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THE ROYALSOCIETY OFCHEMISTRY

136 In Search of more Solutions

29. Identify the metalTime

12 h.

LevelA-level, Higher Grade or equivalent.

Curriculum linksComplex ions. Transition metals.

Group size 23.

Materials and equipment

Materials per groupNi(NH 3)4

2+(aq) (sometimes regarded as Ni(NH 3)62+(aq)): dissolve 2 g of NiSO 4 in

100 cm 3 water, then add dilute ammonia until the solution turns a deep bluecolour.

Cu(NH 3)42+(aq): add ammonia solution dropwise to dilute copper sulphate

solution until a deep blue colour is formed (neither concentration is critical).

VO 2+(aq): dissolve ammonium vanadate (V) (ammonium metavanadate,NH 4VO 3) in 2 mol dm

3 sulphuric acid to make a 0.1 mol dm 3 yellowsolution. Add solid sodium metabisulphite until the solution is blue.

CoCl 42(aq): dissolve 0.5 g CoCl 2.6H 2O in conc HCl to give a blue solution.

Adding water gradually turns the solution pink, and a purple half-way pointcan be reached.

For the tests, the following reagents may be useful:

aqueous sodium carbonate

aqueous sodium hydroxide

aqueous ammonia

conc HCl

solid buta-2,3-dionedioxime

aqueous ethylenediamine

2-hydroxybenzoate

1,2-hydroxybenzoate

EDTA

20 volume hydrogen peroxide

powdered zinc.

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Teachers guide 137

Equipment per grouptest-tubes

test-tube racks

dropping pipettes

full range indicator paper

safety glasses.

SafetyEye protection must be worn.VO 2+(aq) must be used in a fume cupboard because a variety of sulphur compoundsare produced from the metabisulphite. Alternatively, the blue solution can be boiledfor a few minutes to destroy excess metabisulphite.

Risk assessmentA risk assessment must be carried out for this activity.

CommentaryPossible reactions to identify the complexes are:

Ni(NH 3)42+ (aq)

Add dilute hydrochloric acid to the complex solution until it turns green Ni(H 2O) 4

2+(aq). To this add:

(a) aqueous sodium carbonate to give green NiCO 3(s);

(b) sodium hydroxide solution to give green Ni(OH) 2, then add ammonia solutionto give blue Ni(NH 3)6

2+(aq);

(c) conc HCl to give yellow-green NiCl 42(aq);

(d) solid buta-2,3-dionedioxime to give a red precipitate; or

(e) the ligand en dropwise to give first a violet solution, (Ni(H 2O)6en)2+, and then a

pinky-purple solution, (Ni(en) 3)2+(aq).

Other reactions are also possible.

Cu(NH 3)42+

(aq)Add dropwise:(a) water to give a pale blue colour, Cu(H 2O)4

2+(aq);

(b) EDTA to give a pale blue solution;

(c) conc HCl to give green CuCl 42(aq);

(d) 2-hydroxybenzoate to give a green solution; or

(e) 1,2-dihydroxybenzoate to give a green solution.

VO 2+ (aq)Add to the blue complex solution:

(a) 20 vol hydrogen peroxide to give yellow V V; or

(b) powdered zinc to reduce to first green V III and then lilac V II.

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138 In Search of more Solutions

CoCl 42

(aq) To achieve a blue colour in the preparation of this complex, conc HCl is used. Thisaffects the reactions that might be attempted, and is a clue to the identity of theligand.

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1. Food31

In Search of more Solutions 31


28. Which gas is which?In the beaker are test-tubes containing different gases. The gases are carbon dioxide,dinitrogen oxide, oxygen, chlorine and hydrogen.

You may remove a test-tube only once and when you do so you must identify thegas immediately.

Which tube contains which gas?

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Isms 29