hem F acts actshe heet et C hem Number 75
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The Chemistry of Chromium To succeed in this topic you need to understand: the basic basicss of trans transiti ition on metals metals (cov (covered ered in in Factshe Factsheet et 28); 28); • the reaction reactionss of transition transition metal ions with hydroxides hydroxides and ammonia ammonia (covered in Factsheet 46); • the principles principles of oxidati oxidation on and and reduction reduction (covered (covered in in Factsheet Factsheet 11).
The +6 state This is described as an ‘oxidising state’ because most species most species (i.e. molecules and ions) of chromium in this state have a tendency to become reduced to more stable chromium(III) species, usually the Cr3+ ion. The Cr2O72- ion is particularly important: in acidic solution (dilute H2SO4 is usually used), its orange solution is reduced to a green solution of Cr3+(aq):
After working through this Factsheet you will: • know know the the oxid oxidat atio ion n state statess of chr chrom omiu ium; m; • know know that the the +3 state state is is stable, stable, whil whilee +6 is an an oxidisi oxidising ng state state and and +2 is a reducing state; • understan understand d the relationsh relationships ips between between the the principal principal compounds compounds of chromium; • know know the the acidacid-bas basee charac character ter of the oxides oxides;; 3+ • unde unders rsta tand nd the the reac reacti tion onss of of Cr Cr (aq) with OH-(aq), NH3(aq) and CO32-(aq); • unders understan tand d how how to access access the the +6 +6 oxida oxidatio tion n state; state; • know know the inte interco rconver nversio sion n of chrom chromium ium(VI (VI)) compou compounds nds..
•
Reducing agents which can be oxidised by dichromates(VI) include Fe2+ salts (to Fe 3+ salts), sulphites (to sulphates) and ethanol (to ethanal). Remember - Not - Not all chromium(VI) species are oxidants. (A reaction will occur only if a mechanism is available.) In particular, the chromate(VI) ion, CrO 4 2- , is not an oxidant. The +2 state This is a reducing state - a remarkably powerful one. Practical 1 - The action of air on Cr2+ Prepare a chromium(II) compound in the absence of air by boiling a solution of potassium dichromate(VI) with zinc and dilute hydrochloric acid in a conical flask fitted with a bunsen valve (Fig 1).
The element Chromium (atomic number = 24) is the fourth member of the first series of d-block transition elements. •
Cr2O72-(aq) + 14H +(aq) + 6e- → 2Cr3+(aq) + 7H2O(l)
Its Its elec electr tron onic ic con confi figu gura rati tion on is is 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1. This arrangement, in which each of the 3d orbitals is singly occupied, is more stable than the alternative [Ar] 3d4, 4s 2. It is is a hard metal with a bluish bluish tinge, tinge, familiar familiar as chromium chromium plating plating on furniture and the trim of classic cars. The melting point is very high (1890 oC).
Fig 1. A conical flask fitted with a bunsen valve glass rod as a stopper rubber tubing with a slit rubber bung
- Although chromium itself is not in A-level specifications, its aqueous chemistry is a popular topic.
Exam Hint
glass tubing
Oxidation states The oxidation states of chromium are +2, +3 and +6, of which only +3 is stable. Principal compounds and ions are as follows. • CrII The chromium(II) ion, Cr2+. Its electronic configuration configuration is 1s2, 2s2, 6 2 6 4 2p , 3s , 3p , 3d . •
The solution changes colour from orange to green and then to sky blue as chromium is reduced from +6 to +3 and then to +2. Afterwards, remove the bunsen valve and shake the flask. The solution immediately changes from blue to green as oxygen from the air oxidises chromium(II) to chromium(III).
CrIII Chromium(III) oxide, Cr2O3, chromium(III) hydroxide, Cr(OH)3, the chromium(III chromium(III)) ion, Cr3+, and various complex ions such as the hydrated chromium(III) ion, [Cr(H2O)6]3+. The electronic configuration of the Cr3+ ion is 1s2, 2s 2, 2p 6, 3s 2, 3p 6, 3d3.
Remember - The most stable oxidation state of chromium is +3.
