Chromium and its compounds (A-level inorganic chemistry)
Chromium
The electron configuration of chromium.
[Ar]4s13d5
Characteristic of chromium as a transition element
- Forms colored compounds; for instance, Cr3+ is green
- Has variable oxidation states; +2, +3, +4, +5, +6.
- Forms complexes, e.g. Cr(NH3)63+.
- has catalytic properties: for instanceCr2O3 + ZnO catalyzes the formation of methanol from CO and hydrogen.
CO (g) + 2H2(g) → CH3OH
Trial 1
(a) Write the electron configuration of chromium.
(b) Giving examples state the properties that show that chromium is a transition metal.
Oxidation states of chromium
Chromium has oxidation states of +6 (oxidizing), +3 (the most stable) and +2 (reducing).
The reaction of chromium with Air
It is extremely stable and is not affected by moist air. The inertness is probably due to a protective oxide layer. It, however, burns at about 20000C with a brilliant flame to form chromium III oxide
2Cr(s) + 3O2(g) → Cr2O3 (s)
The reaction of chromium with water
Chromium does not react with water but at red heat with steam is to form chromium III oxide
2Cr(s) + 3H2O(g) → Cr2O3 (s) + 2H2(g)
The reaction of chromium with Halogens
F2 and Cl2 combine with hot metal to give chromium III halides.
2Cr(s) + 3X2(g) → 2CrX3 (X = Cl, F)
Chromium reacts directly with fluorine, F2, at 400°C and 200-300 atmospheres to form chromium(VI) fluoride, CrF6. Cr(s) + 3 F2(g) → CrF6(s) [yellow]
The reaction of chromium with alkalis
Chromium does not react with alkalis at ordinary temperatures.
Reaction chromium with ammonia or nitrogen
It forms nitrides
2Cr (s) + 2NH3(g) → 2CrN(s) + 3H2(g)
Cr (s) + N2(g) → CrN(s)
The reaction of chromium with dilute acid
(a) In the absence of air, dil. HCl and dil. H2SO4, chromium reacts with air form chromium II chloride and chromium II sulphate.
Cr (s) + 2HCl(aq) → CrCl2(s) + H2(g)
Cr (s) + 2H2SO4(aq) → CrSO4 (aq) + H2(g)
Reaction chromium with nitric acid
Chromium does not react with nitric acid due to the formation of impervious oxide layer.
Important compounds in oxidation state 6
Chromates (VI) and dichromates (VI)
Sodium and potassium chromates (VI) are yellow solids that dissolve in water to form yellow solutions.
Silver chromate is red
Sodium and potassium dichromates (VI) are orange solids that dissolve in water to form orange solutions.
Uses Chromates (VI) and dichromates (VI)
- They are used as oxidizing agents in the laboratory
Half equations
Cr2O72-(aq) + 6e + 14H+ (aq) → 2Cr3+(aq) + 7H2O(l)
CrO42-(aq) + 3e + 8H+(aq) → Cr3+(aq) + 4H2O(l)
In the process of oxidation, dichromates are reduced to chromium (III) ions and the color changes from orange to green. Note that in acidic solution chromates are converted to dichromates while in alkaline solution dichromates are converted to chromates
(i) Pieces of filter paper impregnated with acidified solutions of sodium dichromate (VI) can be used to test for reducing gases, e.g., sulphur dioxide and hydrogen sulphide.
Cr2O72- (aq) + 8H+ (aq) + 3H2S (g) → 2Cr3+ (aq) + 7H2O (l) + 3S (s)
The orange spot turns green owing to the formation of chromium (III) ions.
