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school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Electrolysis products & electrode equations · Core · Paper 2 style · 1 mark
(a)Molten lead(II) bromide is electrolysed using graphite electrodes. What is formed at each electrode?[1]
Show answer
(a) A cathode: lead; anode: bromine
Positive Pb²⁺ ions move to the cathode and gain electrons to form lead; Br⁻ ions move to the anode and lose electrons to form bromine. B swaps the electrodes; there is no water, so no hydrogen or oxygen can form.
Q2Electrolysis products & electrode equations · Core · Paper 4 style · 7 marks
Zinc chloride, ZnCl₂, is an ionic compound with a fairly low melting point.
(a)Define the term electrolysis.[2]
(b)Name the positive electrode.[1]
(c)Explain why solid zinc chloride does not conduct electricity but molten zinc chloride does.[2]
(d)Name the products at the cathode and at the anode when molten zinc chloride is electrolysed.[2]
Show answer
(a) The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
B1 decomposition/breakdown of ionic compound (molten or aqueous); B1 by electricity.
(b) Anode
B1
(c) In the solid the ions are fixed in a lattice and cannot move; when molten the ions are free to move and carry charge.
Q3Electrolysis products & electrode equations · Core · Paper 4 style · 5 marks
Dilute sulfuric acid is electrolysed using platinum electrodes.
(a)Name the gas formed at the cathode and the gas formed at the anode.[2]
(b)36 cm³ of gas is collected at the cathode. State the volume of gas collected at the anode in the same time.[1]
(c)Complete the half-equation for the anode: 4OH⁻ → O₂ + ___ + ___[2]
Show answer
(a) Cathode: hydrogen; anode: oxygen
B1 each.
(b) 18 cm³
B1 2:1 ratio from 2H₂O → 2H₂ + O₂.
(c) 4OH⁻ → O₂ + 2H₂O + 4e⁻
B1 2H₂O; B1 4e⁻.
Q4Electrolysis products & electrode equations · Core · Paper 4 style · 5 marks
Concentrated aqueous sodium bromide is electrolysed using carbon electrodes.
(a)List the four ions present in the solution.[2]
(b)Name the product at each electrode.[2]
(c)Describe what is seen near the anode.[1]
Show answer
(a) Na⁺, Br⁻, H⁺, OH⁻
B1 for Na⁺ and Br⁻; B1 for H⁺ and OH⁻.
(b) Cathode: hydrogen; anode: bromine
B1 each. Hydrogen, not sodium, because sodium is more reactive than hydrogen; bromide is discharged because the solution is concentrated.
(c) The solution turns orange (yellow-orange / orange-brown) as bromine forms.
B1 orange/yellow-orange/orange-brown colour.
Q5Electrolysis products & electrode equations · Core · Paper 6 style · 5 marks
A student electrolyses concentrated aqueous sodium chloride in an apparatus that collects the gas from each electrode in a separate tube.
(a)Describe what is seen at each electrode.[2]
(b)The student holds damp blue litmus paper at the top of the anode tube. State what happens.[1]
(c)The volume of gas at the anode is a little less than at the cathode, although equal volumes are expected. Suggest why.[1]
(d)Name the gas formed at the cathode.[1]
Show answer
(a) Bubbles of gas at both electrodes; the gas at the anode is pale green / yellow-green.
B1 bubbles at both; B1 pale green at anode.
(b) It turns red and then is bleached (white).
B1 bleached; accept red then white.
(c) Some chlorine dissolves in the water/solution.
B1
(d) Hydrogen
B1
Q6Electrolysis products & electrode equations · Core · Paper 2 style · 1 mark
(a)Which substance conducts electricity when molten but not when solid?[1]
Show answer
(a) B sodium chloride
Sodium chloride is ionic: its ions cannot move in the solid but can move when molten. Copper and graphite conduct when solid; sulfur does not conduct in either state.
Q7Electrolysis products & electrode equations · Core · Paper 4 style · 5 marks
Name the two products formed when each molten compound is electrolysed with inert electrodes.