Remember - Whenever ions are formed from atoms of transition elements, it is always the outer 4s electrons that are lost first. •
CrVI Chromium(VI) oxide, CrO3, the chromate(VI) ion, CrO42-, and the dichromate(VI) ion, Cr2O72-. Exam Hint - Never
say that a chromate(VI) or a dichromate(VI) contains Cr 6+ because this symbolises a non-existent ion. The correct abbreviation for chromium(VI), i.e. chromium in the +6 oxidation state, is Cr VI .
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Redox chart On the redox chart of chromium (p2), the principal compounds and ions have been arranged in order of oxidation number. • Any movement movement up the the chart chart represents represents an oxidation oxidation process, process, for for which which an oxidising agent is required. • Movement Movement down down the the chart chart corre correspo sponds nds to to reducti reduction. on. For this this,, a reducing reducing agent is needed, such as Zn and HCl(aq) in Practical 1. • Movement Movement across across the chart chart is neither neither oxidation oxidation nor nor reductio reduction, n, and and is achieved with reagents such as water, dilute acids, NaOH(aq), NH3(aq) or Na2CO3(aq).
75. The Chemistry of Chromium OXIDATION
REDUCTION
Chem Factsheet
S R + T E 2 A D T U E C I N G
B I B n l a s a s i o l c c k u b s o l e l i d
S S + T T A A 3 T B E L E
C r O
S O + T X 6 A I T D E I S I N G
A I C n G m s r e r p o l e h u n 2 O b s o t l o 3 e e l r i i d c
A S D C o a r c l r i u d k O b i c e r 3 l e i n d w b r a o t w e r n s o l i d
H e a t
[ C r V ( N i o l H e t 3 ) 6 ] 3 +
C o n c . N H 3 ( a q )
A G C h m e r r p e o h n m o p i t C p u e m t r i ( r c I ( I I O ) H H y d ) r 3 o x i N d a e O o H r r o ( N N a q a 2 )
C 3 H N L a i O ( O m a 3 H i ( ) ( a q t e a q d q ) )
[
B C l u r e ( s H o l 2 u O t i o 6 ) n ] 2 +
*
C r O
4
2
−
i s
n o t
A I R ( O 2 ) Z n + H C l ( a q ) N O A I R
[ C r P ( u H r p l 2 O e ) 6 ] 3
C H h r 2 o m C i c r ( V O I 4 ) a c i d
+ Η 2 Ο
[ C r ( O C H h r 4 ) o ( m H a 2 t e O ( ) I 2 I ] I ) −
( H 2 O ) 5 ] 2
r ( O H ) G 5 ( r H e 2 e n O s ) o l 2 ] u 2
+
G r e e ! n [ s C o l r u t G ( O i o r n e H e n )
+
+ H 3 O +
i ! o n s [ C
2
Y C * e h l l o r C o w m r a O t e 4 ( 2 V − I ) i o n i n N O x a 2 a i d O l k 2 a a o t i l o i n r n P e b w s O i o h l 2 t u t i o n
t i !− o n [
C r ( O H ) 6 ] 3 −
a n o x i d a n t
N O O X . I D O F A T C I r O N
o e R . e r g Z . d n F u + e c n d 2 + i g i s l u a l a t g t e s H e n t C s l ( a q )
A l k a l i
A c i d
O D i C r c a h r n r 2 g o e m O a 7 t 2 e − ( V I ) i o n
O U T L I N E C H E M I S T R Y O F C H R O M I U M
75. The Chemistry of Chromium
Chem Factsheet
Oxides Chromium forms three oxides. • Chromi Chromium( um(II) II) oxide, oxide, CrO, CrO, a litt little le known known basi basicc oxide. oxide. • Chro Chromi mium um(I (III II)) oxi oxide de,, Cr Cr2O3, which is amphoteric, dissolving in acids to give chromium(III) salts and in alkalis to give ch romates(III). • Chro Chromi mium um(V (VI) I) oxid oxide, e, CrO CrO3, which is acidic, dissolving in water to give chromic(VI) acid, H2CrO4, and in alkalis to give chromate s(VI). Chromium(III) hydroxide, Cr(OH)3, like the oxide, is amphoteric. The nature of a hydroxide is usually the same as that of the corresponding oxide.