(ii) Potassium dichromate (VI) is used in volumetric analysis to estimate reducing agents, e.g., Fe2+
Cr2O72- (aq) + 14H+ (aq) + 6Fe2+ (aq) → 2Cr3+ (aq) + 6Fe3+ (s) + 7H2O (l)
Dichromates are used in the preparation of carbonyl compounds from alcohol and carboxylic acid from aldehydes and many oxidation processes. Dichromates are preferred to manganates (VII) because they are not a very strong oxidizing agent to cleaves carbon-carbon bonds in organic chemistry
Advantage of potassium dichromate over potassium permanganate in volumetric analysis
- Hydrochloric acid can be used to acidify titration involving potassium dichromate since they are not strong oxidizing agents to oxidize chloride ions to chlorine like permanganate (VII) would do.
- Potassium dichromate can be obtained in stable pure form.
Trial 5
A solution of potassium dichromate (VI) was added to an acidified solution of iron (II) sulphate.
(a) State what was observed. (1mark)
(b) Write an ionic equation for the reaction that took place. (1½ marks)
Trial 6
Potassium dichromate in the presence of sulphuric acid is widely used as an oxidizing agent. Write
(a) Half equation for the reaction between potassium dichromate and potassium iodide (2½ marks)
(b) Write an overall equation for the reaction between potassium dichromate and potassium iodide in acidic solution. (1 ½ mark)
(c) Name one practical application of potassium dichromate as an oxidizing agent. (1 mark)
Conversion dichromate to chromate
In the presence of an alkali dichromate (orange) is converted into chromate (yellow) ions. And chromate ions in acid solution are converted into dichromate solution.
Cr2O72-(aq) + 2OH– (aq) → 2CrO42-(aq) + H2O(l)
2CrO42-(aq) + 2H+ (aq) → Cr2O72-(aq) + H2O(l)
Trial 7
State what is observed and write equations for the reaction that took place when sodium hydroxide is added to potassium dichromate solution followed by excess dilute sulphuric acid
Use of chromate and dichromate to test for lead and barium ions
Potassium chromate and dichromate form yellow precipitates with Pb+2 and Ba2+ and are used to test for these ions
M2+(aq) + CrO42-(aq) → MCrO4(s)
M2+(aq) + Cr2O72-(aq) + H2O(l) → 2MCrO4(s) + 2H+(aq)
Lead chromate is soluble in sodium hydroxide whereas barium chromate is not.
PbCrO4(s) + 4OH–(aq) → PbO22-(aq) +2H2O(l)+ CrO42-(as).
Reaction dichromate with hydrogen peroxide
In dilute acidified solutions, dichromate (VI) reacts with hydrogen peroxide solution to form a blue chromium peroxide CrO(O2)2:
Cr2O72- (aq) + 2H+ (aq) + 4H2O2 (aq) → 2CrO(O2)2 (aq) + 5H2O (l)
The reaction provides a sensitive test. For chromium
Chromium III chloride
It is particularly of interest in exams because it shows the isomerism of inorganic compounds.
Definition:
Isomerism is the existence of compounds with the same molecular formula but different structural formulae
Isomers of chromium III chloride are:
(a) [Cr(H2O)6]3+(Cl–)3 Grey-green
(b) [Cr(H2O)5Cl]2+(Cl–)2.H2OLight green
(c) [Cr(H2O)4Cl]2+(Cl–)3.2H2O
Method of distinguishing the isomers of chromium three chlorides
(a) The structural isomers of chromium (III) chloride can be identified by titration with silver nitrate solution; the isomers precipitate a different number of chloride ions per mole of the isomers
Moles of chloride ion precipitated per mole of the isomers
(a) [Cr(H2O)6]3+(Cl–)3 3
(b) [Cr(H2O)5Cl]2+(Cl–)2.H2O 2
(c) [Cr(H2O)4Cl2]+(Cl–).2H2O 1
(b) By electrical conductivity.
Trial 8
(i) Write the structural formulae of the isomers of chromium (III) chloride, CrCl3.6H2O. (3 marks)
(ii) State one way of distinguishing the isomers. (½ mark)
Trial 9
Chromium (III) chloride-6-water, CrCl3.6H2O, is an example of hydrate isomerism.
(a) Write the formulae of the three possible isomers of CrCl3.6H2O.
