Modern inorganic chemistry

GROUP VII: THE HALOGENS 331

same reaction, which is usually carried a stage further by stronger

heating, i.e.

NaCl + NaHSO 4 -> Na 2 SO 4 + HClt

Anhydrous hydrogen chloride is not particularly reactive, either as

a gas at ordinary temperatures, or a liquid (b.p. 188 K) and does not

react with metals such as iron or zinc, nor with dry oxides. A few

reactive metals such as sodium, will burn in the gas to give the

chloride and hydrogen :

2Na + 2HC1 -> 2NaCl + H 2

However, if heated hydrogen chloride is passed over heated

metals, the chloride is formed ; in the case of a metal exhibiting

variable oxidation state, the lower chloride is obtained :

Sn + 2HC1 -> H 2 + SnCl 2

Fe + 2HC1 -> H 4- 2 - FeCl 2

Aqueous hydrochloric acid

In aqueous solution, hydrogen chloride forms hydrochloric acid.
The concentrated acid contains about 40% hydrogen chloride
(about 12 M). A graph of the boiling point of hydrogen chloride-
water mixtures against composition shows a maximum at about
20 % HC1; hence if either the concentrated or dilute acids be distilled,
then either hydrogen chloride or water respectively distil over,
leaving behind "constant boiling-point' acid.
Hydrochloric acid is a strong monobasic acid, dissolving metals
to form salt and evolving hydrogen. The reaction may be slow if the
chloride formed is insoluble (for example lead and silver are attacked
very slowly). The rate of attack on a metal also depends on concen-
tration ; thus aluminium is attacked most rapidly by 9 M hydro-
chloric acid, while with other metals such as zinc or iron, more
dilute acid is best.
Electrolysis of hydrochloric acid yields hydrogen at the cathode
and oxygen at the anode from the dilute acid, but chlorine at the
anode (of carbon) from the concentrated acid. Electrolysis of the
concentrated acid is used on the large scale to recover chlorine.
If tetramethylammonium chloride is dissolved in hydrochloric
acid, the unstable salt [(CH 3 ) 4 N] [HC1 2 ], can be crystallised out;
here chlorine is showing weak hydrogen bonding (cf. F----H—F~
which is stable, and C1--H—Cl~ which is unstable).