The acidity of oxides increases with oxygen content. (Some people say that acidity increases with oxidation number.)
Aqueous chemistry of chromium(III)
Practical 4 - Reverse changes Add a dilute acid to a chromate(III) solution. Chromium(III) hydroxide is precipitated at first, and then dissolves in e xcess acid to give a bluishgreen solution of a chromium(III) salt. H+(aq) ions from the acid react with OH-(aq) ions (to give H2O) so that the various equilibria are disturbed to the left hand side.
Hydrated chromium(III) ion [Cr(H2O)6]3+, named systematically as the hexaaquachromium(III) ion, is purple, but gives a turquoise coloured acidic solution. Practical 2 - Hydrolysis of chromium(III) salts Dissolve in water a purple chromium(III) salt, such as chrome alum, and test the solution with pH paper. paper. Note the change in colour and the pH of the solution. The hydrated chromium(III) ion can donate a proton from one of its coordinated water molecules to a molecule of solvent water: [Cr(H2O)6]3+ + H2O(l)
Practical 5 - Cr(III) salts with NH 3(aq) Add dilute NH3(aq) dropwise, with shaking, to a solution of a chromium(III) salt. As with NaOH(aq), NaOH(aq), a grey-green precipitate precipitate of hydrated chromium(III) hydroxide is formed in an acid-base reaction:
[Cr(OH)(H2O)5]2+(aq) + H 3O+(aq)
!
An equilibrium is established containing both [Cr(H2O)6]3+, which is purple, and [Cr(OH)(H2O)5]2+, which is green. Consequently, the solution is bluish-green. It is also acidic, because H3O+(aq) ions (“hydrogen ions”) are formed, and the concentration of these is greater than that of OH-(aq) ions.
[Cr(OH)(H2O)5]2+(aq) + 2NH3(aq) ! [Cr(OH)3(H2O)3](s) + 2NH 4+(aq) Now add concentrated NH concentrated NH3(aq). Although the precipitate is insoluble in dilute NH3(aq), it will partially dissolve in the concentrated reagent in a ligand substitution reaction to give a violet solution containing the hexaamminechromium(III) ion:
The reaction is known as an acid-base reaction or reaction or deprotonation 3+ because [Cr(H2O)6] behaves as an acid (proton donor) and H2O as a base (proton acceptor).
[Cr(OH)3(H2O)3](s) + 6NH3(aq) ! [Cr(NH [Cr(NH3)6]3+(aq) + 3OH-(aq) + 3H2O(l)
Other molecules or ions, notably OH-, NH3 and CO32- can also react as bases with [Cr(H 2O)6]3+, but these species are stronger bases than H2O and further deprotonation can occur.
Practical 6 - Cr(III) salts with CO 32-(aq) Add Na2CO3(aq), with shaking, to a solution of a chromium(III) salt. Notice the formation of a grey-green precipitate of hydrated chromium(III) hydroxide, accompanied by effervescence. Collect the gas evolved in a squeezed out teat pipette and bubble it through limewater. Again, there is an acid-base reaction but, when CO32- ions accept protons, carbonic acid is not formed because of its instability. Instead, its decomposition products, carbon dioxide and water, are produced.
Practical 3 - Cr(III) salts with OH -(aq) Add dilute NaOH(aq) dropwise, with shaking, to a solution of a chromium(III) salt. A grey-green precipitate of hydrated chromium(III) hydroxide is formed at first. This dissolves in excess NaOH(aq) to give a grass-green solution of a chromate(III).
[Cr(OH)(H2O)5]2+(aq) + CO32-(aq) → [Cr(OH)3(H2O)3](s) + H2O(l) + CO2(g)
Do not confuse a green chromate(III) with a yellow chromate(VI).
The addition of NaOH(aq), NH 3(aq) or Na 2CO3(aq) to an aqueous solution of a chromium(III) salt gives an identical greygreen precipitate of hydrated chromium(III) hydroxide. In all cases, an acid-base (deprotonation) reaction occurs, in which the hydrated chromium(III) ion acts as an acid on the Brønsted-Lowry theory, i.e. a proton donor.
