Modern inorganic chemistry

414 THE TRANSITION ELEMENTS

Oxidation state +1

In contrast to the +2 state, copper(I) compounds are less frequently

coloured and are diamagnetic, as expected since the 3d level is full.

However, the copper(I) ion, unlike copper(II), is unstable in aqueous

solution where it disproportionates into copper(II) and copper(O)

(i.e. copper metal).

Consider the half-reactions in aqueous solution:

Cu+(aq) + e' -> Cu(s): E* = +0.52 V

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

We see that the Cu+(aq) ion (in the first equation) can oxidise the

Cu+(aq) ion (in the second equation), and hence

Cu+(aq) + Cu+(aq) -> Cu + Cu^2 +(aq)

i.e. 2Cu + (aq)--» Cu + Cu2+(aq), i.e. disproportionation.

In the presence of appropriate ligands, the E^ values may be

affected sufficiently to make Cu(I) stable; but since the likely aquo-

complex which Cu(I) would form is [Cu(H 2 O) 2 ]*, with only two

water ligands, the (hypothetical) hydration energy of Cu* is there-

fore much less than that of the higher charged, more strongly

Equated [Cu(H 2 O) 6 ]^2 +.

Copper{I) oxide, Cu 2 O, occurs naturally as the red cuprite. It is
obtained as an orange-yellow precipitate by the reduction^of a
copper(II) salt in alkaline solution by a mild reducing agent, for
example glucose, hydroxylamine or sodium sulphite:

2Cu^2 + + SOf~ + 4OH~ -> Cu 2 Oi + SOJ- + 2H 2 O

It dissolves in oxo-acids with disproportionation, for example

Cu 2 O + H 2 SO 4 -> CuSO 4 -f Cu| -f H 2 O

Copper(I) chloride, CuCl, is a white solid, insoluble in water. It is
prepared as follows:

1. By warming either copper(I) oxide or a mixture of copper(II)
   chloride and copper with concentrated hydrochloric acid, until a
   deep brown solution is formed.

Cu 20 -f 2HC1 -» 2CuCl + H 2 O

CuCl 2 -f Cu ^2CuCl

In both cases the copper(I) chloride dissolves in the acid to form
the complex [CutlJ^3 ~. On pouring the brown solution into water,