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CLASS-XI (Chemistry) UNIT-9 HYDROGEN
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Position of Hydrogen in the periodic table ; Hydrogen has electronic configuration 1s1,which resembles with alkalimetals as well as with halogens.Hydrogen therefore has resemblanceto both groups---------Properties resembles to Alkali metals
Properties resembles to halogens
1. loses one electron to form Unipositive ion.
1.Gains one electron to form Uninegative ion.
2. Like alkali metals it forms
2. Like halogens it forms a di-
Oxides ,halides and sulphides. 3. It is highly reactive like alkali metals.
atomic molecule 3.Like halogens its ionization enthalpy is high.
Inspite of the fact hydrogen resembles both with alkali metals and halogens It differs From them as well.It is unique in behaviour and is therefore placed separately in the periodic table. Isotopes of Hydrogen: Hydrogen has 3 isotopes; Protium 1H1,deuterium1H2or D(also called heavy Hydrogen) and Tritium1H3or T.Only Tritium is radioactive and emits low energy β particles. Prepration of Hydrogen: 1. Lab prepration:
Granules of Zinc reacts with Hcl
Zn + 2 HCl ------- ZnCl2 + H2 2. By the reaction of zinc with NaOH Zn + 2NaOH ------ Na2ZnO2 + H2 Commercial production of Hydrogen :1. By electrolysis of acidified water using platinum electrodes to give hydrogen. 2H2 O (lq.) electrolysis -> 2 H2(g) + O2(g)
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2. By electrolysis of brine solution .
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Reaction at anode—2Cl (aq) ------Cl2(g) + 2eReaction at cathode- 2H2 O(lq) +2e- --- H2(g) + 2OH(aq.) 3.Hydrocarbons or carbon reacts with steam in presence of catalyst forms Hydrogen. CH4 + H2 O -----
CO + H2O (Water gas or syngas)
PROPERTIES OF HYDROGEN : Hydrogen is a colourless , odourless and combustible gas. It is lighter than air and insoluble in water. CHEMICAL PROPERTIES : H-H bond dissociation enthalpy is highestfor single bond .It is Inert at room temperature.Its chemical reactions are--1. Reactions with halogens: Forrms hydrogen halides. H2 + X2 ------2 HX( reactivity order---F≥Cl≥B≥rI) 2. Reaction with oxygen: Water is prodused ,reaction is highly exothermic. 2H2(g) + O2 ------ 2H2 O 3. Reaction with nitrogen:
- ∆H
Ammonia is produced in presence of catalyst
3H2 + N2 ------ 2 NH3
-∆H
4. Reaction with metals : At high temperature corresponding hydrides are formed. H2(g) + 2M --------2MH 5. Reaction with organic compounds : In presence of catalyst some useful products are obtained. e.g. 1. Hydrogenation of vegetable oils using Ni as catalyst forms fats. 2. Hydroformylation of olefins to form aldehydes. H2 + CO + RCH=CH2 ------
RCH2CH2CHO
Uses of Hydrogen : 1. Synthesis of Ammonia,nitric acid,methanol and nitrogenous fertilizers. 2. In manufecture of vanaspati fats from vegetable oils. 3.Oxyhydrogen torches are used for cutting and welding purpose. 4. It is used as rocket fuel in space and also in fuel cells to generate electricity.
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HYDRIDES : Hydrogen combines with elements exept noble gases to Form binary compounds called hydrides which are classified into three types1. Ionic or saline hydrides 2. Covalent hydrides 3. Metallic hydrides. IONIC HYDRIDES : Formed with metals of s block elements.They are crystalline,nonvolatile and nonconducting in solid state,however their metals conduct electricity and liberate hydrogen gas .They react violently withwaterto form hydrogen gas. NaI
+
H2O ------ NaOH + H2
Some hydrides have covalent character like LiH.BeH2 LiH is unreactive at moderate temperature with oxygen or chlorine and is used in synthesis of other useful hydrides… 8LiH + Al2Cl6 ----------- 2LiAlH4 + 6LiCl 2LiH + B2H6 --------- 2LiBH4 COVALENT HYDRIDES : Formed with most of p block elements.e.g. CH4.NH3 H2O.
