Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 95

can be represented as

2Fe2+(aq) + Br 2 (aq) -» 2Fe^3 +(aq) + 2Br-(aq)

When M is a voltmeter an indication of the energy difference

between the reactants and products is obtained (see below). A

current passes when M is an ammeter, and if a little potassium

thiocyanate is added to the Fe^2 + (aq) a red colour is produced

around the electrode, indicating the formation of iron(III) ions in

solution; the typical bromine colour is slowly discharged as it is

converted to colourless bromide Br~.

A series of experiments can be performed and an order of re-

ducing power established.

STOICHIOMETRY OF REDOX REACTIONS

Since electrical neutrality must be maintained in a redox reaction,
the total number of electrons lost by the reducing agent must equal
the total number of electrons gained by the oxidising agent. For
example, if each atom of the reducing agent gives three electrons,
and each atom of the oxidising agent accepts two electrons, i.e.

(i) A-> A3+ + 3 e~

(ii) B-h2e--»B^2 ~

then the stoichiometry is (i) x 2, and (ii) x 3 so that electrical
neutrality is maintained, i.e. 2A + 3B -> 2A3+ + 3B^2 ~.
We have discussed the simple ionic reaction

2Fe^2 + (aq) 4- Br 2 (aq) -» 2Fe^3 + (aq) + 2Br"(aq)

but when complex ions are involved the use of oxidation states
proves useful. The oxidation state for a simple ion is the charge on
the ion; for the central atom of a complex ion it is the charge the
element in question would have if it was a simple ion, i.e. not co-
ordinated or bonded to other species. Oxidation states can be
deduced from the following assumptions:

Element Oxidation state

Alkali metals, Group I +1

Alkaline earth metals, Group II + 2

Oxygen — 2 (except in peroxides)

Hydrogen + 1 (except in metal hydrides)

Uncombined elements are all given zero oxidation state. Consider
(a) manganese in the permanganate ion, MnO 4 ; there are four