Modern inorganic chemistry

THE TRANSITION ELEMENTS 419

rust-preventing paints. Zinc chromate is a yellow pigment. Lithopone

is also used as a filler in linoleum.

Zinc carbonate and zinc oxide are constituents of calamine lotion.

Zinc oxide, an antiseptic, is present in kzinc' ointment and in cosmetic

powders.

Zinc is important biologically; there are many zinc-protein

complexes, and the human body contains about 2 g. In the human

pancreas, zinc ions appear to play an essential part in the storage

of insulin.

CHEMICAL PROPERTIES OF ZINC COMPOUNDS

Oxidation state + 2

Zinc(II) oxide, ZnO, is prepared by heating the hydroxide Zn(OH) 2

or the carbonate ZnCO 3. It is a white solid, insoluble in water, but

readily soluble in acids to give a solution containing the zinc(II)

cation, and in alkalis to give a hydroxozincate(II) anion:

ZnO + 2H 3 O+ -» Zn^2 + (aq) + 3H 2 O
(e.g.) ZnO + 2OH~ + 3H 2 O -> [Zn(OH) 4 (H 2 O) 2 ]^2 -
Zinc(II) oxide is therefore amphoterie.
On heating, the oxide becomes yellow, reverting to white on
cooling. When zinc oxide is heated, a little oxygen is lost reversibly.
This leaves a non-stoichiometrie compound. The crystal lattice is
disturbed in such a way that electrons from the excess zinc metal
remaining can move in the crystal almost as freely as they can in a
metal. This makes zinc oxide a semiconductor and gives it a yellow
colour, which is lost when oxygen is taken up again on cooling to
give zinc oxide.

Zinc(II) hydroxide is a white gelatinous solid obtained when the

stoichiometric quantity of alkali hydroxide is added to a solution

of a zinc salt:

Zn2+(aq) + 2OH~ -> Zn(OH) 2

It is soluble in alkali, and in ammonia (see below).

Zinc(ll} chloride, ZnCl 2 , is the only important halide—it is prepared
by standard methods, but cannot be obtained directly by heating
the hydrated salt It has a crystal lattice in which each zinc is sur-
rounded tetrahedrally by four chloride ions, but the low melting
point and solubility in organic solvents indicate some covalent