A half-equation isolates one oxidation or reduction process and includes electrons explicitly. Balance atoms and then charge. For a metal forming a cation, electrons appear on the product side; for a cation forming metal, they appear on the reactant side. To combine half-equations, multiply them so electrons lost and gained are equal, add them and cancel matching terms.
In acidic aqueous conditions, first balance atoms other than H and O. Balance O with H2O,H with H+, and charge with electrons. Recheck both atom counts and total charge. For example MnO4−+8H++5e−→Mn2++4H2O is balanced in acid. H+ is supplied by the stated acidic medium; it must not be introduced casually into an explicitly neutral final equation.
Example 1
Combine Al →Al3++3e− and Cu2++2e−→ Cu.
Use six electrons: 2Al →2Al3++6e− and 3Cu2++6e−→3Cu. Overall: 2Al+3Cu2+→2Al3++3Cu.
Example 2
Balance the reduction of H2O2 to water in acid.
H2O2+2H++2e−→2H2O. Both sides have H4O2 and total charge zero.
Watch out: An overall reaction must balance electrons lost and gained; electrons cancel on addition.