Electrolysis
4.1What the syllabus asks of you
- Define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
- Identify in simple electrolytic cells: (a) the anode as the positive electrode (b) the cathode as the negative electrode (c) the electrolyte as the molten or aqueous substance that undergoes electrolysis
- Describe the transfer of charge during electrolysis to include: (a) the movement of electrons in the external circuit (b) the loss or gain of electrons at the electrodes (c) the movement of ions in the electrolyte
- Identify the products formed at the electrodes and describe the observations made during the electrolysis of: (a) molten lead(II) bromide (b) concentrated aqueous sodium chloride (c) dilute sulfuric acid using inert electrodes made of platinum or carbon / graphite
- Identify the products formed at the electrodes and describe the observations made during the electrolysis of aqueous copper(II) sulfate using inert carbon / graphite electrodes and when using copper electrodes
- State that metals or hydrogen are formed at the cathode and that non-metals (other than hydrogen) are formed at the anode
- Predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state
- Predict the identity of the products at each electrode for the electrolysis of a halide compound in dilute or concentrated aqueous solution
- Construct ionic half-equations for reactions at the anode (to show oxidation) and at the cathode (to show reduction)
- State that metal objects are electroplated to improve their appearance and resistance to corrosion
- Describe how metals are electroplated
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Chapter 4: Electrolysis
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64 questions
1May / June 2026 · Paper 11 · Q12Show mark scheme →
ASodium ions are oxidised.
BSodium ions are reduced.
CChloride ions are oxidised.
DChloride ions are reduced.
Mark scheme
2May / June 2026 · Paper 12 · Q10Show mark scheme →
Aconcentrated aqueous XCl: product at the cathode hydrogen, product at the anode chlorine; dilute aqueous XCl: product at the cathode X, product at the anode oxygen
Bconcentrated aqueous XCl: product at the cathode hydrogen, product at the anode oxygen; dilute aqueous XCl: product at the cathode X, product at the anode chlorine
Cconcentrated aqueous XCl: product at the cathode hydrogen, product at the anode chlorine; dilute aqueous XCl: product at the cathode hydrogen, product at the anode oxygen
Dconcentrated aqueous XCl: product at the cathode X, product at the anode oxygen; dilute aqueous XCl: product at the cathode hydrogen, product at the anode chlorine
Mark scheme
3Oct / Nov 2025 · Paper 11 · Q12Also set in Oct / Nov 2018 Paper 11Show mark scheme →
Astatue: cathode; electrolyte: AgCl(aq)
Bstatue: cathode; electrolyte: AgNO3(aq)
Cstatue: anode; electrolyte: AgCl(aq)
Dstatue: anode; electrolyte: AgNO3(aq)
Mark scheme
4Oct / Nov 2025 · Paper 12 · Q11Show mark scheme →
Aratio of volume of gas in X and Y with dilute H2SO4 X : Y = 1 : 2; ratio of volume of gas in X and Y with concentrated aqueous NaCl X : Y = 2 : 1
Bratio of volume of gas in X and Y with dilute H2SO4 X : Y = 1 : 1; ratio of volume of gas in X and Y with concentrated aqueous NaCl X : Y = 1 : 1
Cratio of volume of gas in X and Y with dilute H2SO4 X : Y = 1 : 2; ratio of volume of gas in X and Y with concentrated aqueous NaCl X : Y = 1 : 1
Dratio of volume of gas in X and Y with dilute H2SO4 X : Y = 2 : 1; ratio of volume of gas in X and Y with concentrated aqueous NaCl X : Y = 1 : 2
Mark scheme
5May / June 2025 · Paper 11 · Q14Show mark scheme →
ANegative anions move towards the positive cathode.
BNegative cations move towards the positive cathode.
CPositive anions move towards the negative cathode.
DPositive cations move towards the negative cathode.
Mark scheme
6May / June 2025 · Paper 11 · Q15Show mark scheme →
AL: x, M: x
BL: x, M: ✓
CL: ✓, M: x
DL: ✓, M: ✓
Mark scheme
7May / June 2025 · Paper 12 · Q12Show mark scheme →
Abromide ions, electrons, lead ions, electrons
Belectrons, electrons, bromide ions, lead ions
Celectrons, electrons, bromide ions, electrons
Delectrons, electrons, electrons, electrons
Mark scheme
8May / June 2025 · Paper 12 · Q13Show mark scheme →
Aaqueous copper(II) ions, copper metal
Baqueous copper(II) ions, hydrogen gas
Coxygen gas, copper metal
Doxygen gas, hydrogen gas
Mark scheme
9Oct / Nov 2024 · Paper 11 · Q11Show mark scheme →
Amass of anode: increases; mass of cathode: decreases; colour of solution: becomes paler
Bmass of anode: increases; mass of cathode: decreases; colour of solution: stays the same
Cmass of anode: decreases; mass of cathode: increases; colour of solution: becomes paler
Dmass of anode: decreases; mass of cathode: increases; colour of solution: stays the same
Mark scheme
10Oct / Nov 2024 · Paper 12 · Q12Show mark scheme →
AElectrons move from the cathode to the anode in the external circuit.
