Equation (2) is a balanced equation, but equation (3) is not. As it stands, the left side of the equation has more oxygen atoms than the right side. Since the overall reaction occurs in aqueous acidic solution, water and hydrogen ions are also present in the solution and may be added to the equation to balance it. To account for the two extra oxygen atoms, two molecules of water are added to the right side of equation (3).
Four hydrogen ions then need to be added to the left side to even the number of hydrogen atoms.
Equation (5) is the final form of the half reaction.
