The questions are on the electrolysis test. Take it under real conditions before reading the mark scheme.
Quick answer: Electrolysis uses direct current to split a molten or dissolved ionic compound. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 9-10 test on electrolysis, from reading the cell to electroplating, with the equivalent answers to accept.
Use the marking notes to award marks. Method marks are given for correct ratios and electron balance even when a final number is wrong, and equations must be balanced for charge as well as atoms. Add the marks to a total out of 20.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C4
Question 1 Reading the cell (2 marks)
- Q is the cathode because it is joined to the short plate, the negative terminal. (1 mark)
- The positive copper ions move to Q and gain electrons to form copper metal. (1 mark)
Marking: 1 mark each; accept ‘negative electrode’ for the reason in (a).
Question 2 Molten lithium chloride (3 marks)
- Cathode: lithium; anode: chlorine. (1 mark)
- Li⁺ + e⁻ → Li (1 mark)
- 2Cl⁻ → Cl₂ + 2e⁻ (1 mark)
Marking: balanced equations needed; allow ecf from (a).
Question 3 Copper(II) nitrate solution (3 marks)
- Copper forms; copper is less reactive than hydrogen, so Cu²⁺ ions are discharged more easily than H⁺. (2 marks)
- Oxygen, from hydroxide ions, because no halide ion is present. (1 mark)
Marking: 1 mark for product, 1 for reactivity reason; 1 for oxygen.
Question 4 Splitting acidified water (3 marks)
- Volume of O₂ = 20 cm³. (1 mark)
- Two electrons are needed for each H₂ but four for each O₂, so the same flow of electrons makes twice as many moles of H₂ as O₂. Gas volumes follow mole ratios, so oxygen is 40 ÷ 2 = 20 cm³. (2 marks)
Marking: 1 mark for electron balance, 1 for the 2 : 1 ratio.
Question 5 Making aluminium (3 marks)
- It lowers the melting point of the mixture, saving energy. (1 mark)
- Al³⁺ + 3e⁻ → Al (1 mark)
- Oxygen reacts with the hot carbon to make carbon dioxide, so the anodes burn away. (1 mark)
Marking: allow ‘dissolves the oxide’ in (a) with the energy idea.
Question 6 Uses of brine products (2 marks)
Any two: chlorine for disinfecting water or making PVC; hydrogen as a fuel; sodium hydroxide for soap or paper. Two product-use pairs.Marking: 1 mark per correct product with use.
Question 7 Sodium hydroxide mass (2 marks)
n(NaOH) = 2 × 0.10 = 0.20 mol; M = 40 g/mol; m = 0.20 × 40 = 8.0 g.Marking: 1 mark for the 2 : 1 ratio, 1 for the mass.
Question 8 Electroplating (2 marks)
The key is the cathode because positive nickel ions, Ni²⁺, are attracted to the negative electrode and gain electrons to form a nickel coating. The nickel anode dissolves and replaces the ions used, keeping the solution unchanged. Key = cathode; nickel anode replenishes ions.Marking: 1 mark for cathode reasoning, 1 for the anode replacing ions.
