Modern inorganic chemistry

92 ACIDS AND BASES: OXIDATION AND REDUCTION

reducing agent, being itself oxidised by accepting oxygen. Here we

see immediately that both processes, oxidation and reduction, must

occur simultaneously.

Reduction was then defined as the removal of oxygen or the

addition of hydrogen, whilst oxidation was the addition of oxygen

or the removal of hydrogen.

These definitions are still valuable, especially in organic chemistry ;

for inorganic reactions they require extension. It was soon recognised

that substances other than oxygen can behave as oxidising agents.

The conversion of aqueous sulphur dioxide solution to sulphuric

acid, for example, can be accomplished using mercury(II) oxide or

chlorine water, the equations being most simply represented as

1. SO 2 + H 2 O 4- HgO -> H 2 SO 4 + Hg


2. SO 2 + 2H 2 O -f C1 2 ~» H 2 SO 4 + 2HC1
   and the oxidation being from sulphur(IV) to sulphur(VI). It follows
   that the reaction simply represented as

2FeCl 2 + C1 2 -> 2FeCl 3

can be described as an oxidation of iron(II) to iron(III). Since like
many other inorganic compounds the iron(II) and iron(III) chlorides
form ions in solution, this oxidation could be represented by the
ionic equation

Thus an oxidising agent is identified as an electron acceptor and the
oxidation of iron(II) by chlorine can be written as two 'half equa-
tions, viz.

1. 2Fe2+(aq) -» 2Fe^3 + (aq) -f- 2e

2 O^^^CT2Fe2+(aq) + C1
2 -> 2Fe^3 + (aq) + 2CT
Reduction can now be defined as a process in which electrons are
acquired and oxidation a process in which electrons are released.
These definitions are often difficult to remember and the following
simplification may be helpful:

reduction

Thus, the reducing agent causes reduction to take place, i.e. causes
a reduction in the positive charge; it must therefore supply electrons.
It follows immediately that the oxidising agent must accept electrons.