Modern inorganic chemistry

304 GROUPV!

Fe 2 (SO 4 ) 3 -» Fe 2 O 3 + 3SO 3 T

2Ag 2 SO 4 -+ 4Ag + 2SO 3 f + O 2 T

Iron(II) sulphate is exceptional.

The sulphate ion is detected by addition of barium chloride in

the presence of hydrochloric acid; a white precipitate of barium

sulphate is obtained. The same test can be used to estimate sulphate,

the barium sulphate being filtered off, dried and weighed.

In the sulphate ion, the four oxygen atoms are tetrahedrally

arranged round the sulphur atom, at equal distances; hence all the

S—O bonds are identical, and their short length suggests that they

are double bonds (as in SO 2 , SO 3 , and SOs"):

o

o o c

This structure is perhaps best visualised by regarding it as built

up from a sulphur trioxide molecule and an oxide ion (this happens

in practice).

In pure sulphuric acid, two of the S---O distances are somewhat

longer, and it is believed that the structure is:

-H—O X>.-H—O X)-H—Ox J>~H—Ov O-

\f V V \(

V.H--a

x

\....H^,

X

V-

The dotted lines represent hydrogen bonds. The high boiling point

and viscosity of the pure acid indicate strong interaiolecular forces

of this kind.

OTHER ACIDS

In addition to the simple acids discussed above, sulphur forms two
peroxosulphuric acids containing the —O—O— linkage and a
number of thionic acids containing more than one sulphur atom.

Oxides and oxo-acids of selenium

Selenium dioxide is a volatile solid obtained when selenium is burnt

in air or oxygen. It is very soluble in water, forming a solution of