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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 · 4 marks
(a)Define the term electrolysis.[1]
(b)Solid sodium chloride does not conduct electricity, but molten sodium chloride does. Explain why.[2]
(c)Name the electrode at which sodium is produced and state its charge.[1]
Show answer
(a) The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
Breakdown/decomposition of ionic compound (molten or aqueous) by electricity B1
(b) In the solid the ions are held in fixed positions in the lattice and cannot move; when molten the ions are free to move and carry the charge.
Ions cannot move in solid B1; ions free to move when molten B1
(c) the cathode, negative
B1
Q2Electrolysis: products & electrode equations · Core · Paper 4 style · 5 marks
Molten lead(II) bromide, PbBr₂, is electrolysed using graphite electrodes in a fume cupboard.
(a)Describe what is seen at the anode (positive electrode).[1]
(b)Name the product at the cathode and describe its appearance.[1]
(c)Write ionic half-equations for the reactions at the cathode and at the anode.[2]
(d)Explain why the electrolysis is done in a fume cupboard.[1]
Show answer
(a) red-brown vapour/gas (bromine)
B1
(b) lead; silvery/grey liquid metal (collects as beads at the bottom)
Q3Electrolysis: products & electrode equations · Core · Paper 1 style · 1 mark
(a)Concentrated aqueous sodium chloride is electrolysed with inert electrodes. What are the products?[1]
Show answer
(a) B hydrogen at the cathode, chlorine at the anode
Sodium is more reactive than hydrogen, so H⁺ ions (from water) are discharged at the cathode; at high concentration Cl⁻ is discharged at the anode. A would only happen with molten sodium chloride.
Q4Electrolysis: products & electrode equations · Core · Paper 2 style · 5 marks
Dilute sulfuric acid is electrolysed using platinum electrodes. The gases are collected in two test-tubes.
(a)Name the gas formed at the cathode and the gas formed at the anode.[2]
(b)Describe a test for each gas and the result.[2]
(c)State the ratio of the volume of gas at the cathode to the volume at the anode.[1]
Show answer
(a) cathode: hydrogen; anode: oxygen
Each B1
(b) Hydrogen: lighted splint gives a squeaky pop. Oxygen: relights a glowing splint.
Each test with result B1
(c) 2 : 1
B1
Q5Electrolysis: products & electrode equations · Core · Paper 2 style · 5 marks
(a)In electrolysis, which electrode do positive ions move towards? Name this electrode.[1]
(b)Molten calcium bromide, CaBr₂, is electrolysed. Name the products at the cathode and at the anode.[2]
(c)State whether the ions are oxidised or reduced at each electrode.[2]
Show answer
(a) the negative electrode, the cathode
B1
(b) cathode: calcium; anode: bromine
Each B1
(c) Calcium ions are reduced (gain electrons) at the cathode; bromide ions are oxidised (lose electrons) at the anode.
Reduction at cathode B1; oxidation at anode B1
Q6Electrolysis: products & electrode equations · Core · Paper 1 style · 1 mark
(a)Which substance conducts electricity and is decomposed by it?[1]
Show answer
(a) B molten zinc chloride
Molten zinc chloride is an ionic compound with mobile ions, so it is an electrolyte. Copper conducts but is not decomposed; solid NaCl has fixed ions; ethanol is covalent with no ions.
Q7Electrolysis: products & electrode equations · Core · Paper 2 style · 4 marks
(a)Molten potassium iodide is electrolysed using inert electrodes. Name the product at each electrode.[2]
(b)Electrons flow in the external wires. In which direction do they flow: from the anode to the cathode, or from the cathode to the anode?[1]
(c)Name the particles that carry the current through the molten potassium iodide.[1]
Show answer
(a) cathode: potassium; anode: iodine
Each B1
(b) from the anode to the cathode (through the external circuit)
B1
(c) ions (K⁺ and I⁻)
B1
Q8Electrolysis: products & electrode equations · Standard · Paper 2 style · 5 marks
Aqueous copper(II) sulfate is electrolysed using carbon (graphite) electrodes.
(a)Describe what is seen at the cathode and at the anode.[2]
(b)Write the ionic half-equation for the reaction at the cathode.[1]
(c)Write the ionic half-equation for the reaction at the anode.[1]
(d)The blue colour of the solution fades during the electrolysis. Explain why.[1]
Show answer
(a) cathode: pink-brown solid (copper) forms; anode: bubbles of colourless gas (oxygen)
(d) Cu²⁺ ions (which give the blue colour) are removed from the solution as copper metal.