BHydrogen ions gain electrons at the anode.
CIn the electrolyte, positive ions move to the cathode and negative ions move to the anode.
DThe hydroxide ions in the electrolyte move to the cathode.
Mark scheme
11Oct / Nov 2024 · Paper 12 · Q13Show mark scheme →
ACu → Cu²⁺ + 2e⁻
BCu²⁺ + 2e⁻ → Cu
CAg → Ag⁺ + e⁻
DAg⁺ + e⁻ → Ag
Mark scheme
12May / June 2024 · Paper 11 · Q12Show mark scheme →
Aaqueous sodium chloride ✓, copper ✓, graphite ✗, molten lead(II) bromide ✓
Baqueous sodium chloride ✓, copper ✗, graphite ✗, molten lead(II) bromide ✓
Caqueous sodium chloride ✗, copper ✓, graphite ✓, molten lead(II) bromide ✗
Daqueous sodium chloride ✗, copper ✓, graphite ✗, molten lead(II) bromide ✓
Mark scheme
13May / June 2024 · Paper 11 · Q13Show mark scheme →
Aincrease the concentration of the aqueous copper(II) sulfate
Bincrease the electric current
Creplace the solution with dilute sulfuric acid
Dreverse the electrical connections
Mark scheme
14May / June 2024 · Paper 12 · Q11Show mark scheme →
A1, 2 and 3
B1 and 2 only
C1 and 3 only
D2 only
Mark scheme
15May / June 2024 · Paper 12 · Q12Show mark scheme →
A1, 2 and 3
B1 and 2 only
C1 and 3 only
D2 and 3 only
Mark scheme
16Oct / Nov 2023 · Paper 11 · Q12Show mark scheme →
ACu → Cu²⁺ + 2e⁻
BCu²⁺ + 2e⁻ → Cu
C4OH⁻ → O₂ + 2H₂O + 4e⁻
D2SO₄²⁻ + 2H₂O → 2H₂SO₄ + O₂ + 4e⁻
Mark scheme
17Oct / Nov 2023 · Paper 12 · Q12Show mark scheme →
Aincreases / decreases / remains blue
Bremains the same / decreases / becomes colourless
Cdecreases / increases / remains blue
Dremains the same / increases / becomes colourless
Mark scheme
18May / June 2023 · Paper 11 · Q11Show mark scheme →
Aoxygen is discharged at the anode; OH–(aq) loses electrons more easily than Cl–(aq)
Bduring electrolysis the pH of the electrolyte increases; the electrolysis in aqueous solution involves the reduction of H+(aq) ions
Csolid sodium is produced at the cathode; Na+(aq) is present in aqueous solution
Dthe products stay the same if the aqueous sodium chloride is replaced by molten sodium chloride; Na+ and Cl– are present in both molten and aqueous sodium chloride
Mark scheme
19May / June 2023 · Paper 12 · Q12Also set in Oct / Nov 2020 Paper 11, Oct / Nov 2020 Paper 12Show mark scheme →
Agraph: mass of anode decreasing rapidly then levelling off against time
Bgraph: mass of anode increasing linearly from a non-zero value against time
Cgraph: mass of anode decreasing linearly from a non-zero value against time
Dgraph: mass of anode increasing linearly from zero against time
Mark scheme
20Oct / Nov 2022 · Paper 11 · Q17Show mark scheme →
ACopper is collected at the anode.
BHydrogen is collected at the cathode.
COxygen is collected at the anode.
DSulfur is collected at the cathode.