Being covalent .they are volatile. These hydrides are further divided into… a. electron deficient : they have less electrons in their lewis structure,thus act as lewis acids.i.e.electron acceptors.e.g BH3 b. electron precise:They have required number of electrons in their lewis structure.e.g CH4.SiCl4 . c. electron rich hydrides : They have excess electrons in their lewis structure .They behave as lewis bases i.e.electron donars.e.g. NH3, H2O . METALLIC HYDRIDES : These are formed by many d and f block elements.they conduct Heat and electricity not as efficiently as their parent metal do.
WATER Water is a crucial compound for the survival of all life forms. It is a universal solvent. Properties: It is a colourless and odourless liquid.The unusual properties of water in Condensed phase are due to the presence of extensive hydrogen bonding between Water molecules.This leads to high freezing point.high boiling point .high specific
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heat.thermal conductivity,surface tension and high dipole moment etc. These properties allow water to play a key rple in biosphere.Due to hydrogen bonding it dissolves many covalent compounds like sugar,glucose,alcohol etc.
Structure of water In gaseous phase water is a bent molecule with a bond angle of 104.50.It is highly polar molecule.
O H
H
The crystalline form of water is ice .At atmospheric pressure ice crystallises in hexagonal form .Density of ice is less than water ,therefore it floats on its surface .In winter season ice formed on surface of lake provides thermal insulation which makes the life of aquatic animals more comfortable.This fact is of great ecological significance.
Chemical properties of water
: 1. Amphoteric nature—It has the ability to act
As acid as well as base . H2O(l) + NH3(aq) ↔ OH- (aq) +
NH4+ (aq)
H2O (l) + H2S(aq) ↔ H3O+(aq) +HS-(aq) 2. Autoprotolysis of water takes place as followes— H2O(l) + H2O(l) ↔ H3O+(aq) + OH- (aq) 3. Reduction--- Water can be reduced to hydrogen by highly electropositive metals. 2H2O(l) + 2Na(s) ------ 2NaOH(aq) + H2(g) 4. Oxidation –Water oxidises to O2 during photosynthesis 6CO2 + 12 H2O ---- C6H12O6 + 6H2O + 6O2 5. Hydrolysis—Water has avery high hydrating tendency ,it dissolves many ionic compounds .Certain covalent compounds are hydrolysed with water. P4O10 + 6H2O ------ 4H3PO4 SiCl4 + 2H2O ----- SiO2 + 4HCl 6. Hydrates formation—From aq. Solutions many salts can be crystallised as hydrated salts. e.g. [Cr(H2O)6 ]3+ 3Cl - , CuSO4.5H2O etc.
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HARD AND SOFT WATER Hard water contains calcium and magnecium salts in the form of chlorides,sulfates and Hydrogencarbonates.It does not give lather with soap.It forms scum with soap ,thus not Suitable for laundary. Water free from Calcium and magnecium salts is called soft water.It gives lather with soap very easily. Hardness of water can be of two types--Temporary hardness is due to the presence of magnecium and calcium bicarbonate salts ,which can be removed by --a. By boiling due to which soluble salts are converted to insoluble salts and can be removed by filteration. Mg(HCO3)2 ----∆-------- Mg (OH)2 + 2CO2 2. Clark’s method: Calculated amount of lime is added to hard water ,it precepitates out salts which are removed by filteration. Ca(HCO3)2 + Ca(OH)2 ------ CaCO3 + 2H2O Permanent Hardness is due to chlorides and sulfates of calcium and magnecium Which can be removed by ……… 1. By treating with washing soda to form insoluble carbonates. 2. by ion exchange method 3. Synthetic resin method
HYDROGEN PEROXIDE (H2O2) Prepration : Barium peroxide is first acidified then excess of water is removed by Evaporation under reduced pressure to give hydrogen peroxide. BaO2.8H2O(s) + H2SO4(aq.) ------ BaSO4 (s) + H2O2 (aq) + 8H2O (l)
Properties : It is almost colourless in its pure state and immiscible with water in all Proportions and forms a hydrate H2O2.H2O. Strength of H2O2 solution is expressed in
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Two ways--
1. Percent strength : It expresses the amount of H2O2 by weight present
in 100 ml. of solution e.g. 30% aq. Solution of H2O2 means that 30 gms. of H2O2 are present in 100 ml. of the solution. 2. Volume strength : H2O2 sold in the market labelled as 10 volume ,20 volume, 30 volume etc. 10 volume actually means that 1 ml. of such a solution of H2O2 on decomposition produces 10 ml. of oxygen at N.T.P.