B1
Q9Electrolysis: products & electrode equations · Standard · Paper 2 style · 5 marks
Aqueous copper(II) sulfate is electrolysed using copper electrodes. Before the experiment the anode has a mass of 15.40 g and the cathode 15.10 g. After 30 minutes the anode has a mass of 14.76 g.[Ar: Cu, 64]
(a)Write the half-equation for the reaction at the anode.[1]
(b)Predict the mass of the cathode after 30 minutes.[1]
(c)Calculate the number of moles of electrons that passed through the circuit.[2]
(d)Explain why the blue colour of the solution does not change.[1]
Show answer
(a) Cu → Cu²⁺ + 2e⁻
B1
(b) 15.74 g
Anode lost 0.64 g; same mass gained at cathode: 15.10 + 0.64 = 15.74 B1
(d) Cu²⁺ ions are added at the anode at the same rate as they are removed at the cathode.
B1
Q10Electrolysis: products & electrode equations · Standard · Paper 2 style · 5 marks
(a)Concentrated aqueous sodium chloride and very dilute aqueous sodium chloride are each electrolysed with inert electrodes. Name the gas formed at the anode in each case.[2]
(b)Explain the difference.[1]
(c)Name the gas formed at the cathode in both cases and explain why sodium is not formed.[2]
Show answer
(a) concentrated: chlorine; very dilute: oxygen
Each B1
(b) In concentrated solution there are many chloride ions so Cl⁻ is discharged; in very dilute solution OH⁻ ions (from water) are discharged instead, giving oxygen.
Concentration of Cl⁻ decides; high Cl⁻ → chlorine, low → oxygen from hydroxide/water B1
(c) Hydrogen; sodium is more reactive than hydrogen, so hydrogen ions (from water) are discharged instead.
Hydrogen B1; Na more reactive than H B1
Q11Electrolysis: products & electrode equations · Standard · Paper 2 style · 5 marks
Concentrated hydrochloric acid is electrolysed using carbon electrodes.
(a)Write the ionic half-equation for the reaction at the cathode.[1]
(b)Write the ionic half-equation for the reaction at the anode.[1]
(c)Describe a test for the gas formed at the anode, with the result.[2]
(d)Write the overall equation for the electrolysis.[1]
Show answer
(a) 2H⁺ + 2e⁻ → H₂
B1
(b) 2Cl⁻ → Cl₂ + 2e⁻
B1
(c) Damp blue litmus paper turns red and then is bleached (white).
Damp litmus B1; bleached B1
(d) 2HCl → H₂ + Cl₂
B1
Q12Electrolysis: products & electrode equations · Standard · Paper 2 style · 5 marks
Each electrolyte is electrolysed with inert electrodes.1 molten potassium bromide2 concentrated aqueous potassium bromide3 aqueous silver nitrate4 concentrated aqueous copper(II) chloride
(a)Name the product at the cathode for each electrolyte.[2]
(b)Name the product at the anode for each electrolyte.[2]
(c)Explain why the anode product for electrolyte 3 is different from the others.[1]
Show answer
(a) 1 potassium; 2 hydrogen; 3 silver; 4 copper
All four B2; two or three B1
(b) 1 bromine; 2 bromine; 3 oxygen; 4 chlorine
All four B2; two or three B1
(c) Nitrate ions are not discharged (there is no halide ion), so hydroxide ions/water are discharged, giving oxygen.
No halide present; OH⁻ from water discharged instead B1
Q13Electrolysis: products & electrode equations · Standard · Paper 1 style · 1 mark
(a)Aqueous sodium sulfate is electrolysed with inert electrodes. A few drops of universal indicator are added. What colour is seen near each electrode?[1]
Show answer
(a) B cathode blue/purple, anode red
At the cathode H⁺ is removed (H₂ forms), leaving excess OH⁻, so the solution becomes alkaline (blue/purple). At the anode OH⁻ is removed (O₂ forms), leaving excess H⁺ (red). A has the two reversed.
Q14Electrolysis: products & electrode equations · Standard · Paper 2 style · 4 marks
(a)During the electrolysis of concentrated aqueous sodium chloride, the solution left behind becomes alkaline. Name the alkali formed.[1]
(b)Explain why the solution becomes alkaline.[2]
(c)Write the ionic half-equation for the reaction at the cathode.[1]
Show answer
(a) sodium hydroxide
B1
(b) H⁺ ions are removed at the cathode (as H₂) and Cl⁻ ions at the anode (as Cl₂); Na⁺ and OH⁻ ions are left in solution.