Mark scheme
21Oct / Nov 2022 · Paper 11 · Q18Show mark scheme →
A2Cl⁻ → Cl₂ + 2e⁻
B2Cl⁻ + 2e⁻ → Cl₂
C2H⁺ + 2e⁻ → H₂
D4OH⁻ → O₂ + 2H₂O + 4e⁻
Mark scheme
22Oct / Nov 2022 · Paper 11 · Q19Show mark scheme →
Aanode: 2Br– → Br2 + 2e–; cathode: Ag2+ + 2e– → Ag
Banode: Br2– → Br2 + 2e–; cathode: Ag+ + e– → Ag
Canode: 2Br– → Br2 + 2e–; cathode: Ag+ + e– → Ag
Danode: Ag+ + e– → Ag; cathode: 2Br– → Br2 + 2e–
Mark scheme
23Oct / Nov 2022 · Paper 12 · Q16Show mark scheme →
Aanode: chlorine, cathode: oxygen
Banode: chlorine, cathode: potassium
Canode: oxygen, cathode: hydrogen
Danode: oxygen, cathode: magnesium
Mark scheme
24Oct / Nov 2022 · Paper 12 · Q17Show mark scheme →
Astatement: Aqueous copper(II) sulfate and aqueous copper(II) nitrate are suitable electrolytes when used to copper plate objects. reason: Both solutions contain Cu2+(aq) and can transfer copper from the anode to the cathode.
Bstatement: During the extraction of aluminium from aluminium oxide the carbon anodes have to be replaced regularly. reason: The anodes gradually dissolve in the molten cryolite.
Cstatement: In the electrolysis of concentrated aqueous sodium chloride and of dilute sulfuric acid the same products are formed. reason: H+(aq) is present in both aqueous solutions.
Dstatement: When an aqueous mixture of zinc nitrate and copper(II) sulfate is electrolysed, zinc is formed on the cathode. reason: Zinc is more reactive than copper.
Mark scheme
25May / June 2022 · Paper 11 · Q16Show mark scheme →
AA brown gas is seen at the positive electrode.
BElectrons pass through the solution from one electrode to the other.
CIons pass through the circuit from one electrode to the other.
DThe lead(II) ions are oxidised.
Mark scheme
26May / June 2022 · Paper 12 · Q16Show mark scheme →
Aoxygen produced at the: anode; hydrogen produced at the: cathode; concentration of acid: decreases
Boxygen produced at the: anode; hydrogen produced at the: cathode; concentration of acid: increases
Coxygen produced at the: cathode; hydrogen produced at the: anode; concentration of acid: decreases
Doxygen produced at the: cathode; hydrogen produced at the: anode; concentration of acid: increases
Mark scheme
27Oct / Nov 2021 · Paper 11 · Q14Show mark scheme →
ABromine is formed at the anode when molten lead bromide is electrolysed.
BPositive ions are discharged at the positive electrode.
CSodium is formed at the cathode when aqueous sodium chloride is electrolysed.
DSulfur dioxide is formed as a gas when dilute sulfuric acid is electrolysed.
Mark scheme
28Oct / Nov 2021 · Paper 12 · Q14Show mark scheme →
ADuring the electrolysis of concentrated aqueous sodium chloride solution, hydrogen is produced at the cathode.
BDuring the electrolysis of dilute sulfuric acid, oxygen is produced at the cathode.
CWhen aqueous copper(II) sulfate is electrolysed, the reaction taking place at the cathode is Cu+(aq) + e– → Cu(s).
DWhen aqueous copper(II) sulfate is electrolysed using copper electrodes, the mass of the anode at the end of the reaction will be greater than at the beginning.
Mark scheme
29Oct / Nov 2021 · Paper 12 · Q15Also set in Oct / Nov 2021 Paper 11Show mark scheme →
AIncrease the concentration of the aqueous copper(II) sulfate.
BIncrease the voltage.
CReplace the solution with dilute sulfuric acid.
DReverse the electrical connections.
Mark scheme
30May / June 2021 · Paper 11 · Q13Show mark scheme →
ACu → Cu2+ + 2e–
BCu2+ + 2e– → Cu
CAg → Ag+ + e–
DAg+ + e– → Ag
Mark scheme
31May / June 2021 · Paper 11 · Q14Show mark scheme →
A1, 2 and 3
B1 and 2 only
C2 and 3 only
D1 and 3 only
Mark scheme
32May / June 2021 · Paper 12 · Q14Show mark scheme →
ASodium ions are oxidised.
BSodium ions are reduced.
CChloride ions are oxidised.
DChloride ions are reduced.