(a)molten potassium iodide[1]
(b)molten calcium chloride[1]
(c)molten aluminium oxide[1]
(d)State at which electrode the metal is formed in each case and explain why.[2]
Show answer
(a) Potassium and iodine
B1
(b) Calcium and chlorine
B1
(c) Aluminium and oxygen
B1
(d) The cathode; metal ions are positive and are attracted to the negative electrode, where they gain electrons.
B1 cathode; B1 positive ions attracted / gain electrons.
Q8Electrolysis products & electrode equations · Standard · Paper 4 style · 7 marks
Concentrated aqueous sodium chloride (brine) is electrolysed using inert electrodes.
(a)Name the product at the anode and describe a test for it.[2]
(b)Write the ionic half-equation for the reaction at the cathode.[2]
(c)Name the compound left in solution at the end and explain why it is left.[2]
(d)Explain why sodium is not produced at the cathode.[1]
(c) Sodium hydroxide; H⁺ and Cl⁻ are removed, leaving Na⁺ and OH⁻ ions in solution.
B1 NaOH; B1 explanation.
(d) Sodium is more reactive than hydrogen, so hydrogen ions are discharged instead.
B1
Q9Electrolysis products & electrode equations · Standard · Paper 4 style · 7 marks
Aqueous copper(II) sulfate is electrolysed using graphite electrodes.
(a)Describe what is seen at the cathode.[1]
(b)Describe what is seen at the anode, and name the product.[2]
(c)State and explain what happens to the colour of the solution.[2]
(d)Suggest what happens to the pH of the solution. Explain your answer.[2]
Show answer
(a) A pink-brown (copper-coloured) solid forms on it.
B1
(b) Bubbles (effervescence); oxygen
B1 bubbles; B1 oxygen.
(c) The blue colour fades (becomes paler) because Cu²⁺ ions are removed from the solution as copper.
B1 fades; B1 Cu²⁺ removed.
(d) pH decreases (becomes more acidic) because H⁺ ions are left in solution with sulfate ions — sulfuric acid forms.
B1 decreases; B1 H⁺/sulfuric acid left.
Q10Electrolysis products & electrode equations · Standard · Paper 2 style · 1 mark
(a)Each aqueous solution is electrolysed with inert electrodes. Which one gives hydrogen at the cathode?[1]
Show answer
(a) C aqueous potassium bromide
Potassium is more reactive than hydrogen, so H⁺ ions are discharged and hydrogen forms. Copper and silver are less reactive than hydrogen, so these metals are deposited instead.
Q11Electrolysis products & electrode equations · Standard · Paper 4 style · 5 marks
Sodium metal is manufactured by electrolysing molten sodium chloride.
(a)Write the half-equation for the reaction at the cathode.[1]
(b)Write the half-equation for the reaction at the anode.[2]
(c)State which electrode reaction is oxidation. Explain your answer in terms of electrons.[2]
Show answer
(a) Na⁺ + e⁻ → Na
B1
(b) 2Cl⁻ → Cl₂ + 2e⁻
B1 species; B1 balanced with electrons.
(c) The anode reaction; chloride ions lose electrons.
B1 anode; B1 loss of electrons.
Q12Electrolysis products & electrode equations · Standard · Paper 5 style · 6 marks
A student electrolyses aqueous potassium iodide with carbon electrodes.
(a)Describe what is seen at the cathode.[1]
(b)Describe a test for the gas at the cathode and give the result.[2]
(c)A few drops of universal indicator were added to the solution before the electrolysis. State the colour of the solution near the cathode after a few minutes and explain why.[2]
(d)Describe the colour change near the anode.[1]
Show answer
(a) Bubbles of gas.
B1 'a gas is given off' is not enough.
(b) Lighted splint; it pops.
B1 lighted splint; B1 squeaky pop.
(c) Blue/purple; hydroxide ions are left in solution as hydrogen is discharged, so the solution becomes alkaline.