Exam Hint -
Chromium(III) hydroxide is formed in an acid-base reaction in which three water molecules coordinated to Cr3+ each lose a proton. proton. This can be represented by the equation: [Cr(OH)(H2O)5]2+(aq) + 2OH-(aq) or [Cr(H2O)6]3+(aq) + 3OH-(aq)
[Cr(OH)3(H2O)3](s) + 2H2O(l)
!
Now add excess Na 2CO 3(aq) to the precipitate of chromium(III) hydroxide. For two reasons it will not dissolve. • CO32- is a weaker base than OH-, so a chromate(III) ion cannot be formed. • CO32-, unlike NH3, is not a ligand, so there can be no ligand substitution reaction.
[Cr(OH)3(H2O)3](s) + 3H2O(l)
!
The dissolving of chromium(III) hydroxide in excess NaOH(aq) occurs by a continuation of the same process: [Cr(OH)3(H2O)3](s) + OH -(aq)
[Cr(OH) 4(H2O)2]-(aq) + H2O(l)
!
Remember - With - With Na2CO3(aq), M 2+ ions form precipitates of metal carbonates, MCO3 , while M 3+ ions give metal hydroxides, M(OH) 3 , plus CO2. Whatever the the precipitate, precipitate, it will never will never dissolve dissolve in excess reagent.
[Cr(OH)4(H2O)2]-(aq) + OH-(aq) ! [Cr(OH)5(H2O)]2-(aq) + H2O(l) [Cr(OH)5(H2O)]2-(aq) + OH-(aq)
[Cr(OH)6]3-(aq) + H2O(l)
!
An equilibrium is established in which the dominant ion is [Cr(OH)4(H2O)2]- at low concentrations of OH-, and [Cr(OH)6]3- at high concentrations.
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75. The Chemistry of Chromium
Chem Factsheet
Aqueous chemistry of chromium(VI)
Interconversion of Cr(VI) compounds compounds
Access to the +6 state In general, it is difficult to oxidise Cr(III) compounds to Cr(VI) ones for two reasons. • Most Most species species contai containing ning Cr(V Cr(VI) I) have have a strong strong tendency tendency to becom becomee reduced reduced 2back to the Cr(III) state. However, the CrO4 ion is not an oxidant, so this provides an obvious point of entry to the +6 oxidation level. • Most oxidising oxidising agents agents operate only in in acidic acidic solution solution and, under under these these 22conditions, the CrO4 ion is converted to Cr 2O7 .
Practical 8 - Preparation of sodium dichromate(VI) Acidify the solution of sodium chromate(VI) prepared in Practical 7. Notice the colour change from yellow to orang e. 2CrO42-(aq) + 2H +(aq)
Now make the solution alkaline again and see the colour revert to yellow.
Oxidising agents which function in alkaline conditions and which are powerful enough to convert chromate(III) ions to chromate(VI) ions include sodium peroxide, hydrogen peroxide and lead(IV) oxide.
Cr2O72-(aq) + 2OH-(aq)
→
Potassium dichromate(VI) is made by adding KCl(aq) to Na2Cr2O7(aq) and crystallising the solution. Adding KOH to K2Cr2O7 will give K2CrO 4.
Answers 1 Add zinc (or other other suita suitable ble metal) metal) and boil. boil. The Cr2O72- solution changes colour to green and stays green. The VO2+ solution changes colour to green and then to blue (see Factsheet 46).