Mark scheme
33May / June 2021 · Paper 12 · Q15Also set in May / June 2021 Paper 11Show mark scheme →
Acopper(II) ions only
Bcopper(II) ions and hydrogen ions
Csulfate ions only
Dsulfate ions and hydroxide ions
Mark scheme
34Oct / Nov 2020 · Paper 11 · Q19Show mark scheme →
Aoxygen is discharged at the anode | OH–(aq) loses electrons more easily than does Cl–(aq)
Bduring electrolysis the pH of the electrolyte increases | the electrolysis in aqueous solution involves the discharge of H+(aq) ions
Csolid sodium is discharged at the cathode | Na+(aq) is present in aqueous solution
Dthe products stay the same if the aqueous sodium chloride is replaced by molten sodium chloride | Na+ and Cl– are present in both molten and aqueous sodium chloride
Mark scheme
35Oct / Nov 2020 · Paper 12 · Q19Show mark scheme →
Acopper X
Bcopper Y
Csilver X
Dsilver Y
Mark scheme
36May / June 2020 · Paper 11 · Q30Show mark scheme →
AThe molar ratio of aluminium to oxygen gas formed is 1 : 2.
BThe molar ratio of aluminium to oxygen gas formed is 3 : 4.
COxygen gas is formed at the anode.
DReduction occurs at the anode.
Mark scheme
37May / June 2020 · Paper 12 · Q16Also set in May / June 2020 Paper 11Show mark scheme →
Aelectrolyte: aqueous copper(II) sulfate; anode (positive electrode): piece of pure copper; cathode (negative electrode): steel key
Belectrolyte: aqueous copper(II) sulfate; anode (positive electrode): steel key; cathode (negative electrode): piece of pure copper
Celectrolyte: dilute sulfuric acid; anode (positive electrode): piece of pure copper; cathode (negative electrode): steel key
Delectrolyte: dilute sulfuric acid; anode (positive electrode): steel key; cathode (negative electrode): piece of pure copper
Mark scheme
38May / June 2020 · Paper 12 · Q17Show mark scheme →
Ahydrogen anode
Bhydrogen cathode
Coxygen anode
Doxygen cathode
Mark scheme
39Oct / Nov 2019 · Paper 11 · Q11Show mark scheme →
AL ✓ M ✓
BL ✓ M ✗
CL ✗ M ✓
DL ✗ M ✗
Mark scheme
40Oct / Nov 2019 · Paper 11 · Q12Show mark scheme →
A2Cl– + 2e– → Cl2
B2Cl– – 2e– → Cl2
CNa+ + e– → Na
DNa+ – e– → Na
Mark scheme
41Oct / Nov 2019 · Paper 12 · Q11Show mark scheme →
Aaqueous copper(II) sulfate
Bconcentrated aqueous sodium chloride
Cdilute sulfuric acid
Dmolten lead bromide
Mark scheme
42May / June 2019 · Paper 11 · Q14Show mark scheme →
AA pure copper anode is used.
BA pure copper cathode is used.
CThe colour of the electrolyte fades throughout the process.
DThe electrolyte used is a solution of copper oxide in water.
Mark scheme
43May / June 2019 · Paper 12 · Q14Show mark scheme →
Acathode product: hydrogen; anode product: chlorine
Bcathode product: hydrogen; anode product: oxygen
Ccathode product: sodium; anode product: chlorine
Dcathode product: sodium; anode product: oxygen
Mark scheme
44May / June 2019 · Paper 12 · Q15Also set in May / June 2019 Paper 11Show mark scheme →
Acopper(II) ions and hydrogen ions
Bcopper(II) ions only
Csulfate ions and hydroxide ions
Dsulfate ions only
Mark scheme
45Oct / Nov 2018 · Paper 12 · Q12Also set in Oct / Nov 2018 Paper 11Show mark scheme →
AA pink solid is deposited on the anode.
BBubbles form on the negative electrode.
CThe colour of the solution fades.
DThe negative electrode becomes smaller.
Mark scheme
46Oct / Nov 2018 · Paper 12 · Q13Show mark scheme →
A1 and 2
B2 and 3
C2 and 4
D3 and 4
Mark scheme
47May / June 2018 · Paper 11 · Q7Show mark scheme →
ACu + 2e– → Cu2+
BCu2+ + 2e– → Cu
C2H2O + Cu2+ + 2e– → Cu(OH)2 + O2
DSO4 2– + 4H+ + 2e– → H2SO4 + H2
Mark scheme
48May / June 2018 · Paper 12 · Q13Also set in May / June 2018 Paper 11Show mark scheme →
Aaqueous copper(II) sulfate
Bconcentrated aqueous sodium chloride
Cdilute sulfuric acid
Dethanol
Mark scheme
49May / June 2018 · Paper 12 · Q14Show mark scheme →
Aanode: chlorine, cathode: hydrogen
Banode: chlorine, cathode: magnesium
Canode: magnesium, cathode: chlorine
Danode: oxygen, cathode: hydrogen
Mark scheme
50May / June 2018 · Paper 12 · Q31Show mark scheme →
AAll the metals can also be extracted from their oxides using coke.
