Modern inorganic chemistry

410 THE TRANSITION ELEMENTS

atom is surrounded by four chlorine atoms at the corners of a

square and two chlorine atoms above and below, giving a distorted

octahedral structure:

The anhydrous chloride is prepared by standard methods. It is

readily soluble in water to give a blue-green solution from which the

blue hydrated salt CuCl 2. 2H 2 O can be crystallised; here, two water

molecules replace two of the planar chlorine ligands in the structure

given above. Addition of dilute hydrochloric acid to copper(II)

hydroxide or carbonate also gives a blue-green solution of the

chloride CuCl 2 ; but addition of concentrated hydrochloric acid (or

any source of chloride ion) produces a yellow solution due to

formation of chloro-copper(II) complexes (see below).

In the presence of excess iodide ions, copper(II) salts produce the

white insoluble copper(I) iodide and free iodine, because copper(II)

oxidises iodide under these conditions. The redox potential for the

half-reaction :

Cu^2 + (aq) + e~ -^ Cu +(aq): E^ = +0.15 V

must be modified because the concentration of the reduced species,

Cu + (aq). is greatly diminished in the presence of excess I ~ :

Cu+(aq) + ]

The half-reaction is better written as

Cu^2 + (aq) + I"(aq) + e~ -*CuI(s): £^ = +0.86 V

and hence iodide is readily oxidised:

I 2 (aq) -t• 2e -> 21 iaq): E~ = + 0.54V

Bromide ion is not oxidised in this way.

COPPER(II) OXIDES

Copperdl] oxide. CuO. is a black powder, insoluble in water: it is
prepared by heating either the hydroxide, or the hydrated nitrate.