Electrolysis Explained: Half-Equations Without the Fear

Quick answer: Electrolysis uses electricity to split an ionic compound once it is molten or dissolved, so the ions are free to move. Positive ions travel to the negative cathode and gain electrons (reduction); negative ions travel to the positive anode and lose electrons (oxidation). At the cathode, a metal below hydrogen in reactivity is deposited, otherwise hydrogen forms; at the anode, a halide gives a halogen, otherwise oxygen forms.

The first time a half-equation showed up on my IGCSE chemistry paper, I left it completely blank. It looked like a maths puzzle wearing a lab coat. What finally fixed it wasn't memorising twenty equations — it was realising that electrolysis asks you the same two questions in the same order, every single time.

Once you know that order, you can build a half-equation for a compound you have never seen before. Here is the walkthrough I wish someone had handed 15-year-old me.

What electrolysis actually is

Electrolysis is splitting up an ionic compound using electricity. The catch is that the ions have to be able to move. In a solid they are locked in a lattice and going nowhere, so nothing happens. You have two ways to free them:

• Melt the compound (molten), or
• Dissolve it in water (aqueous).

Then you dip in two electrodes connected to a power supply, and the ions start migrating. That movement, and the electron swap that happens when they arrive, is the whole of electrolysis.

The two electrodes (and the only mnemonics you need)

Two words carry this entire topic:

• PANIC — Positive is Anode, Negative Is Cathode.
• OIL RIG — Oxidation Is Loss of electrons, Reduction Is Gain.

Add one more line: cations (positive ions) are attracted to the cathode. So:

• The cathode is negative. Positive ions arrive and gain electrons. That is reduction.
• The anode is positive. Negative ions arrive and lose electrons. That is oxidation.

Scribble those two words in the margin the second you sit the paper, and you have already secured marks you would otherwise be guessing at.

Molten vs aqueous: the split that decides everything

This is the part most people skim, and it is exactly where the marks hide.

Molten compounds are simple. There are only two ions in the beaker — the metal and the non-metal. Molten lead bromide gives you Pb²⁺ and Br⁻, and that is it. No competition.

Aqueous solutions are sneakier, because water itself supplies a small number of H⁺ and OH⁻ ions. So now there are four ions competing to be discharged, not two. That competition is the whole reason aqueous electrolysis needs rules.

The rules for what actually gets discharged

For a solution, ask these two questions.

At the cathode (positive ions):

• If the metal is more reactive than hydrogen (potassium, sodium, calcium, magnesium, aluminium, zinc, iron...), the metal stays dissolved and hydrogen gas is produced instead.
• If the metal is less reactive than hydrogen (copper, silver, gold), the metal itself is deposited.

At the anode (negative ions):

• If a halide is present (Cl⁻, Br⁻, I⁻), you get the halogen — chlorine, bromine or iodine.
• If there is no halide (for example a sulfate or nitrate), you get oxygen from the hydroxide ions.

That is genuinely the whole decision. Everything below is just applying it.

How to write a half-equation in four steps

1. Write the ion on one side and what it turns into on the other.
2. Balance the atoms.
3. Add electrons (e⁻) to balance the charge. At the cathode they go on the left, because ions gain them; at the anode they go on the right, because ions lose them.
4. Check that the total charge is equal on both sides.

If the electron-balancing step is what trips you up, paste your attempt into the step-by-step explainer and ask it to show the charge check on its own line — that is usually the bit schools rush.

Worked example 1: molten lead(II) bromide

This is the classic first electrolysis experiment, so examiners lean on it.

Step 1 — It is molten, so only two ions exist: Pb²⁺ and Br⁻.

Step 2 — Cathode (reduction). Lead ions arrive and gain electrons:

Pb²⁺ + 2e⁻ → Pb

Silvery molten lead forms at the bottom.

Step 3 — Anode (oxidation). Bromide ions arrive. One bromine atom is Br, but bromine exists as Br₂, so you need two ions:

2Br⁻ → Br₂ + 2e⁻

Orange-brown bromine vapour is given off.

Step 4 — Check the charge. Cathode: left side 2+ and 2- cancel to 0, right side is 0. Balanced. Anode: left side 2-, right side is 2- from the two electrons. Balanced. Done.

