The central idea
Identify oxidation and reduction and combine them into a balanced overall reaction. The number n in the Nernst equation is the electrons transferred by that balanced reaction. Pure solids and pure liquids have activity one and are omitted from the usual reaction quotient.
A worked example
For Zn + Cu²⁺ → Zn²⁺ + Cu, n = 2 and Q = [Zn²⁺]/[Cu²⁺]. At 298 K, E = E° − (0.0592/2) log Q. If the concentration ratio is 100, the cell potential is 0.0592 V below its standard value.
A common mistake
Do not multiply an electrode potential by the coefficient used to balance electrons. Potential is an intensive quantity.
Try the idea
These original practice questions also appear in the foundation mock. Attempt them before opening the explanation.
For Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), E°cell = 1.10 V. At 298 K, [Zn²⁺] = 1.0 M and [Cu²⁺] = 0.010 M. Use 0.0592 V for 2.303RT/F. Ecell is approximately:
Answer: B
Two electrons are transferred and Q = [Zn²⁺]/[Cu²⁺] = 100. E = E° − (0.0592/2)log Q = 1.10 − 0.0592 = 1.0408 V. Pure solids do not enter Q.
How much copper is deposited when 0.100 mol electrons pass through CuSO₄ solution with 100% current efficiency? Use Cu = 63.5.
Answer: A
The cathode reaction is Cu²⁺ + 2e⁻ → Cu. Thus 0.100 mol electrons deposit 0.0500 mol Cu. Its mass is 0.0500 × 63.5 = 3.175 g.
For a galvanic reaction with n = 2 and E°cell = 0.500 V, ΔG° is approximately: F = 96485 C mol⁻¹.
Answer: A
Use ΔG° = −nFE°. This gives −2 × 96485 × 0.500 = −96485 J mol⁻¹, or about −96.5 kJ mol⁻¹. A positive standard cell potential corresponds to a negative standard Gibbs energy change.
For an operating galvanic cell, which statements are correct?
Answer: A, B, C
Oxidation releases electrons at the anode. They travel through the external conductor to the cathode, where reduction consumes them. A salt bridge conducts ions and helps maintain electrical neutrality; electrons do not travel through it as the primary charge carriers.
What to do next
Explain your answer aloud, then find a related question in an official paper. If you can only follow a worked solution but cannot start a fresh question, revisit the central idea and try again without the notes.
Apply it to an official paper