Modern inorganic chemistry

GROUP VII: THE HALOGENS 317

hydrogen fluoride periodically. The fluorine obtained is almost pure,

containing only a little hydrogen fluoride, which is removed by

passage of the gas over sodium fluoride :

NaF -f HF -> NaHF 2

Fluorine boils at 85 K to give a greenish-yellow diatomic gas.

CHLORINE

The most common compound of chlorine is sodium chloride, NaCl,
and this occurs widely in nature. Large deposits are found in
Cheshire and these are extracted by the use of water although some
is mined as rock salt. In many parts of the world sodium chloride
is obtained from sea water. Other chlorides are found in small
quantities both in rocks and sea water, for example carnallite
KC1. MgCl 2. 6H 2 O in the Stassfurt deposits. Chlorine, unlike
fluorine, can be prepared by chemical oxidation of the chloride ion
and this is the method usually used in the laboratory. Strong
oxidising agents are required for the oxidation and amongst those
commonly used are manganese(IV) oxide, MnO 2 , potassium
dichromate(VI), K 2 Cr 2 O 7 , both of which need to be heated with
concentrated hydrochloric acid, and potassium manganate(VII),
KMnO 4 , which evolves chlorine at room temperature when treated
with concentrated hydrochloric acid :

MnO 2 + 4HC1 -> MnCl 2 -f C1 2 + 2H 2 O

14H+ + Cr 2 O?~ + 6C1" -> 2Cr3+ + 7H 2 O + 3C1 2

16H+ + 2MnO + 10C1" -> 2Mn2+ + 8HO + 5C1

Alternatively a mixture of almost any solid chloride and manganese-
(IV) oxide will yield chlorine when warmed with concentrated
sulphuric acid. These are the most common laboratory methods but
there are many others.
On a large scale chlorine is obtained in several ways.

1. By the electrolysis of concentrated sodium chloride solution;
   this process was initially used primarily for the production of
   sodium hydroxide but the demand for chlorine is now so great that
   the chlorine is a primary and not a by-product.


2. By the electrolysis of fused magnesium chloride or fused
   sodium chloride.


3. By the oxidation of hydrogen chloride. A mixture of hydrogen
   chloride with air or oxygen is passed over a catalyst of copper(II)