The chemistry of transition metal compounds

The chemistry of transition metal compounds is characterised by variable oxidation states, coloured complex ions and extensive redox reactions. Any one of the first row transition metals could be used to exemplify these types of reaction.

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The chemistry of copper

Copper is element number 29 in the periodic table. It has an electronic configuration of [Ar] 4s1 3d10, but, perhaps surprisingly, its most common oxidation state is +2.

Copper sulfate pentahydrate, CuSO4.5H2O, is probably the most well known of all of the compounds encountered in chemistry education. It is bright blue crystalline compound which loses water of crystallisation on heating to form the white anhydrous salt. This is a reversible reaction and addition of water caused a release of energy and the blue hydrated copper sulphate is regenerated.

CuSO4.5H2O CuSO4 + 5H2O

Dissolution of copper salts

hexaaquacopper(II) complex ion

Copper salts dissociate in aqueous solution and the metal ions are complexed by the water molecules:

Cu2+ + 6H2O(l) [Cu(H2O)6]2+(aq)

The octahedral hexaaquacopper(II) complex ions are blue in solution. This is the species that is produced by all copper salts when dissolved.

blue copper sulfate solution

Addition of concentrated hydrochloric acid

Hydrochloric acid contains a high concentration of chloride ions. These undergo a ligand replacement reaction with the water ligands in the hexaaquacopper(II) complex.

[Cu(H2O)6]2+ + 4Cl- [CuCl4]2- + 6H2O(l)

The complex changes coordination number from 6 to 4 (the chloride ions cannot fit around the central atom in an octahedral arrangement) and the solution changes from blue to bright green.

Addition of potassium iodide solution

This is a redox reaction with the copper(II) complex being reduced to copper(I) while the iodide ions get oxidised to iodine. The colour change is from a blue solution to a white suspension of copper(I) sulfate mixed with the brownish iodine.

2Cu2+ + 2I- 2Cu+ + I2

This reaction may be used to determine copper solutions. The iodine released can be titrated against sodium thiosulfate using starch solution as an indicator.

2S2O32- + I2 S4O62- + 2I-

When the blue-black colour of the starch indicator disappears the end-point has been reached.

Addition of sodium hydroxide solution

Sodium hydroxide provides hydroxide ions that replace abstract protons from the water ligands around the copper ion. Once two protons have been removed the copper complex becomes neutral and precipitates from solution. This can be represented in a simple format:

Cu2+(aq) + 2OH-(aq) 2Cu(OH)2(s)

There is no further change on addition of excess hydroxide ions.

copper(II) hydroxide precipitate

Addition of ammonia solution

Ammonia is also able to abstract hydrogen ions from the water molecule ligands around the central copper(II) ion:

[Cu(H2O)6]2+(aq) + 2NH3(aq) [Cu(H2O)4(OH)2](s) + 2NH4+(aq)

However, in this case, addition of more ammonia causes the blue precipitate to redissolves as the ammonia molecules replace the water (and hydroxide) ligands around the central copper(II) ion.

blue copper(II) hydroxide precipitate

The final complex contains four ammmonia ligands arranged in a square planar arrangement (the equatorial positions of an octahedral structure) with two axial water ligands.

[Cu(H2O)4(OH)2](s) + 4NH3(aq) [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH-(aq)

The final solution is the deep blue tetraamminediaquacopper(II) ion solution. In many cases this is shortened to the tetraamminecopper(II) complex.

deep blue copper tetrammine complex solution


A2 Inorganic Chemistry experiments

Task Possible context
Carry out a redox titration The analysis of iron tablets by titration using acidified potassium manganate(VII)
Investigate the chemistry of transition metal compounds in a series of experiments The chemistry of copper compounds
Prepare an inorganic complex The preparation of hexaamminecobalt(III) chloride or the preparation of iron(II) ethandioate

A2 Physical Chemistry experiments

Task Possible context
Carry out a kinetic study to determine the order of a reaction An iodine clock experiment e.g. the reaction of sulfite ions with iodate(V) ions
Determine an equilibrium constant Determine a value of Kc for the reaction of ethanol with ethanoic acid.
Investigate how pH changes when a weak acid reacts with a strong base or when a strong acid reacts with a weak base. Determine the pH curve for ethanoic acid reacting with sodium hydroxide

A2 Organic Chemistry experiments

Task Possible context
Prepare a solid organic compound The preparation of aspirin
Purify an organic solid The recrystallisation of impure benzenecarboxylic acid from hot water
Test the purity of an organic solid Determine the melting point of benzenecarboxylic acid