Electrolysis: ions must be free to move
Electrolysis is the decomposition of an ionic compound by an electric current; the molten or aqueous liquid that conducts is the electrolyte. It conducts only when molten or dissolved, because the ions are then free to move and carry charge. As a solid the ions are locked in the lattice, so it does not conduct. Covalent compounds never conduct — they have no ions or free electrons. In the electrolyte the ions move, never the electrons.
Cations → cathode; anions → anode
The anode is the positive electrode and the cathode is the negative electrode. A cation is a positive ion, attracted to the cathode (negative); an anion is a negative ion, attracted to the anode (positive). Opposite charges attract, so positive ions move to the negative electrode and negative ions to the positive electrode. Memory hook: PANIC — Positive Anode, Negative Is Cathode.
Cathode reduction, anode oxidation
Each electrode reaction is an ionic half-equation. At the cathode, positive ions gain electrons — reduction, e.g. . At the anode, negative ions lose electrons — oxidation, e.g. . Remember OIL RIG — Oxidation Is Loss, Reduction Is Gain. A half-equation must balance both atoms and charge.
Drawn from real examiner reports.
Electrons do not move in the electrolyte
Why does solid lead(II) bromide not conduct but molten does? Three marks: (1) in the solid the ions are in fixed positions and cannot move; (2) when molten the ions are free to move; (3) the ions carry the charge / conduct. Classic errors: saying the electrons move (really the ions move), and forgetting the ions are fixed in the solid.
June 2024 Paper 2C Q6(c)(i) — many lost the first mark for not stating the ions are in fixed positions/a lattice, and some wrongly said the electrons move when molten (rejected). June 2024 Paper 2CR Q6(b)(i) — answers that referred to electrons, or said "free to carry charge" without movement, scored 0.
It is the bromide ion, not bromine
In a half-equation the species that reacts is the ion, not the neutral element. At the anode the bromide ion loses electrons: . Writing "bromine loses electrons" scores 0 — is the neutral product, not the reactant. Name the halide ion, not the element.
June 2024 Paper 2C Q6(d)(ii) — "bromine loses electrons" scored 0; the mark required the bromide ion losing electrons.
Wrong ion charge or electrons on wrong side
Two more half-equation slips. (1) Wrong charge: when lead(II) bromide means the ion is , or the hydride instead of . (2) Electrons on the wrong side: reduction has electrons on the left, oxidation on the right. Missing charges stop it balancing — check the total charge is equal on both sides.
June 2024 Paper 2C Q6(c)(iii) — common errors were wrong charges/balancing and writing Pb4+ (the ion is Pb2+). June 2024 Paper 2CR Q6(a)(iv) — candidates used H- instead of H+ and placed electrons on the wrong side.
Redox in electrolysis = electron transfer
At the electrodes, oxidation = loss of electrons and reduction = gain of electrons — defining them only by "gain/loss of oxygen" misses the mark here. The cathode reaction (gain of electrons) is reduction; the anode reaction (loss of electrons) is oxidation. State the electron transfer (OIL RIG) and apply each to the correct electrode.
June 2024 Paper 2CR introduction — examiners noted many candidates struggled to write half-equations and to apply oxidation/reduction in electron terms; oxidation/reduction defined only by oxygen is insufficient for electrode reactions.
"Carbon" is too vague for an electrode
A suitable inert electrode conducts but does not react — use graphite or platinum. Just "carbon" is too vague (it could mean diamond, which does not conduct) and loses the mark. A transition metal electrode is not accepted as inert, and a copper anode would dissolve into the solution rather than stay inert. Name graphite or platinum specifically.
June 2024 Paper 2C Q6(c)(ii) — a suitable inert electrode is graphite/platinum; "carbon" is too vague (could be diamond), a transition metal loses the inert mark, and a copper anode dissolves.
Bromine forms as a brown gas, not orange
At the anode in molten lead(II) bromide the product is bromine as a brown gas/vapour. Marks are lost for "brown liquid", "brown solid" or "orange" — the accepted description is a brown gas/vapour. (Bromine is a liquid at room temperature but is formed here as a vapour at the hot electrode.) Give the state as well as the colour.
June 2024 Paper 2C Q6(d)(i) — the product at the anode is a brown gas/vapour (bromine); "brown liquid/solid" and "orange" were not accepted.
Building an electrode half-equation
Building a half-equation: name the reacting ion and its charge (lead(II) is Pb²⁺). Cathode — positive ion reduced (gains electrons, on the left); anode — negative ion oxidised (loses electrons, on the right). Set the electron number so charge balances on both sides.
Predict the products at each electrode
Two rules for the products. Cathode: a metal more reactive than hydrogen (Na, K) gives hydrogen, otherwise the metal deposits (e.g. Cu). Anode: a halide gives the halogen, otherwise oxygen. In a molten compound only its two ions are present.
Explaining why molten conducts
For "solid vs molten lead(II) bromide", give the chain: in the solid the ions are fixed in the lattice and cannot move, so no charge is carried; when molten the ions are free to move and carry the charge and conduct. Never say the electrons move.
Why does an ionic compound only conduct when molten or in solution? When solid, the ions are in fixed positions in a lattice and cannot move, so it does not conduct. When molten or dissolved, the ions are free to move and carry the charge. Covalent compounds never conduct — they have no ions or free electrons.
Define electrolysis.
Molten lead(II) bromide is electrolysed with inert electrodes. Write the ionic half-equation for the reaction at the cathode, and state whether it is oxidation or reduction.