2CrO42-(aq) + 2H2O(l)
Practice Questions 1 Acidified Acidified solutions solutions containing containing the dichro dichromate( mate(VI) VI) ion, ion, Cr Cr2O72-, and the dioxovanadium(V)ion, VO2+, are both orange and can both act as oxidising agents. Suggest a chemical means of distinguishing between them. Choose one test only, and describe what you would expect to see with each solution. 2
3
4
5
When When cry cryst stal alss of an ora orang ngee soli solid d A were heated they decomposed exothermically, leaving a green solid B which dissolved in nitric acid to give a solution C and also in concentrated concentrated sodium sodium hydroxide solution solution to give a solution D. When sodium hydroxide solution solution was added toA to A, it formed a yellow solution of E and gave off a pungent gas F which turned moist red litmus paper blue. (a) Write down the name and formula formula of A. (b) Identi Identify fy B. (c) State the colours of solutions solutions C and D and write down the formulae of their chromium containing ions. (d) Write an ionic ionic equation equation for the formation formation of E. (e) Write Write an ionic equation equation for the formation formation of F. When iron wire wire was was added added to an acidified acidified soluti solution on of potassium potassium dichromate(VI), the wire dissolved, the solution changed colour, and an odourless, neutral gas was evolved. (a) What colour colour change change was observed? observed? (b) Construct an an ionic equation for the overall reaction between iron and dichromate(VI) ions. (You (You may omit state symbols.) (c) (c) What What gas was evol evolved? ved? (d) In terms of electron transfer, how how do you account for for the formation of this gas? There are three three isomers isomers of chromium chromium(III) (III) chloride-6-w chloride-6-water, ater, with the the following formulae: [Cr(H2O)6]Cl3 [CrCl(H2O)5]Cl2.H2O [CrCl2(H2O)4]Cl.2H2O Suggest a chemical method for distinguishing between them.
2+
[Ar]
Cr 3+
[Ar]
Cr
2
(a) (a) Ammo Ammoni nium um dic dichr hrom omat ate( e(VI VI), ), (NH (NH4)2Cr2O7 (b) Chromium(II Chromium(III) I) oxide, oxide, Cr2O3 (c) C is green/blue-green/turquoise. green/blue-green/turquoise. It contains [Cr(H2O)6]3+ (also [Cr(OH)(H 2O)5]2+). D is green. It contains [Cr(OH)4(H2O)2]-, [Cr(OH)5(H2O)]2- and [Cr(OH)6]3-. (d) Cr2O72-(aq) + 2OH-(aq) → 2CrO42-(aq) + H2O(l) (e) NH4+(aq) + OH -(aq) → NH3(g) + H2O(l)
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(a) (a) From From oran orange ge to dark dark gree green n (b) Fe → Fe3+ + 3e× 2 add 2+ 3+ Cr2O7 + 14H + 6e → 2Cr + 7H2O 2+ 3+ 3+ + 7H2O ⇒ 2Fe + Cr O + 14H → 2Fe + 2Cr 2 7 (c) (c) Hydr Hydrog ogen en (d) Some of the electrons lost by Fe as it dissolves are transferred transferred to hydrogen ions from the acid: 2H+(aq) + 2e- → H2(g)
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Only free Cl- ions will react with AgNO3(aq): not Cl- ions which are coordinated to the central Cr3+ ion. Weigh out equal masses of the three isomers, dissolve each in water, acidify with dilute HNO3 and add excess AgNO3(aq). Filter off the precipitated AgCl, wash, dry and weigh.The masses masses will will be in the the ratio ratio 3:2:1. 3d 4s (a) Cr2+ [Ar]
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Cr 3+
[Ar]
(b) 3d sub-shell electrons electrons are split by ligands into two sets of dorbitals. There are three orbitals in the lower energy set and two in the higher energy set. Cr3+ can have all its 3d electrons in the lower set, but Cr2+ must have at least one 3d electron in the upper set.
(a) Using half-arrows half-arrows to represent represent electrons, electrons, complete complete the the followin following g 2+ 3+ electronic configurations of the Cr and Cr ions. 3d
2CrO42-(aq) + H2O(l)
→
Remember - Chromates(VI) - Chromates(VI) are yellow but dichromates(VI) are orange.
Practical 7 - Preparation of sodium chromate(VI) Pour a solution of a chromium(III) salt into a beaker and add NaOH(aq) with stirring until the initial precipitate of Cr(OH)3 dissolves. Add Na2O2(s), stir and warm until the green solution changes colo ur to yellow. Boil the solution to decompose excess Na2O2, then cool it and allow Na2CrO4 to crystallise out. 2Cr3+(aq) + 3O 22-(aq) + 4OH-(aq)
Cr2O72-(aq) + H2O(l)
→
4s
Ack now led ge men ts: This Factsheet was researched and written by John Brockington. Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU. ChemistryFactsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permiss ion of the publishe r. ISSN 1351 -513 6
(b) Suggest why Cr2+ ions are readily oxidised to Cr3+ ions.
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