The questions are on the electrochemical cells test. Take it under real conditions before reading the mark scheme.
Quick answer: A galvanic cell turns a spontaneous redox reaction into electricity: oxidation at the anode, reduction at the cathode. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on electrochemical cells, from standard conditions to brine electrolysis, with the equivalent answers to accept.
Use these model answers and the marking notes to score yourself out of 20. Method marks are awarded for the correct subtraction of potentials and for the correct mole ratio in the electrolysis calculations.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.2
Question 1 Standard conditions (2 marks)
Any two of: temperature 298 K; ion concentration 1.00 mol/dm³; gas pressure 100 kPa. The potential of the standard hydrogen electrode is 0.00 V.Marking: 1 mark for two correct conditions; 1 mark for 0.00 V.
Question 2 Magnesium–silver cell (3 marks)
- E°cell = 0.80 − (−2.37) = +3.17 V.
- Mg + 2Ag⁺ → Mg²⁺ + 2Ag.
- The anode is magnesium (oxidation).
Marking: 1 mark for each part.
Question 3 Iron(III) and iodide (2 marks)
E°cell = 0.77 − 0.54 = +0.23 V. The value is positive, so the reaction is feasible: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.Marking: 1 mark for +0.23 V; 1 mark for the conclusion.
Question 4 Refining copper (2 marks)
- At the anode (impure copper, right), Cu → Cu²⁺ + 2e⁻.
- At the cathode (pure copper, left), Cu²⁺ + 2e⁻ → Cu.
Marking: 1 mark for each part.
Question 5 Nickel plating (3 marks)
t = 2700 s; Q = 1.50 × 2700 = 4050 C. n(e⁻) = 4050 ÷ 96 500 = 0.04197 mol. Ni²⁺ + 2e⁻ → Ni, so n(Ni) = 0.02098 mol. m = 0.02098 × 58.7 = 1.23 g.Marking: 1 mark for the charge; 1 mark for moles of nickel; 1 mark for the mass.
Question 6 Hydrogen fuel cell (3 marks)
- Anode: H₂ → 2H⁺ + 2e⁻. Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O.
- A fuel cell can run continuously while fuel is supplied and does not need hours of recharging.
Marking: (a) 1 mark for each half-equation. (b) 1 mark for a valid advantage.
Question 7 The salt bridge (2 marks)
The reading would fall to zero. Without the salt bridge the circuit is incomplete, so ions cannot move to balance the charge, and the build-up of positive charge in the zinc half-cell stops the flow of electrons.Marking: 1 mark for the reading falling to zero; 1 mark for the circuit being incomplete or charge building up.
Question 8 Brine electrolysis (3 marks)
Water (E° = −0.83 V) is reduced more easily than Na⁺ (E° = −2.71 V), so the cathode reaction is 2H₂O + 2e⁻ → H₂ + 2OH⁻. At the anode the chloride ions are oxidised because the solution is concentrated: 2Cl⁻ → Cl₂ + 2e⁻, forming chlorine.Marking: 1 mark for comparing E° values; 1 mark for hydrogen formed; 1 mark for chlorine with equation.
