Commercial electrochemical cellsAQA A-Level Chemistry: Subtopic test

1. 1
   A manufacturer makes rechargeable nickel–cadmium cells for cordless power tools. The cell is based on two half-equations with standard electrode potentials: NiO(OH) + H₂O + e⁻ ⇌ Ni(OH)₂ + OH⁻, E = +0.52 V; and Cd(OH)₂ + 2e⁻ ⇌ Cd + 2OH⁻, E = −0.88 V. The electrolyte is aqueous alkali.

   (a)

   What is the EMF of the nickel–cadmium cell under standard conditions?

   [1 mark]

   • A+0.36 V
   • B+1.40 V
   • C+0.88 V
   • D+1.92 V

   (b)

   Which reaction occurs at the negative electrode while the cell is supplying current?

   [1 mark]

   • ACd(OH)₂ + 2e⁻ → Cd + 2OH⁻
   • BNiO(OH) + H₂O + e⁻ → Ni(OH)₂ + OH⁻
   • CNi(OH)₂ + OH⁻ → NiO(OH) + H₂O + e⁻
   • DCd + 2OH⁻ → Cd(OH)₂ + 2e⁻

   (c)

   Explain why this cell can be recharged.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A lithium cell in a mobile phone uses a lithium-containing negative electrode and a cobalt(IV) oxide positive electrode. When the cell supplies current, the simplified electrode reactions are: negative electrode, Li → Li⁺ + e⁻; positive electrode, Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻.

   (a)

   What happens to the cobalt at the positive electrode as the cell supplies current?

   [1 mark]

   • AIt is reduced from oxidation state +4 to +3
   • BIt is oxidised from oxidation state +3 to +4
   • CIt is reduced from oxidation state +3 to +2
   • DIt is oxidised from oxidation state +4 to +5

   (b)

   Which statement describes the flow of charge when the cell supplies current to the phone?

   [1 mark]

   • AElectrons flow in the external circuit from the cobalt oxide electrode to the lithium electrode
   • BElectrons flow through the electrolyte from the lithium electrode to the cobalt oxide electrode
   • CElectrons flow in the external circuit from the lithium electrode to the cobalt oxide electrode
   • DElectrons flow alternately in each direction around the circuit

   (c)

   Explain how the electrode reactions in this cell generate an electric current.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A bus company is trialling hydrogen fuel-cell buses that use an alkaline hydrogen–oxygen fuel cell. Hydrogen and oxygen are fed continuously to separate porous electrodes in contact with aqueous potassium hydroxide. The relevant electrode potentials are: O₂ + 2H₂O + 4e⁻ ⇌ 4OH⁻, E = +0.40 V; and 2H₂O + 2e⁻ ⇌ H₂ + 2OH⁻, E = −0.83 V.

   (a)

   Deduce the equation for the reaction at each electrode when the cell is supplying current, and calculate the EMF of the cell.

   [3 marks]

   (b)

   Evaluate the use of this fuel cell rather than a rechargeable battery to power the buses, giving two benefits and one risk or limitation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A battery supplier compares two types of cell. Cell X is a non-rechargeable zinc–silver oxide cell for hearing aids, with electrode potentials Zn(OH)₂ + 2e⁻ ⇌ Zn + 2OH⁻, E = −1.25 V, and Ag₂O + H₂O + 2e⁻ ⇌ 2Ag + 2OH⁻, E = +0.34 V. Cell Y is a rechargeable lead–acid cell for vehicles, with electrode potentials PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ ⇌ PbSO₄ + 2H₂O, E = +1.69 V, and PbSO₄ + 2e⁻ ⇌ Pb + SO₄²⁻, E = −0.36 V.

   (a)

   Use the data for Cell Y to deduce the electrode reaction at each electrode when it supplies current and the EMF of the cell, and explain how these reactions generate a current and why the cell can be recharged.

   [6 marks]

   (b)

   Use the data for Cell X to deduce the electrode reactions and EMF when it supplies current. Evaluate the use of a non-rechargeable cell such as Cell X, compared with a rechargeable cell, for a hearing aid.

   [6 marks]

   Total for question 4: 12 marks

End of questions