H⁺ and Cl⁻ removed B1; Na⁺ and OH⁻ remain B1
(c) 2H⁺ + 2e⁻ → H₂
B1
Q15Electrolysis: products & electrode equations · Standard · Paper 2 style · 4 marks
(a)Explain why sodium metal can be produced by electrolysing molten sodium chloride but not by electrolysing aqueous sodium chloride.[2]
(b)Explain, in terms of electrons, why the reaction at the cathode is reduction.[1]
(c)Give one reason why electrolysis of molten compounds uses a lot of energy.[1]
Show answer
(a) In aqueous solution, H⁺ ions from the water are also present; hydrogen is less reactive than sodium, so H⁺ is discharged instead of Na⁺. In the melt only Na⁺ is present at the cathode.
Water provides H⁺ ions B1; H⁺ discharged in preference because sodium is more reactive (only Na⁺ in the melt) B1
(b) The ions gain electrons at the cathode.
B1
(c) Energy is needed to melt the compound (high melting point) as well as to drive the current.
B1
Q16Electrolysis: products & electrode equations · Challenge · Paper 2 style · 4 marks
Molten lead(II) bromide is electrolysed until 4.14 g of lead has formed at the cathode.[Ar: Pb, 207; Br, 80]
(a)Calculate the number of moles of lead formed.[1]
(b)Calculate the mass of bromine formed at the anode.[2]
(c)Calculate the number of moles of electrons transferred.[1]
Q17Electrolysis: products & electrode equations · Challenge · Paper 2 style · 5 marks
Dilute sulfuric acid is electrolysed with platinum electrodes. 60 cm³ of hydrogen is collected at the cathode, measured at room temperature and pressure.[One mole of gas occupies 24 dm³ at r.t.p.]
(a)State the volume of oxygen formed at the anode at r.t.p.[1]
(b)Write the half-equation for the formation of oxygen at the anode.[1]
(c)Calculate the number of moles of electrons that passed.[2]
(d)Explain why the concentration of the sulfuric acid increases during the electrolysis.[1]
(d) Water is used up (decomposed into hydrogen and oxygen), so the same amount of acid is in less water.
B1
Q18Electrolysis: products & electrode equations · Challenge · Paper 2 style · 4 marks
Aqueous copper(II) sulfate is electrolysed using inert electrodes. 1.28 g of copper is deposited on the cathode.[Ar: Cu, 64. One mole of gas occupies 24 dm³ at r.t.p.]
(a)Calculate the number of moles of electrons needed to deposit this copper.[2]
(b)The anode reaction is 4OH⁻ → O₂ + 2H₂O + 4e⁻. Calculate the volume of oxygen formed at r.t.p., in cm³.[2]
Show answer
(a) 0.040 mol
1.28 ÷ 64 = 0.020 mol Cu M1; Cu²⁺ + 2e⁻ → Cu, so 0.040 mol e⁻ A1
Q19Electrolysis: products & electrode equations · Challenge · Paper 2 style · 5 marks
A jeweller electrolyses aqueous silver nitrate with inert electrodes. 2.16 g of silver forms on the cathode.[Ar: Ag, 108. One mole of gas occupies 24 dm³ at r.t.p.]
(a)Write the half-equation for the reaction at the cathode.[1]
(b)Calculate the volume of oxygen, at r.t.p., formed at the anode. Give your answer in cm³.[3]
(c)Suggest what happens to the pH of the solution during the electrolysis. Explain.[1]
(c) pH decreases (becomes acidic): OH⁻ removed at anode leaves H⁺ / nitric acid in solution.
