Electrode Potentials Exam Practice
Exam-style practice questions with detailed mark schemes and examiner tips.
Structured Questions
Answer the questions below in the spaces provided. Check your answers with the mark schemes and examiner tips.
Question 1: Electrochemical Cell Calculations & Feasibility
9 marksStandard electrode potentials for three half-cells are shown below:
• (1) Zn^2+(aq) + 2e^- ⇒ Zn(s) (E° = -0.76 V)
• (2) Fe^3+(aq) + e^- ⇒ Fe^2+(aq) (E° = +0.77 V)
• (3) Ag^+(aq) + e^- ⇒ Ag(s) (E° = +0.80 V)
(a) An electrochemical cell is set up by connecting half-cell (1) and half-cell (2).
(i) Write the cell diagram notation for this cell. [2]
(ii) Calculate the standard EMF of this cell. [1]
(iii) Write the overall equation for the reaction that occurs when this cell is delivering current. [2]
(b) Predict, with a calculation and a reason, whether silver metal (Ag) will react with iron(III) ions (Fe^3+) to form iron(II) ions (Fe^2+) under standard conditions. [4]
Show Mark Scheme
(a)
- (i) Zn(s) | Zn^2+(aq) || Fe^3+(aq), Fe^2+(aq) | Pt(s) [2] (1 mark for Zn half-cell correctly on left; 1 mark for Fe^3+, Fe^2+ half-cell with Pt electrode on right)
- (ii) E°cell = E°(RHS) − E°(LHS) = +0.77 − (-0.76) = +1.53 V [1]
- (iii) Zn(s) + 2Fe^3+(aq) → Zn^2+(aq) + 2Fe^2+(aq) [2] (1 mark for correct reactants and products; 1 mark for correct balancing)
(b)
- The potential reaction is: Ag(s) + Fe^3+(aq) → Ag^+(aq) + Fe^2+(aq) [1]
- Fe^3+ is reduced (cathode) and Ag is oxidised (anode). E°cell = E°(Fe^3+/Fe^2+) − E°(Ag^+/Ag) [1]
- E°cell = +0.77 V − (+0.80 V) = -0.03 V [1]
- Since E°cell is negative, the reaction is not thermodynamically feasible under standard conditions [1]
Question 2: The Standard Hydrogen Reference Electrode
6 marks(a) Outline the components and describe the standard conditions required for the Standard Hydrogen Electrode (SHE) to act as a reference electrode. [4]
(b) Explain the role of the platinum black coating on the platinum electrode in the Standard Hydrogen Electrode. [2]
Show Mark Scheme
(a)
- Hydrogen gas bubbled at a pressure of 100 kPa (or 1 bar) [1]
- Solution containing H^+ ions at a concentration of 1.00 mol dm^-3 (e.g. 1.00 mol dm^-3 HCl or 0.50 mol dm^-3 H2SO4) [1]
- Inert platinum electrode [1]
- Temperature maintained at 298 K (or 25 °C) [1]
(b)
- Platinum is chemically inert so it will not react with the acid or gas [1]
- The porous platinum black coating provides a very high surface area to catalyse/establish the equilibrium between H2(g) and H^+(aq) rapidly [1]
Question 3: Hydrogen-Oxygen Fuel Cells
7 marksThe hydrogen-oxygen fuel cell is used in some modern eco-vehicles.
(a) Write the half-equations for the reactions occurring at the anode and cathode of an alkaline hydrogen-oxygen fuel cell, and combine them to give the overall cell reaction. [3]
(b) State two advantages and two disadvantages of using hydrogen-oxygen fuel cells to power vehicles compared to conventional internal combustion engines. [4]
Show Mark Scheme
(a)
- Anode (oxidation): H2 + 2OH^- → 2H2O + 2e^- [1]
- Cathode (reduction): O2 + 2H2O + 4e^- → 4OH^- [1]
- Overall cell reaction: 2H2(g) + O2(g) → 2H2O(l) [1]
(b)
Advantages (any two from):
- Only waste product is water, making it non-polluting at the point of use [1]
- Significantly more efficient at converting chemical energy into electrical energy than combustion engines [1]
- Provides continuous power as long as fuel is supplied, eliminating the need to wait for recharging [1]
Disadvantages (any two from):
- Hydrogen is a gas with low energy density, making it difficult/dangerous to store and transport (requires high-pressure cylinders or cryogenics) [1]
- Most hydrogen is currently produced by steam reforming of methane, which releases carbon dioxide and fossil fuels [1]
- Uses expensive platinum catalysts in the electrodes, increasing production costs [1]
- Refuelling infrastructure is currently very limited compared to petrol stations [1]