of halide ion is expected to increase in the order of increasing in the order of increasing size of the halide ions F–, Cl–, Br– and I–. The electrode to bt removed from F– is very near to nucleus and therefore it is most difficult to remove and easy for I–. Therefore, HI should be strong reducing agent BLEACHING POWDER CaOCl2 .H 2O Bleaching powder is also called calcium chlorohypochlorite because it is considered as a mixed salt of hydrochloric acid and hypochlorous acid. Ca(OCl)Cl Preparation Ca OH 2 Cl 2
40o C
Ca OCl Cl H 2O
Properties (i) It is a pale yellow powder. It has a strong smell of chlorine. It is soluble in water but a clear solution is never formed due to the presence of impurities. (ii) On long standing, it undergoes auto-oxidation into calcium chlorate and calcium chloride. 6CaOCl 2 Ca ClO3 2 5CaCl2 (iii) (iv)
2CaOCl2 CoCl2 2CaCl 2 O2 In presence of a little amount of a dilute acid, it loses oxygen. 2CaOCl2 H 2SO CaCl 2 CaSO 4 2HClO 4 HClO HCl O On account of the liberation of nascent oxygen, it shows oxidising and bleaching properties. (a) Oxidising properties CaOCl 2 H 2S CaCl 2 H 2O S CaOCl2 2Kl 2HCl 3CaOCl2 2NH 3
(v)
CaCl 2 2KCl H 2O I 2 3CaCl 2 3H 2O N 2
It oxidizes NO 2 to NO3 , AsO33 to AsO34 and Fe 2 to Fe 3 (b) Bleaching action When bleaching powder reacts with dilute acids or CO 2 it liberates chlorine which is known as available chlorine.
Page No.-62 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds CaOCl 2 2HCl
CaCl 2
CaOCl 2 H 2SO 4 CaOCl 2 CO 2
H 2O Cl2
CaSO 4 H 2O Cl 2 CaCO 3 Cl 2
HNO3 is a strong oxidising acid to be avoided, here ESTIMATION OF AVAILABLE CHLORINE: Let the weight of sample of bleaching powder be Wg. Add into a beaker containing acetic acid solution and excess KI. A yellow brown solution is formed I3 the titrant. Note the volume where the blue colour disappear.
Add to beaker W gm. Containing B.P CH3COOH & Excess KI
Yellow brown Solution
Blue Add Hypo Reaction involved : CaOCl 2 2CH 3COOH Cl2 2KI
Add a drop of Starch
Iodometric Titration
CH3COO 2 Ca
At end point blue colour disapper
H 2O Cl 2
2KCl I2
I 2 2S2 O32
S4O62
2I
1 M hypo Vhypo 71 Calculation : 2 100 %Cl W (vi) Bleaching powder converts acetone or ethyl alcohol into CHCl3 CaOCl 2 H 2O CH3COCH 3 3Cl 2
Ca OH 2 Cl2 CCl3COCH 3 3HCl
2CCl3COCH 3 Ca OH 2
CH3COO 2 Ca
2CHCl3
Illustration - 39 The property of halogens, that indicated incorrect is (a) F > Cl > Br > I....... ionisation energy
(b) F > Cl > Br > I....... electron affinity
(c) F > Cl > Br > I ...... electronegativity
(d) I > Br > Cl > F...... density in liquid state
Ans. (b) Hint : [The electron affinity of Cl is maximum. The trend is : Cl > F > Br > I]
Illustration - 40 C o ld a n d d ilu t e N a O H
C l2
H od and conc. N aO H
( A ) + N a C l + H 2O ( B ) + N a C l + H 2O
Compounds (A) and (B) are : Page No.-63 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds (a) NaClO3, NaClO
(b) NaOCl2, NaOCl
(c) NaClO4, NaClO3
(d) NaOCl, NaClO3
Ans. (d) Hint :
Cl2 2NaOH Cold and dil.
3Cl2 6NaOH Hot and conc.
NaCl
NaOCl
5NaCl
H2O
(A )
NaClO 3 (B)
3H2O]
Illustration - 41 The property of halogen acids, that indicated incorrect is (a) HF > HCl > HBr > HI.......acidic strength (b) HI > HBr > HCl > HF.....reducing strength (c) HI > HBr > HCl > HF........bond length (d) HF > HCl > HBr > HI......thermal stability Ans. (a) Hint : [HI is the strongest acid while HF is the weakest acid. The order of acidic strength is HI > HBr > HCl > HF]
Illustration - 42 The halogen that is most readily reduced is (a) fluorine
(b) chlorine
(c) bromine
(d) iodine
Ans. (a) Hint : [The reduction potential of fluorine is maximum and thus, it is easily reduced, i.e., it acts as strongest oxidising agent].
