Electrolysis

A typical Paper 3 or 4 electrolysis question hands you a diagram of a cell and asks for the electrode names, the product at each electrode and a reason: 4 to 6 marks that are pure recall plus one rule. Examiner reports repeat the same complaint every series: candidates know the products but swap the electrodes, so half the marks vanish on the first line.

The definitions that earn single marks (Core)

Each of these is a one-mark question in its own right. Learn them word for word.

Term	Exam definition
Electrolysis	Decomposition of an ionic compound, molten or aqueous, by the passage of an electric current
Electrolyte	The molten or aqueous ionic compound that is decomposed
Cathode	The negative electrode
Anode	The positive electrode
Inert electrode	An electrode that does not react: carbon (graphite) or platinum

Cations are positive ions; they move to the cathode. Anions are negative ions; they move to the anode. Opposite charges attract, so each ion travels to the electrode of opposite sign. That single sentence is what “explain why the ions move” is asking for.

Why solids do not conduct (Core)

A solid ionic compound has its ions locked in fixed positions in a giant lattice, so they cannot move and no current flows. Melt it, or dissolve it in water, and the ions become mobile. The mark scheme wants “ions are free to move” (“electrons are free to move” scores zero here, because in an electrolyte the ions carry the charge). This links straight back to ionic bonding in Atoms, Elements and Compounds.

Molten binary compounds: products are forced (Core)

A binary compound contains exactly two elements, so there is no competition at either electrode. The rule never bends: metal at the cathode, non-metal at the anode.

The syllabus set-piece is molten lead(II) bromide:

• Cathode: lead. A shiny grey liquid metal collects below the electrode.
• Anode: bromine, an orange-brown vapour.

The same logic answers any molten example thrown at you. Molten aluminium oxide gives aluminium at the cathode and oxygen at the anode (the basis of aluminium extraction, covered under Metals). Molten zinc chloride gives zinc and chlorine.

Half-equations and charge transfer (Supplement)

Extended candidates write the half-equation at each electrode for molten lead(II) bromide:

• Cathode (reduction): Pb2+ + 2e− → Pb
• Anode (oxidation): 2Br− → Br2 + 2e−

Two checks before you move on. First, electrons sit on the left at the cathode (the ion gains them) and on the right at the anode (the ion loses them). Second, the atoms and charges balance: bromine is diatomic, so two bromide ions are needed.

The charge-transfer mark is its own marking point: electrons flow through the external wires and electrodes, while ions move through the electrolyte. Electrons never travel through the liquid.

Reduction happens at the cathode, oxidation at the anode. Electrolysis is a forced redox reaction, and the OILRIG bookkeeping is explained fully in Redox and Oxidation Numbers.

What changes in aqueous solution

Once water is present, H+ and OH− ions compete with the ions of the compound, and the products depend on reactivity and concentration. That decision routine is Extended material with its own page: Electrolysis of Aqueous Solutions. For Core, memorise the two named results: concentrated aqueous sodium chloride gives hydrogen at the cathode and chlorine at the anode; dilute sulfuric acid gives hydrogen at the cathode and oxygen at the anode.

Worked exam question

Molten lead(II) bromide is electrolysed using inert carbon electrodes. (a) Explain why lead(II) bromide must be molten for electrolysis to occur. [2] (b) Name the product formed at the cathode and at the anode. [2] (c) Write the half-equation for the reaction at the anode. [1]

Model answer: (a) In the solid the ions are in fixed positions (1); when molten the ions are free to move and carry the current (1). (b) Cathode: lead (1). Anode: bromine (1). (c) 2Br− → Br2 + 2e− (1).

Mark-by-mark: (a) is two separate ideas: fixed ions in the solid, mobile ions in the liquid. Writing only “the ions can move when molten” usually earns one of the two. (b) is the metal-cathode rule; “lead bromide” at either electrode scores nothing because electrolysis decomposes the compound into elements. (c) needs Br2 (diatomic) and the electrons on the right; Br− → Br + e− fails both checks. Part (c) appears only on Paper 4.

The mistakes that cost marks

1. Swapping cathode and anode. The cathode is negative. The memory hook PANIC (Positive Anode, Negative Is Cathode) costs five seconds and saves two marks.
2. Writing “electrons move through the electrolyte”. Ions carry the current in the liquid; electrons stay in the wires and electrodes.
3. Naming a compound as a product. Electrolysis splits the electrolyte into elements: lead and bromine, never “lead bromide” at an electrode.
4. Unbalanced half-equations. Bromine, chlorine and oxygen are diatomic, and the total charge must match on both sides.

How examiners want it phrased

Student wording	Mark-scheme wording
”It needs to be hot so it works"	"When molten, the ions are free to move and carry the current"
"The lead goes to the minus side"	"Pb2+ ions are attracted to the cathode and gain electrons: Pb2+ + 2e− → Pb"
"Bromine comes off at the top"	"Bromide ions are oxidised at the anode: 2Br− → Br2 + 2e−"
"Electricity flows through everything"	"Electrons flow through the external circuit; ions move through the electrolyte”

The reliable answer pattern is ion → electrode → electron gain or loss. Build every electrolysis explanation from those three pieces and you match the marking points automatically. If the decision routine for aqueous solutions still feels like guesswork, the Electrochemistry pillar walks through the full topic, and a free 1-hour trial lesson with a Chemistry specialist will drill the cathode/anode routine on real past-paper cells.

Test yourself

Answer all three before clicking. Each answer stays hidden until you do.

Q1 (2 marks). Molten zinc chloride is electrolysed using inert carbon electrodes. Name the product formed at each electrode.

Show answer

• Cathode: zinc [1] • Anode: chlorine [1]

Q2 (2 marks). Define the term electrolysis.

Show answer

• The decomposition of an ionic compound [1] • When molten or in aqueous solution, by the passage of an electric current [1]

Q3 (2 marks, Supplement). Molten aluminium oxide is electrolysed during the extraction of aluminium. Write the half-equation for the reaction at each electrode.

Show answer

• Cathode: Al3+ + 3e− → Al [1] • Anode: 2O2− → O2 + 4e− [1]