In redox half equations, hydrogen and oxygen may be balanced using major species that are available in aqueous solution. The major species available depends on the pH of the solution.
In acidic aqueous solution H+ and H2O are major species.
In alkaline aqueous solution OH– and H2O are major species.
To balance zO (z = any number)
in acidic solution
add z H
2O to the other side of the equation
H2O is the more oxygen-rich (higher mass %O) of the major species available in acidic aqueous solution.
in alkaline solution
add z OH
– to the other side of the equation
OH– is the more oxygen-rich (higher mass %O) of the major species available in alkaline aqueous solution.
To balance zH
in acidic aqueous solution,
add zH
+ to the opposite side of the equation
H+ is the more hydrogen-rich of the major species available in acidic aqueous solution.
in alkaline solution,
add zH
2O to the side of the equation deficient in H
then add zOH
– to opposite side of the equation.
H2O is the more hydrogen-rich of the major species available in alkaline aqueous solution.
Why do we add OH
– to balance both oxygen and hydrogen in alkaline solution?
It is inappropriate to add H+ to balance H because H+ is NOT a major species.
Balancing zH using zH2O (zH-OH) upsets oxygen balance.
Oxygen balance is restored if OH– is added to the other side of the equation.
Example of balancing a half equation in alkaline solution:
Balance atoms other than O and H.
Add OH– to balance O. Now there are 3 extra H at left.
Add 3H2O to balance 3H and then 3OH– to other side.
Add electrons to balance charge.
Fe2O3
Fe2O3 + OH–
Fe2O3 + OH– + 3H2O
Fe2O3 + 3H2O + 2e–
2Fe(OH)2→
2Fe(OH)2 →
3OH– + 2Fe(OH)2 →
2OH– + 2Fe(OH)2 →
Note that this balancing process has been done in steps to show you how the equation builds. In practice it would be all done on a single line.
OH
– appears on both sides of this half equation. OH
– will appear on only one side of the equation when this half equation is combined with a second half equation in the equation for the overall reaction.