Redox reactions that occur in alkaline solution MUST NOT have H+(aq) as a reactant or product.
The oxygen and hydrogen for a reaction in aqueous solution in alkaline aqueous solution is balanced by appropriate numbers of OH– and H2O.
One way to get around this difficulty is to use the reactants and products of the alkaline reaction
The oxygen and hydrogen for a reaction in aqueous solution in alkaline aqueous solution is balanced by appropriate numbers of OH– and H2O.
The balancing of such half equations can be complicated by the fact that both OH– and H2O contain both hydrogen and oxygen.
One way to get around this difficulty is to use the reactants and products of the alkaline reaction
balance the equation with H+ and H2O as for acidic solution THEN
add sufficient OH– ions to both sides of the final equation to neutralise the H+ THEN
combine H+ and OH– on the same side of the equation to give H2O
adjust the original coefficient on the H2O appropriately.
add sufficient OH– ions to both sides of the final equation to neutralise the H+ THEN
combine H+ and OH– on the same side of the equation to give H2O
adjust the original coefficient on the H2O appropriately.
| Fe(OH)2 |  | Fe2O3 | Balance atoms other than O and H. |
| 2Fe(OH)2 | | Fe2O3 | Balance oxygen using H2O. |
| 2Fe(OH)2 | | Fe2O3 + H2O | Balance hydrogen using H+ |
| 2Fe(OH)2 | | Fe2O3 + H2O + 2H+ | Balance electrons. |
| 2Fe(OH)2 | | Fe2O3 + H2O + 2H+ + 2e– | Add same number of OH– as H+ to both sides of the equation |
| 2Fe(OH)2 + 2OH– | | Fe2O3 + H2O + 2H+ + 2e– + 2OH– | Adjust numbers of H2O |
| 2Fe(OH)2 + 2OH– | | Fe2O3 + 3H2O + 2e– |