B1
Q20Electrolysis: products & electrode equations · Challenge · Paper 4 style · 5 marks
A student electrolyses dilute sulfuric acid with carbon electrodes and collects the gases in two inverted measuring cylinders full of the acid.Time / min | 1 | 2 | 3 | 4 | 5Volume at cathode / cm³ | 8 | 16 | 24 | 32 | 40Volume at anode / cm³ | 3 | 7 | 11 | 15 | 19
(a)Calculate the rate at which gas forms at the cathode, in cm³/min.[1]
(b)Predict the volume at the anode after 5 minutes if the gases behaved exactly as the equation predicts.[1]
(c)After the first minute, the anode volume increases by 4 cm³ each minute. Use this to show that the ratio of the rates of formation is as expected.[1]
(d)Suggest two reasons why less oxygen was collected than expected.[2]
Show answer
(a) 8 cm³/min
40 ÷ 5 = 8 B1
(b) 20 cm³
Half of 40 B1
(c) 8 : 4 = 2 : 1, the expected ratio of H₂ to O₂
B1
(d) Any two of: oxygen is more soluble in water than hydrogen, so some dissolves in the acid at first; some oxygen reacts with the carbon anode to form carbon dioxide; a leak/gas lost before collection began.
1 mark each for any two: oxygen dissolves in the solution (more soluble than H₂); oxygen reacts with/oxidises the carbon anode (CO₂ formed); gas lost/leak
Q21Electrolysis: products & electrode equations · Challenge · Paper 2 style · 5 marks
An aqueous solution of an unknown salt Z is electrolysed with carbon electrodes. A pink-brown solid forms on the cathode. A pale yellow-green gas forms at the anode; it bleaches damp litmus paper.
(a)Identify the products at each electrode.[2]
(b)Suggest the name of salt Z and state whether the solution is concentrated or dilute. Explain.[2]
(c)Copper forms at the cathode but hydrogen does not. Explain why.[1]
Show answer
(a) cathode: copper; anode: chlorine
Each B1
(b) Copper(II) chloride, concentrated: chlorine (not oxygen) is formed, so the chloride ion concentration must be high.
Copper(II) chloride B1; concentrated, because Cl⁻ discharged rather than OH⁻ B1
(c) Copper is less reactive than hydrogen, so Cu²⁺ ions are discharged in preference to H⁺ ions.
B1
Q22Electrolysis: products & electrode equations · Challenge · Paper 4 style · 5 marks
A student electrolyses concentrated aqueous potassium iodide with carbon electrodes in a U-tube. Observations:cathode: bubbles of a colourless gas; a drop of phenolphthalein near this electrode turns pinkanode: the colourless solution turns yellow-brown; no gas is seen
(a)Name the product at the anode and write its half-equation.[2]
(b)Name the gas at the cathode and explain why the phenolphthalein turns pink.[2]
(c)Suggest how the student could confirm that the yellow-brown substance is iodine.[1]
Show answer
(a) iodine; 2I⁻ → I₂ + 2e⁻
Name B1; equation B1
(b) Hydrogen; H⁺ ions from water are discharged, leaving OH⁻ ions (with K⁺), so the solution near the cathode becomes alkaline.
Hydrogen B1; OH⁻ left in excess / potassium hydroxide forms B1
(c) Add starch solution: it turns blue-black.
B1
Q23Electrolysis: products & electrode equations · Challenge · Paper 2 style · 5 marks
(a)Aqueous sodium sulfate is electrolysed with inert electrodes. Write half-equations for the reactions at the cathode and at the anode.[2]
(b)Explain why neither sodium nor any sulfur-containing product forms.[2]
(c)Explain why the total amount of sodium sulfate in the solution does not change, even though the electrolysis continues for an hour.[1]
(b) Sodium is more reactive than hydrogen so H⁺ is discharged instead of Na⁺; sulfate ions are not discharged, so hydroxide ions/water are discharged instead.
Na⁺ not discharged as Na is more reactive than H B1; SO₄²⁻ not discharged, OH⁻ discharged instead B1
(c) Only water is decomposed; the Na⁺ and SO₄²⁻ ions stay in solution.
B1
Q24Cells & hydrogen fuel cells · Core · Paper 2 style · 3 marks
(a)Write the overall equation for the reaction in a hydrogen–oxygen fuel cell.[1]
(b)State one advantage of a hydrogen fuel cell car compared with a petrol car.[1]
(c)State one disadvantage of hydrogen fuel cell cars.[1]
Show answer
(a) 2H₂ + O₂ → 2H₂O
B1
(b) The only product is water (no CO₂, CO or oxides of nitrogen from the car).
B1
(c) Any one of: hydrogen is difficult/expensive to store (high pressure); few refuelling stations; hydrogen is flammable/explosive; much hydrogen is made from fossil fuels.
