IGCSE Chemistry 0620 Paper 4: Electrolysis, Electroplating and Charge Calculations

Level: Year 10-11 (IGCSE) | Subject: Chemistry 0620 | Time: 75 minutes | Total: 100 marks | Parts: 31

Paper at a glance

Paper	Time	Marks	Calculator
Paper 4 (Extended, structured)	1 h 15 min	75	Not allowed

Questions

Question 1 (16 marks) – Principles of electrolysis and electrolytes

(a) Define electrolysis and state the three conditions needed for it to occur. (3 marks)

(b) Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. (3 marks)

(c) State which terminal of the supply the anode and the cathode are connected to, and the type of reaction at each electrode. (3 marks)

(d) Describe the direction of movement of cations and anions, and its relation to the current in the external circuit. (3 marks)

(e) Five substances are tested: molten lead(II) bromide, ethanol, sugar solution, dilute sulfuric acid and a copper strip. State which conduct and explain. (4 marks)

Question 2 (17 marks) – Predicting the products of electrolysis

(a) Molten lead(II) bromide is electrolysed with carbon electrodes. State the product at each electrode. (3 marks)

(b) Write the half-equation for the cathode and for the anode in that electrolysis. (3 marks)

(c) Copper(II) sulfate solution is electrolysed with copper electrodes. Describe the change at each electrode. (3 marks)

(d) The same solution is electrolysed with platinum electrodes. State how the products differ and explain why. (4 marks)

(e) Explain why the order of discharge of ions has to be considered in aqueous solutions. (4 marks)

Question 3 (16 marks) – Electroplating: principle and applications

(a) State the three requirements for setting up electroplating. (3 marks)

(b) Describe the apparatus for silver plating an iron spoon and the change seen after the current is switched on. (3 marks)

(c) Why is pure silver used as the anode, and why does the silver ion concentration stay roughly constant? (4 marks)

(d) A current of 0.50 A is passed for 3600 s to plate silver (Ag+ + e- → Ag). Calculate the mass of silver deposited (Ag = 108). (3 marks)

(e) Why is the surface rubbed with abrasive paper and then cleaned with alkali before plating? (3 marks)

Question 4 (18 marks) – Calculations on charge, moles and mass deposited

(a) A current of 2.0 A flows for 1800 s. Calculate the charge passed. (3 marks)

(b) From the answer above, calculate the number of moles of electrons passed (F = 96500 C/mol). (3 marks)

(c) If the cathode reaction is Cu2+ + 2e- → Cu, calculate the mass of copper deposited (Cu = 63.5). (3 marks)

(d) Calculate the charge and the time needed at 2.0 A to deposit 1.00 g of copper. (3 marks)

(e) A current of 4.0 A is passed for 7200 s through molten aluminium oxide (Al3+ + 3e- → Al). Calculate the mass of aluminium formed (Al = 27). (3 marks)

(f) A cell runs at 5.0 V and 2.0 A for 1800 s. Calculate the energy used in J and in kWh, and the cost at 1.20 dollars per kWh. (3 marks)

Question 5 (17 marks) – Electrolysis of concentrated sodium chloride solution and its industrial use

(a) Concentrated sodium chloride solution is electrolysed with inert electrodes. State the product at each electrode and the substance formed in the solution. (3 marks)

(b) Write the half-equation at each electrode in that electrolysis. (3 marks)

(c) A current of 1.0 A is passed for 3600 s. Calculate the volume of chlorine at room temperature and pressure (molar volume 24 dm³/mol). (3 marks)

(d) In the same electrolysis, what is the volume ratio of hydrogen at the cathode to chlorine at the anode? If the solution volume is 500 cm³, calculate the concentration of hydroxide ions. (4 marks)

(e) Explain why industry uses a diaphragm to separate the anode and cathode compartments. (4 marks)

Question 6 (16 marks) – Practical technique, redox and safety

(a) In the laboratory, how can the anode be identified? Give two observations you could use. (3 marks)

