Modern inorganic chemistry

THE ELEMENTS OF GROUPS IB AND MB 437

the mercury ions are in pairs with the chloride ions adjacent, i.e.

CP *Hg—Hg+. Cl~. (It is now known that mercury can

also form species Hg^ up to Hgg+ ; cadmium also gives Cd^+, and

other polymetallic cations, for example Bi^ are known.) The ion

Hg|+(aq) tends to disproportionate, especially if the concentration

of Hg^2 +(aq) is reduced, for example by precipitation or by complex

formation. However, the equilibrium can be moved to the left by

using excess of mercury, or by avoiding aqueous solution. Thus,

heating a mixture of mercury and solid mercury(II) chloride gives

mercury(I) chloride, which sublimes off:

Hg + HgCl 2 -> Hg 2 Cl 2

The product, commonly called calomel, is a white solid, insoluble in
water; in its reactions (as expected) it shows a tendency to produce
mercury(II) and mercury. Thus under the action of light, the sub-
stance darkens because mercury is formed; addition of aqueous
ammonia produces the substance H 2 N—Hg—Hg—Cl, but this also
darkens on standing, giving H 2 N—Hg—Cl and a black deposit of
mercury.
Mercury(I) ions can be produced in solution by dissolving excess
mercury in dilute nitric acid:

6Hg + 8H+ + 2NO 3 ~ -» 3Hg|+ + 2NO + 4H 2 O

From the acid solution white hydrated mercury(I) nitrate

Hg 2 (NO 3 ) 2 .2H 2 O

can be crystallised out; this contains the ion

[H 2 O-Hg-Hg-H 2 O]^2 +

which is acidic (due to hydrolysis) in aqueous solution. Addition of
chloride ion precipitates mercury(I) chloride.

Oxidation state + 2

Mercury(II) oxide, HgO, occurs in both yellow and red forms; the
yellow form is precipitated by addition of hydroxide ion to a
solution containing mercury(II) ions, and becomes red on heating.
Mercury(II) oxide loses oxygen on heating.

Mercury(II) chloride is obtained in solution by dissolving mercury(II)

oxide in hydrochloric acid; the white solid is obtained as a sublimate

by heating mercury(II) sulphate and solid sodium chloride:

HgSO 4 + 2NaCl -» HgCl 2 + Na 2 SO 4