BThe anode and cathode should be made of the metal being extracted.
CThe pure metal is always produced at the cathode.
DThe same mass of each metal is formed.
Mark scheme
51Oct / Nov 2017 · Paper 11 · Q14Show mark scheme →
AAl → Al3+ + 3e–
BAl3+ + 3e– → Al
CO2 + 4e– → 2O2–
D2O2– → O2 + 4e–
Mark scheme
52Oct / Nov 2017 · Paper 11 · Q15Show mark scheme →
Aelectrode becomes smaller; electrode becomes bigger; constant
Belectrode becomes smaller; gas given off; fades
Cgas given off; electrode becomes bigger; fades
Dgas given off; gas given off; constant
Mark scheme
53Oct / Nov 2017 · Paper 12 · Q16Show mark scheme →
Apositive electrode: 2Cl– → Cl2 + 2e–; negative electrode: 2H+ + 2e– → H2
Bpositive electrode: Cl2 + 2e– → 2Cl–; negative electrode: Mg2+ + 2e– → Mg
Cpositive electrode: 2Cl– → Cl2 + 2e–; negative electrode: Mg2+ + 2e– → Mg
Dpositive electrode: 2Cl– → Cl2 + 2e–; negative electrode: Mg2+ + 2e– → 2Mg
Mark scheme
54Oct / Nov 2017 · Paper 12 · Q17Also set in Oct / Nov 2017 Paper 11Show mark scheme →
A1, 2 and 3
B1 and 2 only
C1 only
D2 and 3 only
Mark scheme
55May / June 2017 · Paper 11 · Q14Show mark scheme →
Aiodine; calcium
Biodine; hydrogen
Coxygen; calcium
Doxygen; hydrogen
Mark scheme
56May / June 2017 · Paper 11 · Q15Show mark scheme →
Aanode; Al3+ + 3e– → Al
Banode; Al3+ – 3e– → Al
Ccathode; Al3+ + 3e– → Al
Dcathode; Al3+ – 3e– → Al
Mark scheme
57May / June 2017 · Paper 12 · Q11Also set in May / June 2017 Paper 11Show mark scheme →
A1, 2 and 4
B1 and 2 only
C2 and 3
D3 and 4
Mark scheme
58May / June 2017 · Paper 12 · Q12Show mark scheme →
Acaesium | hydrochloric acid
Bchlorine | caesium hydroxide
Chydrogen | caesium hydroxide
Dhydrogen | hydrochloric acid
Mark scheme
59May / June 2017 · Paper 12 · Q13Show mark scheme →
A1 and 2
B1 and 3
C2 and 4
D3 and 4
Mark scheme
60Oct / Nov 2016 · Paper 11 · Q14Show mark scheme →
Aelectrode dissolves | copper deposited | stays a constant blue colour
Belectrode dissolves | hydrogen gas given off | blue colour becomes more intense
Chydrogen gas given off | oxygen gas given off | stays a constant blue colour
Doxygen gas given off | hydrogen gas given off | stays a constant blue colour
Mark scheme
61Oct / Nov 2016 · Paper 12 · Q14Also set in Oct / Nov 2016 Paper 11Show mark scheme →
Aaluminium
Bcalcium
Ciron
Dsodium
Mark scheme
62May / June 2016 · Paper 11 · Q15Show mark scheme →
Aat the positive electrode: colourless gas forms; electrolyte: blue colour fades; at the negative electrode: pink solid forms
Bat the positive electrode: colourless gas forms; electrolyte: no change; at the negative electrode: colourless gas forms
Cat the positive electrode: electrode decreases in mass; electrolyte: blue colour fades; at the negative electrode: colourless gas forms
Dat the positive electrode: electrode decreases in mass; electrolyte: no change; at the negative electrode: pink solid forms
Mark scheme
63May / June 2016 · Paper 12 · Q12Show mark scheme →
ABoth electrodes contain copper.
BCopper is both oxidised and reduced in the process.
CPure copper is deposited on the positive electrode.
DThe electrolyte is aqueous copper(II) sulfate.
Mark scheme
64May / June 2016 · Paper 12 · Q13Show mark scheme →
A1
B4
C7
D11
Mark scheme