Structure of H2O2 H2O2 has a nonplanar structure .two oxygen atoms are linked to each other by single Covalent bond (peroxide bond) and each oxygen is bonded to one hydrogen by single covalent bond.
H O
O H
Chemical properties : H2O2 acts as oxidising as well reducing agent in both acidic and basic medium. Oxidising action : (Acidic medium) 2Fe2+ (aq) + 2H+ (aq) + H2O (aq) --- 2Fe3+ (aq) + 2H2O (l) PbS(s) + 4H2O (aq) --- PbSO4(s) + 4H2O(l) (Basic medium) 2Fe2+ H2O2 ----- 2Fe3+ + 2OHMn2+ + H2O2---- Mn2+ + 2OHReducing action : (Acidic medium) 2MnO4 + 6H+ 5H2O2----- 2Mn2+ + 8 H2O + 5O2 HOCl + H2O2 ------ H3O+ + Cl- + O2 (Basic medium) I2
+
H2O2 + I2 + 2OH- -------- 2I - + 2H2O + O2
2MnO4 + 3H2O2
--------- 2MnO2
+ 3O2 + 2H2O + 2OH-
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USES OF H2O2 1. In industries it is used as bleaching agent for textiles, paper ,leather etc. 2. Used in the synthesis of certain food products and medicines. 3. In daily life it is used as a hair bleach and mild disinfectant,as antiseptic it is sold in Market as perhydrol. 4. Nowadays it is also used in environmental i.e. green chemistry,in pollution control treatment and sewage treatment. HEAVY WATER (D2O) Heavy water is extensively used as a moderator in nuclear reactors. It can be prepared by exhaustive electrolysis of water or as a by product in some fertilizer industries. USES OF HYDROGEN AS FUEL AND HYDROGEN ECONOMY Hydrogen releases a large amount of heat on combustion which is more than the energy released from petrol and also pollutants are less than petrol.It can be converted into liquid state by cooling to 20 k.In this way Hydrogen economy is an alternative.The basic principle of hydrogen economy is the transportation and storage of energy in the form of liquid or gaseous hydrogen.It is for the first time in India that a pilot project using hydrogen as fuel was launched in oct.2005 for running automobiles.5% of H 2 is mixed in CNG for vehicles,its percentage would be gradually increased to reach the optimum level.Nowadays it is also used in fuel cell to generate electricity.It is expected that safe sources of hydrogen will be identified in near future. -----------------------------------------
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QUESTION BANK 1. Why does hydrogen occur in a diatomic form rather than in a monoatomic form? 2. What are the consequences of high enthalpy of H—H bond in terms of its chemical reactivity? 3. What do you understand by electron deficient,electron precise and electron rich compounds of hydrogen?Give one example of each. 4. Among NH3,H2O and HF which one is expected to to have highest magnitude of hydrogen bonding and why? 5. What is autopyrolysis of water and what is its significance? 6. What are lewis acids and lewis base.Give one example of each. 7. Draw structure of water and explain what properties of water makes it useful as solvent? 8. What are hydrides ?How they are classified?Write one example of each. 9. Differentiate between hydrolysis and hydration,Explain the terms with example. 10. What causes the temporary and permanent hardness of water? What methods are used to remove this hardness? 11. Writa a reaction which shows amphoteric nature of water. 12. A 30% solution is marked as 100 vol. H2O2,what does it mean? 13. Write one chemical reaction for each to justify H2O2 can act as oxidising as well as reducing agent. 14. What do you understand by the terms------a. Hydrogen economy b. Syngas c. Fuel cell d. Heavy water e. Hydrogenation. 15. Arrange the following -----a. LiH,NaH, and CsH in order of their ionic character b. H-H, D-D,and F-F in order of increasing bond enthalpy. 16. Complete the following reactions-----a. CO + H2 --------- b. Zn + NaOH ------- c. PbS + H2O2------ d.CaO + H2O ------- e. Alcl3 + H2O-----
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