MYP Chemistry Electrolysis: FAQs on Concepts, Exam Answers and Revision

State the electrolyte, identify which ions are present, then work out which ion is discharged at each electrode using the reactivity/activity series and concentration. Always name the product, write a half-equation if asked, and link the answer back to electron transfer — that's what MYP examiners actually reward.

Electrolysis is the decomposition of an ionic compound using electrical energy, splitting it into its elements at two electrodes. It only works when ions are free to move — so the compound must be molten or dissolved in water. No movement, no current, no reaction.

An electrolyte is the substance being broken down — a molten or aqueous ionic compound that conducts electricity because its ions can move. An electrode is the solid conductor (often carbon or platinum) dipped into the electrolyte to carry current in and out of the circuit.

The cathode is connected to the negative terminal of the power supply, so it carries a surplus of electrons. Positive ions (cations) are attracted to that negative electrode, gain electrons there, and are reduced — which is exactly why the cathode is where reduction happens.

Electrolysing acidified or salted water produces hydrogen gas at the cathode and oxygen gas at the anode, in a 2:1 volume ratio. You confirm hydrogen with a lit splint (a squeaky pop) and oxygen with a glowing splint (it relights).

Molten lead bromide gives only two possible ions, so you get liquid lead at the cathode and orange bromine gas at the anode. Aqueous copper sulfate has water present too, so you get copper metal deposited at the cathode but oxygen gas — not sulfate — at the anode.

Expect "identify" (name the product or ion), "describe" (state what you'd observe, like bubbling or colour change), and "explain" (give the reason, using electron transfer or the discharge rules). MYP: From Principles into Practice groups these under Criterion A knowledge and Criterion D application — mixing them up costs marks.

The most frequent error is swapping cathode and anode reactions — writing oxidation at the cathode or reduction at the anode. The second most common is forgetting that water contributes H⁺ and OH⁻ ions in any aqueous electrolysis, not just the salt's own ions.

At the cathode, less reactive metals (like copper) are discharged before hydrogen, and hydrogen is discharged before very reactive metals (like sodium). At the anode, halide ions are usually discharged before hydroxide/water, and concentration can shift the outcome — a dilute solution favours water discharge.

Electrolysis uses an external power supply to force a non-spontaneous reaction; a galvanic cell generates its own electricity from a spontaneous redox reaction, no power source needed. Electrode names also swap in meaning between diagrams, which is why students often mix the two up in exams.

MYP covers electrolysis qualitatively — identifying products, writing simple half-equations, and explaining observations. DP Chemistry (first exams 2025 under the current guide) adds quantitative work: Faraday's constant, calculating mass or volume of product from charge passed, and standard electrode potentials for predicting discharge order.

The best help isn't re-explaining the theory — it's making them practise writing full explanations, not just labelling diagrams. Ask your child to talk through a molten-versus-aqueous example out loud; if they can explain why the products differ, they've actually understood the discharge rules, not just memorised them.

Look for resources that pair clear notes with actual practice questions marked against MYP criteria, not just diagrams to memorise. On revisionprep.com, the MYP Chemistry Revision Notes and Topical Worksheets are built around exactly this — concept explanation followed by graded practice, matching the Criterion A/D style used in class assessments.