How can I balance galvanic cell equations?

Here are the steps to balance a galvanic cell equation.

EXAMPLE

Write the balanced equation for a galvanic cell that uses the reduction half-reactions
O₂ + 4H⁺ + 4e⁻ → 2H₂O
Ni²⁺ + 2e⁻ → Ni

Solution

1: Reverse one of the half-reactions so that the cell voltage is positive. We reverse the nickel half-reaction. The cell half-reactions are
O₂ + 4H⁺ + 4e⁻ → 2H₂O
Ni → Ni²⁺ + 2e⁻

From here on, the process is the same as for any redox reaction. See

http://socratic.org/questions/how-can-i-balance-redox-chemical-equations-step-by-step?source=search

2: Equalize electrons transferred.
1 ×[O₂ + 4H⁺ + 4e⁻ → 2H₂O]
2 × [Ni → Ni²⁺ + 2e⁻]

3: Add the two half-reactions.
1 O₂ +2 Ni + 4 H⁺ → 2 Ni²⁺ + 2 H₂O

4: Check mass balance.
On the left: 2 O; 2 Ni; 4 H
On the right: 2 Ni; 4 H; 2 O

5: Check charge balance.
On the left: 4+
On the right: 4+

The balanced equation is
O₂ +2Ni + 4H⁺ → 2Ni²⁺ + 2H₂O