Modern inorganic chemistry

GROUP IV 199

Tin(II) chloride is slowly oxidised in air, but keeping a piece of tin

metal in the solution prevents this.

TIN(IV) CHLORIDE, SnCl 4

Stannic chloride is prepared by treating metallic tin with chlorine:

Sn + 2C1 -+ SnCl 4

(This reaction has been used to recover tin from scrap tinplate.)

Tin(IV) chloride is a colourless liquid, which fumes in air due to

hydrolysis:

SnCl 4 + 2H 2 O ^--- SnO 2 + 4HC1

hyd rated

It is soluble in organic solvents (a characteristic of a covalent com-

pound), but dissolves in water and can form hydrates (a character-

istic of an ionic compound). Hence the hydra ted Sn4+ must be

formed in water and undergo hydrolysis thus (cf. aluminium):

Kn[Sn.xH vH OH* 2 O] - Sn _ +°H T+ +H H "- 2)H 2 O

This process goes on until (if alkali is added) the final product is

[Sn(OH) 6 ]^2 ~. (If alkali is not added, hydrolysis ultimately gives the

hydrated oxide in accordance with the equation above.) The

hydrolysis can be suppressed by addition of hydrochloric acid, and

with excess of this, hexachlorostannic(IV) acid is formed:

SnCl 4 4- 2HC1 -> H 2 SnIVCl 6

Salts of this acid are known and ammonium hexachlorostan-

nate(IV) (NH 4 ) 2 SnQ 6 , is used as a mordant.

Lead

LEAD(II) CHLORIDE

The solid is essentially ionic, made up of Pb2+ and Cl~ ions. The
vapour contains bent molecules of PbCl 2 (cf. SnCU). Lead chloride
is precipitated when hydrochloric acid (or a solution of a chloride)
is added to a cold solution of a lead(II) salt. It dissolves in hot water
but on cooling, is slowly precipitated in crystalline form. It dissolves
in excess of concentrated hydrochloric acid to give the acid
H 2 [Pb"Cl 4 ].