Page 119 - Basic physical chemistry for the atmospheric sciences

Oxidation-reduction reactions 1 0 5

oxidant is reduced, that is, it gains electrons). Similarly, if a substance
A causes the reduction of substance B , substance A is called the
reducing agent or reductant. In Eq. (6. 3 ) Cu(s) is the reductant, be­
cause it causes Ag + (aq) to gain electrons (note that the reductant i s
i
oxidized, that s , it loses electrons). 1

6.2 Oxidation numbers
In order to deal with oxidation-reduction reactions that are more
complex than the simple ones discussed so far, we must introduce the
concept of oxidation numbers (sometimes called oxidation states or
valence states). Oxidation numbers permit us to identify and balance
redox reactions and to determine the oxidant and reductant.
Consider the redox reaction in solution of the ferric ion Fe 3 + (aq)
and hydrogen sulfite HS0 3 (aq)
3
2Fe + ( aq) + H S 0 3 (aq) + H20(1)-
2Fe 2 + (aq) + HSO 4 (aq) + 2H + (aq) (6.4)

The half-reactions are
HS0 3 ( aq) + H20(1)- HS0 4 ( aq) + 2H + ( aq) + 2e - (6.5)

and,
2
2Fe3 + (aq) + 2e - - 2Fe + (aq) (6.6)
In this case, it is d i fficult to say exactly where the two electrons on the
right side of Reaction (6.5) come fro m . Therefore, we need a method
for keeping track of the electrons in reactions such as th s . This can be
i
done by making some assumptions. It is assumed that the h y drogen
atom in HS0 3 (aq) carries one unit of positive charge, and that each
oxygen atom carries two units of negative charge. Since the total
charge on HS0 3 (aq) is one unit of negative charge, the charge on
the sulfur atom is four units of positive charge. [Charge on sulfur
atom = molecular charge - charge on one hydrogen atom - charge
4
- 1 - 1 - 3( - 2) = + . ] This fictitious
on three oxygen atoms =
charge is called the oxidation number of sulfur in HS0 3 (aq).
Following the same procedure, the oxidation number of sulfur in
HS0 4 ( aq) is: - 1 - 1 - 4 ( - 2 ) = + 6 . Thus, according to this method
of bookkeeping, the two electrons on the right side of Reaction (6. 5 )
originate from the sulfur atom, which changes its oxidation number