Worked example 2: the electrolysis of brine

Brine is concentrated sodium chloride solution, and it is one of the most tested industrial processes on GCSE and IGCSE papers — AQA, Edexcel and Cambridge all use it.

Ions present: Na⁺ and Cl⁻ from the salt, plus H⁺ and OH⁻ from the water. Four ions.

Step 1 — Cathode. Sodium is more reactive than hydrogen, so sodium stays in solution and hydrogen is discharged:

2H⁺ + 2e⁻ → H₂

Step 2 — Anode. A halide (Cl⁻) is present and concentrated, so chlorine wins over oxygen:

2Cl⁻ → Cl₂ + 2e⁻

Step 3 — What is left behind? Na⁺ and OH⁻ ions, which means the liquid left in the beaker is sodium hydroxide, NaOH.

So brine gives you three useful products from one salty solution: hydrogen, chlorine and sodium hydroxide — used for margarine, bleach and soap. One exam-trap detail: if the solution were dilute rather than concentrated, the anode would give oxygen instead of chlorine. Concentration matters here, and mark schemes check that you noticed.

Copper: the disappearing anode

Electrolysing copper(II) sulfate solution has two versions, and mixing them up is a classic dropped mark.

With inert electrodes (carbon or platinum):

• Cathode: copper sits below hydrogen, so copper is deposited — Cu²⁺ + 2e⁻ → Cu. The blue colour fades as Cu²⁺ leaves the solution.
• Anode: sulfate is not a halide, so oxygen forms — 4OH⁻ → O₂ + 2H₂O + 4e⁻.

With copper electrodes (how copper is purified):

• Cathode: Cu²⁺ + 2e⁻ → Cu. Pure copper plates on, so the cathode gains mass.
• Anode: the copper electrode itself dissolves — Cu → Cu²⁺ + 2e⁻ — so the anode loses mass.

That second version is the whole principle behind purifying copper and electroplating: impure copper dissolves off the anode and pure copper builds up on the cathode. If any of that felt fast, drop the exact reaction into the AI explainer and ask it to redo the half-equations one line at a time.

The mistakes that cost real marks

• Forgetting the electrons entirely, or putting them on the wrong side.
• Writing Cl⁻ → Cl₂ without the 2 in front. Unbalanced atoms lose the mark straight away.
• Forgetting that water adds H⁺ and OH⁻ in aqueous solutions.
• Swapping anode and cathode — write PANIC down first.
• Saying chlorine for a dilute solution when it should be oxygen.

Examiners often give one mark for balanced atoms and a separate mark for balanced charge, so a half-equation that is right except for the electrons still throws away half the marks.

Test yourself

1. Molten aluminium oxide is electrolysed. Write the cathode half-equation. Aluminium ions are Al³⁺.
2. In the electrolysis of concentrated potassium bromide solution, what is produced at the anode, and why?
3. Copper(II) sulfate is electrolysed with carbon electrodes. What forms at the anode?

Quick answers:

1. Al³⁺ + 3e⁻ → Al — three electrons because the ion carries a 3+ charge.
2. Bromine, because bromide is a halide and it is discharged in preference to oxygen.
3. Oxygen, because sulfate is not a halide, so the hydroxide ions are discharged instead.

Want a full set marked instantly? Generate a round on this exact topic with the quiz maker, or turn every rule above into revision cards with flashcards.

FAQ

Why does the cathode attract positive ions if it is negative?

Opposite charges attract. The cathode is the negative electrode, so positive ions (cations) are pulled toward it, where they gain electrons and are reduced.

How do I know if hydrogen or the metal forms at the cathode?

Check the reactivity series. If the metal is more reactive than hydrogen, hydrogen is produced and the metal stays dissolved. If it is less reactive — copper, silver or gold — the metal is deposited.

What is the difference between electrolysis of molten and aqueous compounds?

Molten compounds contain only two ions, so the products are always just that metal and non-metal. Aqueous solutions also contain H⁺ and OH⁻ from water, so you have to apply the discharge rules to decide the winner.

Do I need state symbols in half-equations?

Usually not for the marks, but they are never wrong to include. Get balanced atoms and balanced charge first — that is what the mark scheme is actually checking.

In short: Electrolysis is just two questions — what happens at the cathode, and what happens at the anode — answered with the reactivity series and the halide rule, then balanced with electrons.