Page 123 - Basic physical chemistry for the atmospheric sciences

Oxidation-reduction reactions 1 0 9

balance the hydrogen atoms. 02 and H2 are not used to bal­
ance the oxygen and hydrogen atoms unless they are known
s
to participate in the reaction. For basic solution , OH - (aq)
can be used to balance the half-reactio s . For each excess
n
oxygen atom on one side of the equation, balance is achieved
by adding one H20 to the other side . Hydrogen balance is
achieved by adding one OH - for each excess hydrogen atom
on the same side of the equation as the excess exists, and one
H 0 on the other side of the equation. If both oxygen and
2
hydrogen atoms are i n excess on the same side of the equa­
tion , add an OH - on the other side of the equation for each
pair of oxygen and hydrogen atoms that are in excess. ]
Step 4. For each of the half-reaction , balance the electric charge by
s
adding electrons to the right side of the oxidation half-reaction
and to the left side of the reduction half-reaction.
Step 5. Multiply each half-reaction by a number that makes the total
number of electrons lost by the reductant equal to the number
of electrons gained by the oxidant.
Step 6. Add the two half-reactions to get the overall reaction. (Cancel
any terms that are identical on both sides of the reaction; all
electrons should cancel . )
Step 7 . Check the overall reaction for conservation o f the atoms of
each element and net charge.

The following two exercises l lustrate the application of this method,
i
first to an acidic reaction and then to a basic reaction.
Exercise 6.2. Balance the equation for the following redox reaction
in an aqueous solution

Solution.
Step I . The unbalanced equation is given above, which, for an aque­
ous solution, may also be written

+
H ( aq) + N0 3(aq) + H S(aq)- NO(g) + S(s) + H20(1) (6.7)
2
i
u
Step 2 . The oxidant is NO ] (aq), because t conta n s N , which n der­
i
goes a decrease in oxidation number [from + 5 on the right side of
Reaction (6.7) to + 2 on the left side]. Therefore, the unbalanced
reduction half-reaction is
N0 3 (aq)- NO(g) (6.7a)