Questions on Electrochemistry, rechargeable batteries & fuel cells

1. A lead-acid battery usually consists of six cells in series producing a total of 12.00 volts. The electrolyte is an aqueous solution of sulfuric acid. One of the electrodes is made of lead and the other of lead(IV) oxide.

(a) Identify the anode (negative electrode).

(b) State the half-equations for the reactions occurring at each electrode and deduce the overall equation for the reaction taking place when the battery is producing electricity.

(c) State the equation for the reaction that takes place when the battery is recharging.

(d) Assuming the battery produces electricity under standard conditions, use Section 24 of the data booklet to deduce the standard electrode potential for the half-reaction that takes place at the cathode (positive electrode).

(e) State two advantages of a lead-acid battery compared to a lithium-ion battery.

2. The overall equation for a hydrogen fuel cell is:

H₂(g) + ½O₂(g) → H₂O(l) ΔH^⦵ = − 286 kJ mol^-1

(a) State the equations for the half-reactions taking place at the anode (−) and the cathode (+) when sodium hydroxide is used as the electrolyte.

(b) State the equations for the half-reactions taking place at the anode (−) and the cathode (+) when phosphoric acid is used as the electrolyte.

(c) The theoretical thermodynamic efficiency of a hydrogen fuel cell is 83%. Deduce the value of ΔG^⦵ for a hydrogen fuel cell.

(d) Explain why the value of ΔG^⦵ is less quantitatively than the ΔH^⦵ value for the hydrogen fuel cell.

3. The Nernst equation can be used to determine how changing the concentration affects the electromotive force (EMF) of a cell. The equation is:

E = E^⦵ − (RT / nF) ln Q

(a) Explain why the Nernst equation cannot also be used to determine how changing the temperature affects the EMF of the cell.

(b) Consider a cell made up from a nickel half-cell (Ni(s)/Ni²⁺(aq)) and a silver half-cell (Ag(s)/Ag⁺(aq)).

(i) Use Section 24 of the data booklet to determine the EMF of the cell under standard conditions.

(ii) Calculate the EMF of the cell at 298 K if the concentration of the nickel ions in solution is 0.25 mol dm⁻³ and the concentration of the silver ions in solution is 2.00 mol dm⁻³.

(c) Determine the EMF of a concentration cell made from silver where the concentration of silver ions in one of the half-cells is 0.01 mol dm⁻³ and in the other is 1.00 mol dm⁻³.

4. The Geobacter species of bacteria can be used to oxidise ethanoate ions to carbon dioxide in a microbial fuel cell.

(a) Explain how a microbial fuel cell works.

(b) State the half-equation for the reaction that occurs at the anode.

Electrochemistry, rechargeable batteries & fuel cells questions