B1 blue/purple; B1 OH⁻ left / alkaline.
(d) Colourless solution turns brown (iodine forms).
B1
Q13Electrolysis products & electrode equations · Standard · Paper 4 style · 5 marks
Molten lead(II) bromide is electrolysed using graphite electrodes.
(a)Explain why the lead(II) bromide must be heated before a current flows.[1]
(b)Suggest why the experiment is done in a fume cupboard.[1]
(c)Write the half-equation for the formation of lead.[2]
(d)Describe what is seen at the anode.[1]
Show answer
(a) Ions must be able to move; they are free to move only when molten.
B1
(b) Bromine vapour is toxic.
B1
(c) Pb²⁺ + 2e⁻ → Pb
B1 species; B1 2e⁻.
(d) Red-brown / orange-brown vapour (gas).
B1
Q14Electrolysis products & electrode equations · Standard · Paper 2 style · 1 mark
(a)Aqueous copper(II) sulfate is electrolysed using carbon electrodes. Which statement is correct?[1]
Show answer
(a) C Oxygen forms at the anode and the solution becomes more acidic.
OH⁻ ions are discharged at the anode as oxygen and H⁺ ions remain, so the solution becomes acidic. B happens only with copper anodes; copper (not hydrogen) forms at the cathode; the blue colour fades.
Q15Electrolysis products & electrode equations · Standard · Paper 4 style · 5 marks
Aqueous sodium sulfate containing universal indicator (green) is electrolysed in a U-tube with inert electrodes. Hydrogen forms at the cathode and oxygen at the anode.
(a)State the colour near the anode and near the cathode after a few minutes.[2]
(b)Explain the colour near the anode.[2]
(c)Explain why sulfate ions are not discharged.[1]
(d) Copper ions are formed at the anode at the same rate as they are removed at the cathode, so the concentration of Cu²⁺ stays the same.
B1 Cu²⁺ made at anode; B1 removed at cathode at same rate.
Q17Electrolysis products & electrode equations · Challenge · Paper 2 style · 1 mark
(a)Dilute aqueous sodium chloride is electrolysed with inert electrodes. Which row shows the main products?[1]
Show answer
(a) A cathode: hydrogen; anode: oxygen
In a dilute solution of a halide, OH⁻ (from water) is discharged at the anode giving oxygen; hydrogen forms at the cathode because sodium is more reactive than hydrogen. C is the result for the concentrated solution.
Q18Electrolysis products & electrode equations · Challenge · Paper 4 style · 8 marks
Name the product at the cathode and the product at the anode when each of the following is electrolysed with inert electrodes.
(a)Molten calcium bromide[2]
(b)Concentrated hydrochloric acid[2]
(c)Aqueous silver nitrate[2]
(d)Dilute aqueous magnesium sulfate[2]
Show answer
(a) Cathode: calcium; anode: bromine
B1 each.
(b) Cathode: hydrogen; anode: chlorine
B1 each.
(c) Cathode: silver; anode: oxygen
B1 each.
(d) Cathode: hydrogen; anode: oxygen
B1 each.
Q19Electrolysis products & electrode equations · Challenge · Paper 4 style · 6 marks
During electrolysis an electric current passes through both the external circuit and the electrolyte.
(a)Name the particles that carry the current in the metal wires.[1]
(b)Name the particles that carry the current in the electrolyte.[1]
(c)Describe the transfer of charge at the electrodes and in the external circuit when molten potassium chloride is electrolysed.[4]
Show answer
(a) Electrons
B1
(b) Ions
B1
(c) K⁺ ions move to the cathode and gain electrons forming potassium; Cl⁻ ions move to the anode and lose electrons forming chlorine; electrons flow through the external circuit from the anode to the cathode (pushed by the power supply).
B1 cations to cathode, gain electrons; B1 anions to anode, lose electrons; B1 electrons leave at anode; B1 electrons travel through wire to cathode.