Illustration - 43 A g C lO
3
+ (A ) (B ) + (C ) + (D )
M nO 2 + HC l C onc.
The substances (A), (B), (C) and (D) are (a) Cl2, AgCl, ClO2, O2
(b) Cl2, Ag, Cl2O6, O2
(c) H2, AgCl, H2O, O2
(d) HClO, AgCl, Cl2O, O2
Page No.-64 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Ans. (c) Hint : 2AgClO3 Cl2 (A)
2 AgCl (B)
2ClO 2 (C)
O2 ] (D)
Page No.-65 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-66 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-67 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds PSEUDOHALOGENS AND PSEUDOHALIDES A few ions are known, consisting of two or more atoms of which at least one is N, that have properties similar to those of the halide ions. They are therefore called pseudohalide ions. Pseudohalide ions are univalent, and these form salts resembling the halide salts. For example, the sodium salts are soluble in water, but the silver salts are insoluble. The hydrogen compounds are acids like the halogen acids HX. Some of the pseudohalide ions combine to form dimmers comparable with the halogen molecules X2. These include cyanogens CN 2 , thiocyanogen SCN 2 and selenocyanogen SeCN 2 Anion Acid Dimer HCN hydrogen cyanide cyanide ion CN 2 CN cyanogens HSCN thiocyanic acid thiocyanate ion SCN SCN 2 thiocyanogen selenocyanate ion SeCN SeCN 2 selenocyanogen HOCN cyanic acid cyanate ion OCN H 2 NCN cyanamide cyanamide ion NCN 2 HN 3 N3 azide ion hydrogen azide The best known pseudohalide is CN . This resembles Cl , Br and I in the following respects. 1. It forms an acid HCN 2. It can be oxidized to form a molecule cyanogens CN 2 3. 4. 5. 6.
It forms insoluble salts with Ag , Pb 2 and Hg Interpseudohalogen compounds ClCN, BrCN and ICN can be formed. AgCN is insoluble in water but soluble in ammonia, as is AgCl. It forms a large number of complexes similar to halide complexes. e.g. Cu CN 2
2
and CuCl4
2
and CO CN 6
3
and CoCl6
3
General Properties (i) Physical state The interhalogen compounds may be covalent gases (e.g. CIF, BrF, ClF3 , IF7 ), liquids (e.g. BrF3 , BrF5 ) or solids (e.g. ICl, IBr, IF3 , ICl3 ) (ii) Colour Although many of the interhalogen compounds containing fluorine are colourless, yet those made up of heavier halogens are coloured. The colour becomes deeper with the increase of the molecular weight of the compound. In this behaviour these compounds resemble the halogens themselves. (iii) Diamagnetic nature Since all the valence electrons in interhalogen compounds are present as bonding or nonbonding (i.e., lone pairs) electron pairs, these compounds are diamagnetic in nature. (iv) Heats of formation All these compounds have low heats of formation. For example, these values for IBr and BrCl are —2.5 and —0.34 kcal/mole respectively. These are extremely volatile compounds. In general compounds containing fluorine are more volatile than those containing chlorine, bromine or iodine. (v) Boiling points For each type of interhalogen compounds, the boiling points increase with the increase in the electronegativity difference between A and B atoms.
