Electrolyte, electrode, anode and cathode
Electrolysis is the breakdown of an ionic compound by an electric current. Four definitions each need two elements: an electrolyte conducts electricity and is decomposed by it; an electrode is the conductor carrying current in and out; the anode is the positive electrode, attracting anions; the cathode is the negative electrode, attracting cations. Hook: Anode-Anions, Cathode-Cations; electrons flow through the external circuit from anode to cathode.
Electrode products: molten vs aqueous
In a molten compound only its own ions are present: the cathode reduces the metal ion to metal, the anode oxidises the non-metal ion to non-metal (molten gives lead and bromine). An aqueous solution also holds and from water. Cathode: a metal below hydrogen is deposited, else hydrogen. Anode: a concentrated halide gives the halogen; a dilute halide or non-halide gives oxygen.
Half-equations and electroplating
(Extended) Half-equations balance atoms and charge at one electrode. Cathode (reduction, electrons gained): . Anode (oxidation, electrons lost): , or . Electroplating (Core): object = cathode, plating metal = anode, electrolyte = a solution of the plating-metal salt; the anode dissolves to replace discharged ions.
Drawn from real examiner reports.
Labelling the wire, not the electrode
In diagram questions the label line must point to the electrode rod dipped in the electrolyte, not to the connecting wire or the battery terminal — a line to the wire scores zero. A related slip is swapping anode and cathode, so the rod on the positive terminal is wrongly labelled cathode. Point the line at the rod itself and mark the anode positive, the cathode negative.
S22 Paper 31 Q7(c)(i); S23 Paper 31 Q4(d)(i); S23 Paper 32 Q4(f)(i)
Naming the electrode, not the product
"State the product at the cathode" wants a substance — copper, hydrogen, lead — not the word "cathode". Many answer with the electrode name and score zero. Related: an observation question wants what is seen ("bubbles of colourless gas"), whereas "hydrogen is produced" names the product, not an observation. Read whether the stem asks for a product or an observation.
S22 Paper 31 Q7(c)(ii); W22 Paper 32 Q7(c)(ii)
Chlorine given for a dilute halide
A halide's anode product depends on concentration. Concentrated chloride gives chlorine (); dilute chloride gives oxygen (), because hydroxide from water is discharged ahead of the few chloride ions. Memorising "chloride to chlorine" without checking the stem loses the dilute case — the word dilute or concentrated is the trigger.
S23 Paper 41 Q5(a)(vi)
Electrolyte confused with electrode
An electrolyte is the conducting liquid or solution that is decomposed; an electrode is the solid conductor carrying current in and out of it. Swapping the two in a definition scores zero. Remember: the electrolyte is what breaks down, the electrode is what the current passes through. Each definition still needs its two mark-scheme elements.
Oxidation and reduction swapped
Reduction is a gain of electrons and happens at the cathode; oxidation is a loss of electrons and happens at the anode (OIL RIG). Candidates often give the opposite definition, or place oxidation at the cathode. Anchor it: cations gain electrons at the negative cathode (reduction); anions lose electrons at the positive anode (oxidation).
W22 Paper 31 Q4(b)(ii); W22 Paper 32 Q4(b)(ii)
Ion and electron directions muddled
Cations (positive) move through the electrolyte to the cathode; anions (negative) move to the anode. Electrons flow only in the external circuit, from anode to cathode — never through the electrolyte. Candidates often get the electron direction right but send the positive ions the wrong way. Keep ion movement inside the cell separate from electron flow in the wire.
W22 Paper 21 Q10
(Extended) Half-equation in four steps
Build any half-equation in four steps: (1) identify species and product; (2) balance atoms — diatomic products need a 2 (); (3) add electrons — left for a cathode (reduction), right for an anode (oxidation); (4) check the charges match.
Answer the exact command word
Match the wording: "State the product" wants the substance (copper, hydrogen); "Describe the observation" wants what is seen (bubbles, a colour). Gas tests: hydrogen gives a squeaky pop with a burning splint, oxygen relights a glowing splint, chlorine bleaches damp litmus.
Label the electrolysis diagram fully
Diagram marks need: label lines to the electrode rods (not the wires); anode marked positive, cathode negative; ion arrows (cations to the cathode, anions to the anode); and products named as substances. These are easy marks routinely dropped.
Electrolysis is the decomposition of a substance by the passage of electricity.
| Term | Two-element mark-scheme definition |
|---|---|
| Electrolyte | A liquid or solution that conducts electricity and is decomposed by it |
| Electrode | An electrical conductor through which current enters or leaves the electrolyte |
| Anode | The positive electrode; anions travel towards it |
| Cathode | The negative electrode; cations travel towards it |
Ion movement: cations () travel to the cathode (); anions () travel to the anode (). Electrons travel through the external circuit from anode to cathode.
(Extended) Write the half-equation for the reaction at the cathode during electrolysis of molten lead(II) bromide.
Draw and label a diagram of the apparatus used to electrolyse molten lead(II) bromide. Include: a d.c. power supply, two electrodes, the electrolyte, electrode labels (anode/cathode with polarity), and arrows showing the direction of movement of ions and ions. [4]
