EXAMPLE #1 Balancing Redox
Reactions Using the
Oxidation Number Method
Balance the following redox equation using either the
inspection technique or the oxidation number method. Be sure to
check that the atoms and the charge are balanced.
HNO3(aq) + H3AsO3(aq)
--> NO(g) + H3AsO4(aq) + H2O(l)
Step #1: Try to balance the atoms
The H and O atoms are difficult to balance in this
equation. You might arrive at the correct balanced equation using a trial and
error technique, but if you do not discover the correct coefficients fairly
quickly, proceed to Step #3.
Step #3: Is the reaction redox?
The N atoms change from +5 to +2, so they are reduced. This
information is enough to tell us that the reaction is redox. (The As atoms,
which change from +3 to +5, are oxidized.)
Step #4: Determine the net increase in oxidation number for the element that is
oxidized and the net decrease in oxidation number for the element that is
+3 to +5 Net Change = +2
N +5 to +2 Net Change = -3
Step #5: Determine
a ratio of oxidized to reduced atoms that would yield a net increase in
oxidation number equal to the net decrease in oxidation number.
As atoms would yield a net increase in oxidation
number of +6. (Six electrons would be lost by three arsenic atoms.) 2 N atoms
would yield a net decrease of -6. (Two nitrogen atoms would gain six electrons.)
Thus the ratio of As atoms to N atoms is 3:2.
Step #6: To get the ratio
identified in Step 5, add coefficients to the formulas which contain the
elements whose oxidation number is changing.
--> NO(g) +
Step #7: Balance
the rest of the equation by inspection.
2HNO3(aq) + 3H3AsO3(aq)
2NO(g) + 3H3AsO4(aq) + H2O(l)
here to see an exercise that will allow you to try this task yourself.
Return to the Balancing Redox