Modern inorganic chemistry

326 GROUP VII: THE HALOGENS

In aqueous solution sulphur dioxide (sulphurous acid) is oxidised

to sulphuric acid :

SO 2 + C1 2 + 2H 2 O -* H 2 SO 4 4- 2HC1

Chlorine reacts with some metallic oxides to yield chlorides, for

example

2Fe 2 O 3 + 6C1 2 -» 4FeCl 3 + 3O 2

Bromine has many oxidising reactions (E^ = + 1.07 V) and like

chlorine it will oxidise sulphur dioxide in aqueous solution to

sulphuric acid, and hydrogen sulphide to sulphur.

Iodine has the lowest standard electrode potential of any of the

common halogens (E^ = +0.54 V) and is consequently the least

powerful oxidising agent. Indeed, the iodide ion can be oxidised to

iodine by many reagents including air which will oxidise an acidified

solution of iodide ions. However, iodine will oxidise arsenate(III) to

arsenate(V) in alkaline solution (the presence of sodium carbonate

makes the solution sufficiently alkaline) but the reaction is reversible.

for example by removal of iodine,

_ ~ + I 2 + 2OH~ ^ AsOr + 2I~ 4- H 2 C

arsenate(III) arsenate^V)

The oxidation of the thiosulphate ion S 2 Oj" to tetrathionate ion,

S 4 Ol^, is used to estimate iodine:

The disappearance of iodine at the end point is detected by the

addition of fresh starch solution which gives a blue complex as long

as iodine is present.

HALOGEN COMPOUNDS

THE HYDRIDES (HYDROGEN HALIDES)

Physical properties

All the halogens form hydrides by direct combination of the
elements. The hydrogen halides formed are covalently bonded, and
when pure are colourless gases at room temperature. Some import-
ant physical properties of the hydrogen halides are given in Table
11.3 below. The data in Table 113 clearly reveal unexpected
properties for hydrogen fluoride. A graph of atomic number of the
halogen against b.p. for the hydrogen halides has been given on