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]