Will the reaction run?
Reduction potentials are tabulated against the standard hydrogen electrode, assigned 0 V. The more positive reduction potential is favoured for reduction when the two half-cells are connected under standard conditions.
E°cell = E°cathode − E°anode using reduction potentials for both. Do not multiply electrode potentials when multiplying a half-equation: voltage is not proportional to the amount of substance.
A positive E°cell predicts thermodynamic feasibility in the written direction under the stated standard conditions, commonly 298 K, 1 M solutions and 100 kPa gases. It does not guarantee a fast observable reaction or the same voltage under different conditions.
Follow the electrons
Metal/Cu galvanic cell under standard conditions. Ion and electron movement is schematic. The salt bridge carries ions; the external wire carries electrons.
Explore: Compare half-cells and use their standard potentials to predict the spontaneous direction.
E°(Cu²⁺/Cu) = +0.34 V and E°(Zn²⁺/Zn) = −0.76 V. Calculate E° for the spontaneous cell.
- E°cell = 0.34 − (−0.76).
- E°cell = +1.10 V.
Answer: 1.1 V
Assumed knowledge
Subtract signed numbers; values below are standard reduction potentials.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Calculate E°cell from reduction potentials.
- Predict spontaneous direction under standard conditions.
- Recognise the limits of standard predictions.