9.10 Electrolysis and Faraday's Law

Electrolysis basics

Electrolysis uses an external power source (electricity) to force a nonspontaneous redox reaction to occur, in contrast to a galvanic cell which produces electricity spontaneously. In an electrolytic cell, the power source drives electrons so that oxidation occurs at the anode and reduction occurs at the cathode — same electrode definitions as galvanic cells, but here the anode is + and cathode is −.

Faraday's Law: connecting charge to moles

Faraday's Law relates the amount of substance produced/consumed to the electric charge passed: Q = It, and moles of electrons = Q / F, where F = 96,485 C/mol e⁻. Using the balanced half-reaction's stoichiometry, you can convert moles of electrons to moles of metal deposited or gas produced, then to mass or volume.

Quantitative electrolysis problems

Typical problems give current (I) and time (t) to find charge (Q = It), then moles of e⁻ (Q/F), then moles of product using the half-reaction coefficients, and finally mass (using molar mass) or volume (using ideal gas law, PV = nRT). Always check the number of electrons transferred per ion in the half-reaction — it's the key stoichiometric link.

Key terms

Electrolysis

A process that uses an external electrical power source to drive a nonspontaneous redox reaction.

Electrolytic cell electrode signs

Anode is positive (+) and cathode is negative (−); oxidation still occurs at the anode, reduction at the cathode.

Faraday's constant (F)

96,485 C/mol e⁻ — the charge of one mole of electrons.

Q = It

Charge (C) equals current (A) multiplied by time (s); used to find total charge passed in electrolysis.

Steps to find mass deposited in electrolysis

1) Q = It 2) mol e⁻ = Q/F 3) mol substance (via half-reaction stoichiometry) 4) mass = mol × molar mass.

Try an AP-style question

A current of 2.00 A flows for 965 s. How many moles of electrons pass?

1. 0.00200 mol
2. 0.0100 mol
3. 1,930 mol
4. 0.0200 mol