Modern inorganic chemistry

THE TRANSITION ELEMENTS 411

or the basic carbonate of copper(II). It dissolves in acids to give
solutions of copper(II) salts. It is readily reduced to the metal by
heating with hydrogen and is used to determine carbon and hydrogen
in organic compounds (the carbon as carbon monoxide reduces the
copper(II) oxide to copper).

Hydrated copper(II) hydroxide, Cu(OH) 2 , is precipitated as a pale

blue solid when alkali is added to an aqueous solution of a copper(II)

salt:

alkali

[Cu(H 2 O) 6 ]^2 + + H 2 O ^=

acid

,,

CU

(OH)

»H 252 0)

(OH) 2

CU(H

2 O) 4

It is readily dehydrated on warming, to give the black oxide CuO.
It dissolves in excess of concentrated alkali to form blue hydroxo-
cuprate(II) ions, of variable composition; it is therefore slightly
amphoteric. If aqueous ammonia is used to precipitate the hydroxide,
the latter dissolves in excess ammonia to give the deep blue ammino
complexes, for example [Cu(NH 3 ) 4 (H 2 O) 2 ]^2 +

COPPER(ll) CARBONATE

The 'normal' carbonate CuCO 3 is not known; two naturally

occurring basic carbonates have already been mentioned. If a

solution of, for example, sodium carbonate is added to a solution of

a copper(II) salt, a green basic carbonate is precipitated; the

reactions are:

[Cu(H 2 O) 6 ]2+ + H 2 O ^ [Cu(OH)(H 2 0) 5 ] + + H 3 O +

2[Cu(OH)(H 2 0) 5 ] + + COr -> [Cu(OH)(H 2 O) 5 ] 2 CO 3 ,

i.e. CuCO 3. Cu(OH) 2 aq.

On heating, the basic carbonate readily yields the black copper(II)

oxide.

COPPER(II) SULPHATE

This substance is familiar as the blue crystalline pentahydrate
CuSO 4 .5H 2 O. In this crystal, each Cu2+ ion is surrounded by
four water molecules at the corners of a square, while the fifth water
molecule is held by hydrogen bonds (see Figure 13.8).