Q20Electrolysis products & electrode equations · Challenge · Paper 4 style · 5 marks
Concentrated aqueous copper(II) chloride is electrolysed using inert electrodes. Ar: Cu = 64. 1 mol of gas occupies 24 dm³ at room temperature and pressure.
(a)Name the products at the cathode and at the anode.[2]
(b)0.32 g of copper is deposited. Calculate the volume of chlorine, in cm³ at room temperature and pressure, formed at the same time.[3]
Show answer
(a) Cathode: copper; anode: chlorine
B1 each.
(b) 120 cm³
M1 mol Cu = 0.32 ÷ 64 = 0.0050; M1 Cu²⁺ + 2e⁻ and 2Cl⁻ − 2e⁻, so mol Cl₂ = 0.0050; A1 0.0050 × 24 000 = 120 cm³.
Q21Electrolysis products & electrode equations · Challenge · Paper 6 style · 6 marks
The mass of copper deposited at the cathode during electrolysis depends on the current.
(a)Plan an experiment to investigate how the current affects the mass of copper deposited on the cathode when aqueous copper(II) sulfate is electrolysed for a fixed time. You may use common laboratory apparatus, including a variable resistor and an ammeter.[6]
Show answer
(a) Weigh cathode, electrolyse at set current and time, wash, dry, reweigh, repeat at different currents.
1 mark each, max 6: clean and weigh the copper cathode (balance); set up cathode and anode in copper(II) sulfate solution connected to a power supply, ammeter and variable resistor in series; adjust the resistor to a set current (e.g. 0.2 A) and keep it constant; electrolyse for a fixed time (e.g. 20 min, stop-watch); remove, rinse with distilled water (and propanone), dry and reweigh cathode; repeat with different currents (e.g. 0.4, 0.6, 0.8 A) using the same volume/concentration of solution, same time; calculate mass gained and plot against current.
Q22Electrolysis products & electrode equations · Challenge · Paper 4 style · 6 marks
Aqueous silver nitrate is electrolysed using platinum electrodes. Ar: Ag = 108. 1 mol of gas occupies 24 dm³ at room temperature and pressure.
(a)Write the half-equation for the reaction at the cathode.[1]
(b)Write the half-equation for the reaction at the anode.[2]
(c)1.08 g of silver is deposited. Calculate the volume of oxygen formed at room temperature and pressure, in cm³.[3]
Q23Cells & hydrogen fuel cells · Core · Paper 2 style · 1 mark
(a)What is the only product of the reaction in a hydrogen–oxygen fuel cell?[1]
Show answer
(a) B water
2H₂ + O₂ → 2H₂O; there is no carbon in the reactants, so no carbon dioxide can form.
Q24Cells & hydrogen fuel cells · Standard · Paper 4 style · 6 marks
A city transport company is considering replacing diesel buses with hydrogen fuel cell buses.
(a)Write the overall equation for the reaction in a hydrogen–oxygen fuel cell.[2]
(b)Give two advantages of a hydrogen fuel cell bus compared with a diesel bus.[2]
(c)Give two disadvantages.[2]
Show answer
(a) 2H₂ + O₂ → 2H₂O
B1 formulae; B1 balanced.
(b) Only water is produced; no carbon dioxide (greenhouse gas), carbon monoxide, oxides of nitrogen or particulates released from the bus; quieter / more efficient.
1 mark each for any two.
(c) Hydrogen is difficult/costly to store (needs high pressure, flammable/explosive); few refuelling stations; hydrogen is often made from fossil fuels or using electricity from fossil fuels; fuel cells are expensive.
1 mark each for any two.
Q25Cells & hydrogen fuel cells · Standard · Paper 4 style · 4 marks
Energy released when 1 mol of fuel reacts with oxygen: hydrogen, H₂, 286 kJ; methane, CH₄, 890 kJ. Mr: H₂ = 2, CH₄ = 16.