Any one valid disadvantage B1
Q25Cells & hydrogen fuel cells · Core · Paper 1 style · 1 mark
(a)What is produced, apart from electrical energy, in a hydrogen–oxygen fuel cell?[1]
Show answer
(a) B water only
Hydrogen combines with oxygen to form water; there is no carbon in the fuel, so no carbon dioxide is made (unlike option C, from burning a hydrocarbon).
Q26Cells & hydrogen fuel cells · Standard · Paper 2 style · 4 marks
(a)A delivery company in Islamabad is choosing between petrol vans and hydrogen fuel cell vans. Give two environmental advantages of the fuel cell vans.[2]
(b)Explain why using fuel cell vans may still cause some carbon dioxide emission overall.[1]
(c)Give one practical disadvantage for the company.[1]
Show answer
(a) No carbon dioxide (greenhouse gas) from the van; no carbon monoxide, oxides of nitrogen or particulates (less air pollution / smog).
1 mark each for any two: no CO₂; no CO; no NOₓ/SO₂; no particulates; only water produced
(b) The hydrogen is often made using fossil fuels (e.g. from methane) or electricity from fossil-fuel power stations.
B1
(c) Hydrogen is hard to store / few filling stations / vans more expensive.
B1
Q27Cells & hydrogen fuel cells · Challenge · Paper 2 style · 5 marks
A fuel cell bus uses 0.40 kg of hydrogen on a short journey.[Ar: H, 1; O, 16. One mole of gas occupies 24 dm³ at r.t.p.]
(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, at r.t.p., in dm³.[1]
(d)In the fuel cell, hydrogen loses electrons at one electrode. State whether hydrogen is oxidised or reduced. Explain.[1]
Show answer
(a) 200 mol
400 g ÷ 2 = 200 B1
(b) 3.6 kg
moles H₂O = moles H₂ = 200 M1; 200 × 18 = 3600 g = 3.6 kg A1
(c) 2400 dm³
100 mol × 24 = 2400 B1
(d) Oxidised, because it loses electrons.
B1
Q28Electroplating & industrial electrolysis · Core · Paper 2 style · 5 marks
A craftsman in Peshawar electroplates a steel spoon with silver.
(a)State what should be used as the cathode and what should be used as the anode.[2]
(b)Suggest a suitable electrolyte.[1]
(c)Give two reasons why metals are electroplated.[2]
Show answer
(a) cathode: the steel spoon; anode: a piece of (pure) silver
Each B1
(b) aqueous silver nitrate (a solution containing silver ions)
B1
(c) To improve appearance; to resist corrosion.
1 mark each: appearance; prevent corrosion/rusting
Q29Electroplating & industrial electrolysis · Standard · Paper 2 style · 5 marks
Aluminium is extracted by electrolysis of aluminium oxide, Al₂O₃, dissolved in molten cryolite. The electrodes are made of carbon (graphite).
(a)Explain why the aluminium oxide is dissolved in molten cryolite.[1]
(b)Write the half-equations for the reactions at the cathode and at the anode.[2]
(c)Explain why the carbon anodes must be replaced regularly.[2]
Show answer
(a) It lowers the operating temperature (aluminium oxide has a very high melting point), saving energy.
B1
(b)Cathode: Al³⁺ + 3e⁻ → AlAnode: 2O²⁻ → O₂ + 4e⁻
Each B1
(c) The oxygen formed reacts with the hot carbon anodes to form carbon dioxide, so the anodes burn away.
Oxygen reacts with carbon B1; forms CO₂ so anode wears away B1
Q30Electroplating & industrial electrolysis · Challenge · Paper 2 style · 5 marks
Impure copper is purified by electrolysis using aqueous copper(II) sulfate. In one cell the impure copper anode loses 12.8 kg and the pure copper cathode gains 12.2 kg. Assume all the copper that leaves the anode is deposited on the cathode, and that the impurities do not dissolve but fall to the bottom of the cell.
(a)State what the cathode is made of at the start.[1]
(b)Write the half-equation for the reaction at the anode.[1]
(c)Calculate the percentage of copper in the part of the anode that was used up. Give your answer to 3 significant figures.[2]
(d)Calculate the mass of impurities (sludge) collected below the anode.[1]
Show answer
(a) a thin sheet of pure copper
B1
(b) Cu → Cu²⁺ + 2e⁻
B1
(c) 95.3 %
12.2 ÷ 12.8 M1; × 100 = 95.3 % A1
(d) 0.6 kg
12.8 − 12.2 = 0.6 kg B1
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).
