Modern inorganic chemistry

GROUP VII: THE HALOGENS 325

OTHER DISPLACEMENT AND OXIDATION REACTIONS

Many of the reactions of halogens can be considered as either
oxidation or displacement reactions; the redox potentials (Table
11.2) give a clear indication of their relative oxidising power in
aqueous solution. Fluorine, chlorine and bromine have the ability
to displace hydrogen from hydrocarbons, but in addition each
halogen is able to displace other elements which are less electro-
negative than itself. Thus fluorine can displace all the other halogens
from both ionic and covalent compounds, for example

2NaCl 4- F 2 ~» 2NaF + C1 2

2 --7— Cl + F -> 2 ~C— F + C1 2

and oxygen from water and silica :

SiO 2 4- 2F 2 -» SiF 4 + O 2

The reaction with silica explains why fluorine reacts with glass and
quartz, but if these are rigorously freed from adsorbed water, the
reaction is very slow ; hence dry fluorine can be manipulated in dry
glass apparatus but all glass taps must be lubricated with fluoro-
earbon grease since hydrocarbon greases would be attacked. The
very strong oxidising properties of fluorine in aqueous systems are
seen in reactions such as the conversion of chlorate(V) to chlorate-
(VII), chromium(III) to dichromate(VI) and the oxidation of the
hydrogensulphate ion, HSO^, to peroxodisulphate :

2HSO4 + F 2 -> S 2 Oi~ + 2HF

Also, in anhydrous conditions, silver reacts with fluorine and forms
silver difluoride AgF 2 and cobalt gives cobalt(III) fluoride, CoF 3 ,
these metals showing higher oxidation states than is usual in their
simple salts.
Chlorine has a lower electrode potential and electronegativity
than fluorine but will displace bromine and iodine from aqueous
solutions of bromide and iodide ions respectively :

C1 2 + 2Br~ -> 2Cr -f Br 2

Chlorine reacts directly with carbon monoxide to give carbonyl
chloride (phosgene) :

CO + C1 2 -* COC1 2

and sulphur dioxide to give sulphur dichloride dioxide:

SO 2 + Cl' 2 -* SO 2 C1 2