IB Chemistry Electrolysis: Everything Students and Parents Ask

Electrolysis uses electrical energy to force a non-spontaneous redox reaction, splitting molten or aqueous compounds into their elements at the anode and cathode. It sits in Reactivity 3.4: Electron transfer reactions, tested as short Paper 1 items on both SL and HL papers, plus longer Paper 2 calculations — Faraday's law included — that only HL students sit.

A voltaic cell generates electrical energy from a spontaneous redox reaction; an electrolytic cell consumes electrical energy to drive a non-spontaneous one. Oxidation always happens at the anode in both — but the anode is negative in a voltaic cell and positive in an electrolytic cell, a swap examiners test constantly.

Predicting electrolysis products means listing every ion present, checking standard electrode potentials to see which species is easiest to oxidise or reduce, then writing half-equations. In concentrated aqueous solutions, though, concentration can override electrode potential — concentrated NaCl produces chlorine gas at the anode instead of oxygen, for instance.

Electrolysis of a molten salt only involves the compound's own ions, so the metal deposits at the cathode and the non-metal is released at the anode with no competition. In aqueous solution, water molecules can also be oxidised or reduced, so you must compare electrode potentials before naming the actual products.

Faraday's law links charge passed to amount of substance produced: moles of electrons = (current × time) ÷ 96,500 C mol⁻¹, the Faraday constant. Divide by the number of electrons in the half-equation to find moles of product, then multiply by molar mass for the mass deposited — this calculation is HL only.

Both — the core idea of electrolytic cells, discharge order and half-equations is common to SL and HL under Reactivity 3.4: Electron transfer reactions. What's HL-only is the quantitative side: Faraday's law calculations and using standard electrode potential values to justify which ion is preferentially discharged in aqueous solution.

SL students write electrode half-equations and identify products qualitatively using electrode potentials and a simple discharge-order rule. HL students go further, quantifying the process with Faraday's law — linking current, time and mass — content that appears on HL Paper 2 and can resurface in Paper 3 data-based questions.

Electrolysis trips students up because it demands three skills at once: solid redox logic, correct half-equation writing, and — at HL — a Faraday's law calculation. The most common failure I see in mock scripts is confusing electrode polarity the moment the cell type flips from voltaic to electrolytic.

The mistake I mark down most is forgetting that anode charge flips between cell types: negative in a voltaic cell, positive in an electrolytic one — though oxidation always happens there. Get that reversed and every half-equation, and the direction of ion migration, follows incorrectly.

Start with half-equations: practise writing them for at least five different molten and aqueous electrolytes until the discharge order is automatic. Then move to Faraday's law calculations if you're HL, working backwards from mass to current or time as often as forwards, since examiners like reversing the question.

The most frequently tested scenarios are electroplating and purification of copper, extraction of aluminium from molten Al₂O₃ (the Hall-Héroult process), and aqueous electrolysis of brine producing chlorine, hydrogen and sodium hydroxide. HL papers add Faraday's law calculations layered onto any one of these industrial contexts.

IB Chemistry treats electrolysis with more quantitative depth than most A-Level specifications, particularly at HL where Faraday's law calculations are compulsory rather than an optional extension. A-Level students often meet electrolysis once in Year 12; IB HL students revisit it across Reactivity 3.4 and again in practical coursework.

Look for resources that pair short concept notes with plenty of half-equation practice and mixed calculation questions, since electrolysis rewards repetition more than memorisation. On revisionprep.com, the IB Chemistry Revision Notes cover Reactivity 3.4 step by step, and the Topical Worksheets give graded electrolysis calculation practice before mocks.