Oxidation–reduction and electrochemistry
Electrochemistry questions test oxidation-number assignment, balancing in acidic or basic medium, cell notation and EMF, the Nernst equation, and quantitative electrolysis. You need both the electron bookkeeping and the thermodynamic link ΔG° = −nFE°.
Oxidation numbers and balancing
Element = 0; monatomic ion = its charge; O usually −2 (peroxide −1); H usually +1 (hydride −1); the sum equals the overall charge. Balance by the ion–electron method: split into half-reactions, balance atoms other than O and H, balance O with H₂O, balance H with H⁺, balance charge with electrons, equalise electrons, then add and cancel. In basic medium, add OH⁻ to both sides to neutralise H⁺.
Galvanic cells
Anode is oxidation and negative in a galvanic cell; cathode is reduction and positive. E°cell = E°cathode − E°anode, using reduction potentials for both. A positive E°cell means spontaneity and ΔG° < 0. Notation: anode | anolyte || catholyte | cathode. The salt bridge preserves electroneutrality; the standard hydrogen electrode is the 0.00 V reference.
Nernst equation
E = E° − (0.0592/n)·log Q at 25 °C. EMF falls as products accumulate; at equilibrium E = 0 and Q = K, so log K = nE°/0.0592. A concentration cell has E° = 0 but a non-zero E driven by the concentration difference.
Electrolysis and Faraday's laws
An electrolytic cell is driven by an external supply, but the cathode is still where reduction occurs. Q = I·t, moles of electrons = Q/F with F = 96 485 C/mol, and mass deposited = (M/n)(Q/F). Competing half-reactions are decided by comparing potentials, with overpotential sometimes favouring water decomposition.
Worked example
For Zn(s) | Zn²⁺(1 M) || Cu²⁺(1 M) | Cu(s), E° = 0.34 − (−0.76) = 1.10 V. ΔG° = −nFE° = −2 × 96 485 × 1.10 = −2.12 × 10⁵ J/mol, so the reaction is spontaneous. If [Cu²⁺] falls to 0.010 M with [Zn²⁺] = 1 M, E = 1.10 − (0.0592/2)·log(1/0.010) = 1.10 − 0.059 = 1.04 V.
Traps
- Multiplying E° by the stoichiometric coefficient; electrode potential is intensive.
- Mixing up anode sign conventions between galvanic and electrolytic cells.
- Adding half-reactions without equalising the electrons first.
- Using mass instead of moles of electrons in Faraday calculations.