Page No.-68 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds (vi)
(vii)
(viii)
Thermal stability Thermal stability of AB type interhalogen compounds decreases with the decrease in electronegativity difference between A and B atoms. Thus the order of stability of some AB compounds is as IF (1.5)> BrF (1.2) > CIF (1.0) > ICl (0.5) > IBr (0.3)> BrCl (0.2) In parentheses are given the electronegativity difference between A and B atoms. The above order is also explained by saying that greater is the difference between the electronegativity values of A and B, the more polar is the A–B bond and hence greater is the thermal stability of AB compound. Reactivity AB type compounds are more reactive than A 2 and B2 molecules, since A—B bond in AB compounds is weaker than A—A and B—B bonds in A 2 and B2 respectively. Thus AB type compounds convert the metals into a mixture of two halides. For example ICl 2Na NaI NaCl The order of reactivity of some interhalogen compounds has been found as ClF3 BrF3 IF7 BrF5 BrF . Hydrolysis Hydrolysis gives halogen acid and oxy-halogen acid. The oxy-halogen acid is of larger (i.e., central) halogen atom. Examples are BrCl H 2O HCl (halogen acid) + HOBr (oxy-halogen acid) ICl H 2 O
HCl HIO
ICl3 2H 2O
3HCl
HIO2
3H 2O
5HF
HIO3
BrF5 3H 2O
5HF
HBrO3
6H 2O
7HF
H 5 IO 6
IF5
IF7
ZERO GROUP The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) belong to group 18 of the periodic table. All these elements are gaseous under ordinary conditions of temperature and pressure. The last member of the group i.e., radon is obtained from radioactive disintegration of radium. All others are present in air in traces. They are also known as rare gases, because they are found in very small amounts in nature. They are highly non-reactive and baring few exceptions, they don’t take part in chemical reactions and are therefore, called inert gases or noble gases. They always occur in free state because of the inert nature. The valence shell electronic configuration of helium is 1s 2 and for other members of this group is ns 2 np6 . Thus, except helium, other noble gases have a closed octet of electrons in their outermost shell. This electronic configuration is very stable and is related to their chemical inertness. Their physical properties show a regular gradation. Some of the physical properties are given in the following table:
Page No.-69 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Elements Helium Neon Argon Krypton Xenon Radon
Ionisation energy kJ mol1
Density at STP g cm 3
Melting point (K)
Boiling point (K)
2373 2080 1520 1351 1170 1037
1.8 104 9.0 104 1.8 103 3.7 103 5.9 103 9.7 103
— 24.6 83.8 115.9 161.3 202
4.2 27.1 87.2 119.7 165.0 211
GENERAL TRENDS IN PHYSICAL PROPERTIES (i) State All of them are monoatomic, colourless, odourless and tasteless gases. (ii) Solubility They are sparingly soluble in water. The solubility generally increases with increase in atomic number. (iii) Boiling point and melting point Due to weak intermolecular van der Waal’s forces between them they possess very low boiling point and melting point in comparison to those of other substances of comparable atomic and molecular masses. However, the boiling point and melting point increase with increase in atomic number because van der Waal’s forces become stronger with increase in size of the atoms or molecules. Therefore, among noble gases radon has the highest melting point and boiling point whereas helium has the least melting point and boiling point. (iv) Liquefication It is extremely difficult to liquefy these gases as there are only weak van der Waal’s forces which hold atoms together. Since these forces increase with the increase in atomic size and population of electrons, ease of liquefication increases down the group from He to Rn. (v) Atomic radii In the case of noble gases, the atomic radii corresponds to van der Waal’s radii. Therefore, these are quite large as compared with atomic radii of the other atoms belonging to the same period. As we go down the group, the van der Waal’s radius increases due to the addition of new electronic shells and increase in screening effect. (vi) Ionisation energies The ionisation energies of noble gases are very high. This is due to the stable configurations of noble gases. However, the ionisation energies decrease with increase in atomic number from He to Rn due to increasing atomic size and decrease in effective nuclear charge. (vii) Electron affinities
(viii)
Due to the stable ns 2 np6 electronic configurations, noble gas atoms have no tendency to accept additional electron. Therefore, their electron affinities are almost zero. Enthalpy of fusion and enthalpy of vapourization In general, the enthalpies of fusion and the enthalpies of vaporization are low and increase down the group.
Page No.-70 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-71 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds
Page No.-72 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25
Preparation & Properties of Compounds Illustration - 44 What are the products formed in the reaction of xenon hexafluoride with silica ? (a) XeSiO4 + HF
(b) XeF2 + SiF4
(c) XeOF4 + SiF4
(d) XeO3 + SiF4
Ans. (c) Hint : [ 2XeF6 SiO2
2XeOF 4
SiF4 ]
Illustration - 45 Which of the following two are isostructural ? (a) XeF2, IF2–
(b) NH3, BF3
(c) CO32–, SO32–
(d) PCl5, ICl5
Ans. (a) Hint : [XeF2 is a linear molecule (Xe and sp3d hybridized with three equatorial positions occupied by line pairs). IF2– is a linear molecule (I is sp3d hyhrbdized, with three positions occupied by line pairs),]
Illustration - 46 XeF2 reacts with SbF5 to form (a) [XeF] [SbF6] (b) [XeF3] [SbF4] Ans (a)
(c) XeO3
(d) XeF3
Page No.-73 NARAYANA DWARKA CENTRE 201-A, Dimension Durga Tower, Plot No. – 10, Sector 4 Market, Dwarka New Delhi – 110075, Ph : 45621724 / 25