Modern inorganic chemistry

334 GROUP VII:THE HALOGENS

Hence hydrogen iodide cannot be produced by the reaction of

sulphuric acid with an iodide. Hydriodic acid is slowly oxidised by

air (more rapidly in light) liberating iodine:

4HI + O 2 -» 2H 2 O + 2I 2

Other examples of its reducing action are:

1. Reduction of dinitrogen oxide to ammonia (which gives the
   ammonium ion with the acid):

N 2 O 4- 10HI -> 2NH^ + 21" + H 2 O + (^412)

2. Reduction of nitric to nitrous acid:

HNO 3 + 2HI -> HNO 2 -f I 2 -f H 2 O

OXIDES

None of the halogens reacts directly with oxygen but all form oxides

by indirect methods.

Fluorine oxides

The oxides of fluorine are more correctly called oxygen fluorides

because of the greater electronegativity of fluorine.

Oxygen difluoride OF 2 is obtained when a rapid stream of gaseous

fluorine is passed through 2 % caustic soda solution:

2F 2 -f 2NaOH -» 2NaF + OF 2 + H 2 O

It is a gas at room temperature with a boiling point of 128 K. It is a
strong oxidising agent, some reactions occurring with explosive
violence. Water hydrolyses it slowly at room temperature, but the
reaction evolving oxygen is rapid in the presence of a base, and
explosive with steam:

OF 2 -h H 2 O -> O 2 + 2HF

Fluorine is known to form three other oxides, O 2 F 2 , O 3 F 2 and

O 4 F 2 but all these decompose below 200 K.

Chlorine oxides

Chlorine forms several very reactive, unstable oxides. Dichlorine
monoxide C1 2 O is a yellowish gas at room temperature, the liquid