(b) Why must a direct current be used for electrolysis instead of an alternating current? (3 marks)

(c) State which electrode is where oxidation occurs and which is where reduction occurs, and explain how electrons and ions complete the circuit. (4 marks)

(d) After some time the electrolyte warms up. What effect does this have on the electrolysis? (3 marks)

(e) State three safety precautions for carrying out an electrolysis experiment. (3 marks)

Answers and marking notes

Question 1 – Principles of electrolysis and electrolytes

(a) Electrolysis is the breakdown of an electrolyte by passing a direct current through it (1 mark); a direct current supply is needed (1 mark); an electrolyte that is molten or in aqueous solution plus two electrodes in a closed circuit are needed (1 mark). [3 marks]

(b) In the solid state the sodium and chloride ions are held in a fixed lattice and cannot move (1 mark); when molten the ions are free to move (1 mark); so the molten salt can carry charge and conduct (1 mark). [3 marks]

(c) The anode is connected to the positive terminal and oxidation happens there (1 mark); the cathode is connected to the negative terminal and reduction happens there (1 mark); electrons flow through the external circuit from anode to cathode (1 mark). [3 marks]

(d) Cations move towards the cathode (1 mark); anions move towards the anode (1 mark); this ion movement together with the electron flow in the external circuit completes the circuit (1 mark). [3 marks]

(e) Molten lead(II) bromide and dilute sulfuric acid conduct (2 marks); both contain ions that are free to move (1 mark); ethanol and sugar solution contain only molecules and copper conducts by free electrons rather than ions, so they are not electrolytes (1 mark). [4 marks]

Question 2 – Predicting the products of electrolysis

(a) Lead is formed at the cathode (1 mark); bromine is formed at the anode (1 mark); together they are the elements present in lead(II) bromide (1 mark). [3 marks]

(b) Cathode: Pb2+ + 2e- → Pb (2 marks, one for the equation and one for balancing); anode: 2Br- → Br2 + 2e- (1 mark). [3 marks]

(c) A red-brown layer of copper is deposited at the cathode (1 mark); the copper anode dissolves (1 mark); the blue colour stays about the same and the concentration is roughly unchanged (1 mark). [3 marks]

(d) Copper is still deposited at the cathode (1 mark); oxygen is produced at the anode instead (1 mark); platinum is inert and does not dissolve (1 mark); hydroxide ions are discharged in preference to sulfate ions (1 mark). [4 marks]

(e) Several ions are present in an aqueous solution at the same time (1 mark); only the ion that is easiest to oxidise or reduce reacts first at the electrode (1 mark); the reactivity series and the relative ease of discharge of halide and hydroxide ions decide the products (1 mark); concentration also affects the order of discharge (1 mark). [4 marks]

Question 3 – Electroplating: principle and applications

(a) The object to be plated is made the cathode (1 mark); the plating metal is made the anode (1 mark); the electrolyte contains ions of that metal (1 mark). [3 marks]

(b) The spoon is the cathode, pure silver is the anode and silver nitrate solution is the electrolyte (2 marks); a silver layer gradually covers the spoon (1 mark). [3 marks]

(c) Silver dissolves from the anode at the same rate as silver is deposited at the cathode (2 marks); so the silver ion concentration stays almost constant (1 mark); pure silver keeps impurities out of the coating (1 mark). [4 marks]

(d) Charge = 0.50 × 3600 = 1800 C (1 mark); moles of electrons = 1800 ÷ 96500 = 0.0187 mol, the same as the moles of silver (1 mark); mass = 0.0187 × 108 = 2.01 g (1 mark). [3 marks]

(e) To remove oxide and grease from the surface (1 mark); so that the surface conducts well (1 mark); so that the coating sticks firmly and evenly (1 mark). [3 marks]

