Page 122 - Basic physical chemistry for the atmospheric sciences

108 Basic physical chemistry
for N ; for H S(g): + 1 for H and therefore - 2 for S. For the atoms on
2
the right side of the reaction the oxidation numbers are for NO(g): - 2
for oxygen and therefore + 2 for N ; for S the oxidation number is O ;
and fo r H20(1): + 1 fo r H and - 2 fo r oxygen. Hence, the reaction
decreases the oxidation number for nitrogen (from + 5 to + 2) ; there­
fore nitrogen is reduced . The oxidation number of sulfur, on the other
hand, is increased (from - 2 to O); therefore sulfur is oxidized . The
oxidation numbers of the hydrogen and oxygen remain unchanged .
(b) The oxidation numbers of the atoms on the left side of the
reaction are + 2 for Cu, + 1 for H , and - 2 for oxygen. On the right
side of the reaction they are 0 for Cu, 0 for oxygen, and + 1 for H .
Hence, the reaction decreases the oxidation number of u (from + 2
C
to 0) ; therefore copper is reduced . The oxidation number of oxygen is
increased (from - 2 to O) ; therefore oxygen s oxidized.
i

6.3 Balancing oxidation-reduction reactions
A balanced chemical equation must have the same number and types
of atoms on both sides of the equation , and the sum of the electric
charges must be the same for the reactants as for the products of the
reaction. If all the reactants and products are known, the equation for
a redox reaction may be balanced by the half-reaction method. (An­
other method, called the oxidation-number method, may also be used ,
but for our purposes knowledge of one method is sufficient .)
The half-reaction method involves application of the following se­
p
quential ste s .
t
Step 1 . Write down h e overall unbalanced equation for the reaction.
2
Step . Write down the unbalanced equations for the oxidation half­
reaction and for the reduction half-reaction (species should
not be written as free atoms or ions unless they exist in
these forms).
s
Step 3. For each of the half-reaction , first balance the atoms that
undergo oxidation and reduction. Then balance atoms other
than oxygen and hydrogen. Finally, balance the oxygen and
s
hydrogen atom . [In neutral or acidic solutions , H20 and
H + (aq) may be added to balance the oxygen and hydrogen
3
atoms. The oxygen atoms are balanced first . For each excess
oxygen atom on one side of the equation, balance is achieved
by adding one H 20 to the other side. Then H + (aq) is used to