(a)Calculate the energy released per gram of each fuel. Give your answers to 3 significant figures.[2]
(b)Use your answers to suggest one advantage of hydrogen as a fuel for vehicles.[1]
(c)Hydrogen has a very low density. Suggest why this is a problem for its use in vehicles.[1]
Show answer
(a) Hydrogen 143 kJ/g; methane 55.6 kJ/g
B1 286 ÷ 2 = 143; B1 890 ÷ 16 = 55.6.
(b) It releases more energy per gram (so a lighter fuel load is needed).
B1
(c) A large volume is needed / it must be stored under high pressure in strong, heavy tanks.
B1
Q26Cells & hydrogen fuel cells · Challenge · Paper 4 style · 5 marks
A fuel cell bus uses 8.0 kg of hydrogen in one day. 2H₂ + O₂ → 2H₂O. Mr: H₂ = 2, H₂O = 18. 1 mol of gas occupies 24 dm³ at room temperature and pressure.
(a)Calculate the number of moles of hydrogen used.[1]
(b)Calculate the mass of water produced, in kg.[2]
(c)Calculate the volume of oxygen used, in dm³ at room temperature and pressure.[2]
Q27Electroplating & industrial electrolysis · Core · Paper 2 style · 1 mark
(a)[1]
Show answer
(a) A cathode: spoon; anode: silver; electrolyte: aqueous silver nitrate
The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal (Ag⁺). B reverses the electrodes, which would dissolve the spoon.
Q28Electroplating & industrial electrolysis · Standard · Paper 4 style · 5 marks
A bicycle workshop in Sialkot plates steel handlebars with nickel.
(a)Give two reasons why metal objects are electroplated.[2]
(b)State which electrode the handlebar should be, and name a suitable electrolyte.[2]
(c)Write the half-equation for the reaction at the handlebar.[1]
Show answer
(a) To improve appearance; to resist corrosion (rusting).
1 mark each.
(b) Cathode; aqueous nickel(II) sulfate (or another soluble nickel salt).
B1 cathode; B1 solution containing Ni²⁺ ions.
(c) Ni²⁺ + 2e⁻ → Ni
B1
Q29Electroplating & industrial electrolysis · Standard · Paper 4 style · 5 marks
Impure copper is purified by electrolysis. Pure copper is deposited on the cathode.
(a)State which electrode is made of the impure copper.[1]
(b)Name a suitable electrolyte.[1]
(c)The impure anode loses 120 g. The anode contained 6.0% by mass of impurities, which collect below it. Calculate the mass gained by the cathode.[2]
(d)Suggest why copper used in electrical wiring is purified.[1]
Show answer
(a) Anode
B1
(b) Aqueous copper(II) sulfate
B1
(c) 112.8 g
M1 120 × 0.94; A1 112.8 g.
(d) Impurities reduce its electrical conductivity.
B1
Q30Electroplating & industrial electrolysis · Challenge · Paper 4 style · 7 marks
Aluminium is extracted by the electrolysis of aluminium oxide dissolved in molten cryolite, using carbon electrodes. Ar: Al = 27, O = 16.
(a)Explain why the aluminium oxide is dissolved in molten cryolite.[2]
(b)Write the half-equation for the reaction at the cathode.[1]
(c)Explain why the carbon anodes must be replaced regularly.[2]
(d)Calculate the maximum mass of aluminium, in tonnes, that can be obtained from 1.02 tonnes of aluminium oxide.[2]
Show answer
(a) It lowers the operating temperature (aluminium oxide has a very high melting point), so less energy is needed; the solution also conducts better.
B1 lower temperature / melting point of electrolyte; B1 saves energy/cost.
(b) Al³⁺ + 3e⁻ → Al
B1
(c) Oxygen is formed at the anode; it reacts with the hot carbon to form carbon dioxide, so the anodes burn away.
B1 oxygen reacts with carbon; B1 CO₂ forms.
(d) 0.54 tonnes
M1 Mr Al₂O₃ = 102, contains 54 of Al; A1 1.02 × 54/102 = 0.54 t.
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