(c) The co-ordination number of chromium is the same in the isomers of CrCl3.6H2O.
(i) Explain briefly what is meant by the term co-ordination number.
(ii) What is the co-ordination number of chromium in CrCl3.6H2O?
Hydrolysis of chromium (III) salts
Chromium (III) salts hydrolyze in water to give acidic solutions, which turn blue litmus paper red and liberate carbon dioxide from carbonates
Cr3+(aq) + 3H2O(l) → Cr(OH)3(s) + 3H+(aq)
Consequently, a solution of chromium (III) salt
- Has a pH less than 7
- Change the color of blue litmus paper to pink
- Liberates carbon dioxide (effervescence) from carbonate and hydrogen carbonates.
Trial 10
Chromium (III) sulphate was dissolved in water and a few drops of concentrated sodium carbonate solution added to the solution.
(i) State what was observed. (2marks)
(ii) Give a reason for your answer and equations for the reaction (2½ marks)
Answer
- Effervescence and green precipitate formed
- Chromium three ions are hydrolyzed to produce an acidic solutions that liberates carbon dioxide from the carbonate. The green precipitate is chromium (III) hydroxide formed.
2Cr3+(aq) + 3CO32-(aq) + 3H2O (l) → 2Cr(OH)3(s) + 3CO2(g)
Qualitative analysis of chromium (III) salts
With sodium hydroxide solution:
It forms a green precipitate soluble in excess
Cr3+ (aq) + 3OH– (aq) → Cr(OH)3 (s)
Cr(OH)3(s) + OH–(aq) → Cr(OH)4– (aq)
With dilute ammonia solution:
It forms a green precipitate insoluble in excess
Cr3+ (aq) + 3OH– (aq) → Cr(OH)3 (s)
Confirmatory test;
It forms a green precipitate with sodium hydroxide soluble in excess turns yellow with the hydrogen peroxide solution and temporary blue when a few drops of concentrated sulphuric acid are added to the mixture.
Equations
Cr3+ (aq) + 3OH– (aq) → Cr(OH)3 (s)
Cr(OH)3(s) + OH–(aq) → Cr(OH)4– (aq
Cr2O72- (aq) + 2H+ (aq) + 4H2O2 (aq) → 2CrO(O2)2 (aq) + 5H2O (l)
Trial 11
Dilute sodium hydroxide solution was added dropwise until in excess to aqueous chromium (III) chloride
State what was observed. (1½ mark)
Write an equation for the reaction. (2 marks)
To the product in (a) was added a few drops of hydrogen peroxide and the mixture boiled
State what was observed. (1mark)
Write an equation for the reaction. (1½ marks)
Trial 12
A dark purple crystalline solid Z dissolved in water to give a violet solution. When aqueous sodium hydroxide was added to the solution of Z dropwise, a green precipitate was formed and it dissolved in excess sodium hydroxide to give a green solution.
Identify the cation in Z. (1 mark)
Write the formula of the species responsible for the formation of:
The violet color (1 mark)
Green solution (1 mark)
Trial 13
A dilute solution of chrome alum, K2Cr2(SO4)4.2H2O was prepared and divided into two portions.
The first portion turned blue litmus red. Explain the observation and write an equation to illustrate your answer. (4 marks)
To the second portion was added a few drops of sodium carbonate solution. State what was observed and explain your answer. (3marks)
Trial 14
(a) When about 1cm3 of dilute sulphuric acid was added to about 1cm3 of potassium chromate, an orange solution was observed. Addition of an excess dilute sodium hydroxide solution to the mixture gave a yellow solution. Write an equation for the reaction that took place. (3 marks)
(b) Briefly describe a test that can be carried out in the laboratory to confirm that the potassium chromate contains chromium ions. (3mark)
Trial 15
(b) When hydrogen peroxide was added to the green solution, a yellow solution was formed, which on treatment with lead ethanoate solution formed a yellow solid. Write an equation for the reaction leading to the formation of:
(i) The Green solution.
(ii) The yellow solution.
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