When you multiply a half-equation to balance electrons, keep its standard electrode potential unchanged. To calculate the standard cell potential from two tabulated reduction potentials, subtract the anode reduction potential from the cathode reduction potential.
The 2027 H2 Chemistry 9476 syllabus covers standard electrode and cell potentials. This worked example shows how a half-equation with a coefficient of two can still use the original value in volts.
- Write both values as reduction potentials.
- Identify the reduction and oxidation for the proposed cell reaction.
- Balance the electrons in the chemical equation.
- Subtract the original reduction potentials.
CHAPTER 1 OF 4 · Back to contents
Keep the two calculations separate
Use the standard reduction potentials +0.77 V for Fe³⁺ + e⁻ → Fe²⁺ and +0.34 V for Cu²⁺ + 2e⁻ → Cu. For the spontaneous direction under standard conditions, Fe³⁺ is reduced and copper metal is oxidised.
CHAPTER 2 OF 4 · Back to contents
Worked example | Two iron ions for each copper atom
Oxidising one Cu atom releases two electrons. Each Fe³⁺ ion takes one electron, so double the iron half-equation before adding it to the copper oxidation equation. The net reaction is 2Fe³⁺(aq) + Cu(s) → 2Fe²⁺(aq) + Cu²⁺(aq).
Calculate E°cell using +0.77 V − (+0.34 V) = +0.43 V. Doubling the iron equation doubles the amount reacting and the charge transferred. It does not double the potential difference. You can also add +0.77 V to the oxidation potential of −0.34 V.
Contents · Previous chapter · Next chapter
CHAPTER 3 OF 4 · Back to contents
Reverse the proposed reaction
If the question asks about the reverse net reaction, its standard cell potential is −0.43 V. A negative value indicates that the reverse direction is not thermodynamically spontaneous under standard conditions. The voltage does not give the reaction rate.
Contents · Previous chapter · Next chapter
CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Why does doubling the reaction leave the voltage unchanged?
Potential difference is energy transferred per unit charge. Doubling the reaction doubles both the associated energy change and the charge, so the ratio stays the same.
Can I subtract the oxidation potential from the reduction potential?
Subtract the tabulated reduction potential at the anode. If you have already reversed its sign to get an oxidation potential, add it instead.