Question 4 – Calculations on charge, moles and mass deposited

(a) Q = I t = 2.0 × 1800 (1 mark); = 3600 C (2 marks, one for the value and one for the unit). [3 marks]

(b) n = 3600 ÷ 96500 (1 mark); = 0.0373 mol (2 marks). [3 marks]

(c) moles of copper = 0.0373 ÷ 2 = 0.0187 mol (1 mark); mass = 0.0187 × 63.5 (1 mark); = 1.18 g (1 mark). [3 marks]

(d) moles of copper = 1.00 ÷ 63.5 = 0.0157 mol (1 mark); charge = 0.0157 × 2 × 96500 = 3.04 × 10³ C (1 mark); time = 3040 ÷ 2.0 = 1520 s (1 mark). [3 marks]

(e) charge = 4.0 × 7200 = 28800 C (1 mark); moles of electrons = 28800 ÷ 96500 = 0.298 mol and moles of aluminium = 0.0995 mol (1 mark); mass = 0.0995 × 27 = 2.69 g (1 mark). [3 marks]

(f) power = 5.0 × 2.0 = 10 W (1 mark); energy = 10 × 1800 = 18000 J = 0.0050 kWh (1 mark); cost = 0.0050 × 1.20 = 0.0060 dollars (1 mark). [3 marks]

Question 5 – Electrolysis of concentrated sodium chloride solution and its industrial use

(a) Hydrogen is released at the cathode (1 mark); chlorine is released at the anode (1 mark); sodium hydroxide is formed in the solution (1 mark). [3 marks]

(b) Cathode: 2H+ + 2e- → H2, or 2H2O + 2e- → H2 + 2OH- (2 marks); anode: 2Cl- → Cl2 + 2e- (1 mark). [3 marks]

(c) charge = 3600 C and moles of electrons = 0.0373 mol (1 mark); moles of chlorine = 0.0373 ÷ 2 = 0.0187 mol (1 mark); volume = 0.0187 × 24 = 0.448 dm³, that is 448 cm³ (1 mark). [3 marks]

(d) The same number of moles of electrons passes at both electrodes and each mole of gas needs 2 mol of electrons (1 mark); so the ratio is 1:1 and the hydrogen volume is also 448 cm³ (1 mark); hydroxide ions = 0.0373 mol (1 mark); concentration = 0.0373 ÷ 0.500 = 0.0746 mol/dm³ (1 mark). [4 marks]

(e) To stop chlorine from mixing with the sodium hydroxide solution (1 mark); the two react to form sodium chlorate(I), which lowers the value of the products (1 mark); separating them allows chlorine and hydrogen to be collected and gives a purer sodium hydroxide (1 mark); it also avoids an explosive mixture of hydrogen and chlorine (1 mark). [4 marks]

Question 6 – Practical technique, redox and safety

(a) The electrode connected to the positive terminal (1 mark); bubbles or dissolving of the electrode and colour changes can be observed (1 mark); damp starch iodide paper turns blue-black with chlorine (1 mark). [3 marks]

(b) An alternating current keeps reversing direction (1 mark); the reactions at the electrodes reverse every half cycle (1 mark); the products cannot build up at a fixed electrode, so the expected products are not obtained (1 mark). [3 marks]

(c) Oxidation occurs at the anode (1 mark); reduction occurs at the cathode (1 mark); electrons flow through the external circuit from anode to cathode (1 mark); ions move through the electrolyte and together they keep the circuit electrically neutral (1 mark). [4 marks]

(d) The resistance of the electrolyte turns some electrical energy into heat (1 mark); the ions move faster and the current rises slightly (1 mark); but the solution may evaporate, its concentration may change and the quality of the product may fall (1 mark). [3 marks]

(e) Work in a fume cupboard to avoid breathing in chlorine (1 mark); use a low voltage direct current supply and avoid short circuits (1 mark); wear gloves and goggles to avoid contact with corrosive electrolytes (1 mark). [3 marks]

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