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) Zn²⁺(aq) + 2e⁻ ⇌ Zn(s) (\(E^{\ominus} = -0.76\text{ V}\))
• (2) Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq) (\(E^{\ominus} = +0.77\text{ V}\))
• (3) Ag⁺(aq) + e⁻ ⇌ Ag(s) (\(E^{\ominus} = +0.80\text{ V}\))
(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 (Fe³⁺) to form iron(II) ions (Fe²⁺) under standard conditions. [4]
Show Mark Scheme
(a)
- (i) \(\text{Zn}(\text{s}) \mid \text{Zn}^{2+}(\text{aq}) \parallel \text{Fe}^{3+}(\text{aq}), \text{Fe}^{2+}(\text{aq}) \mid \text{Pt}(\text{s})\) [2] (1 mark for Zn half-cell correctly on left; 1 mark for Fe³⁺,Fe²⁺ half-cell with Pt electrode on right)
- (ii) \(E^{\ominus}_{\text{cell}} = E^{\ominus}(\text{RHS}) - E^{\ominus}(\text{LHS}) = +0.77 - (-0.76) =\) +1.53 V [1]
- (iii) Zn(s) + 2Fe³⁺(aq) → Zn²⁺(aq) + 2Fe²⁺(aq) [2] (1 mark for correct reactants and products; 1 mark for correct balancing)
(b)
- The potential reaction is: Ag(s) + Fe³⁺(aq) → Ag⁺(aq) + Fe²⁺(aq) [1]
- Fe³⁺ is reduced (cathode) and Ag is oxidised (anode). \(E^{\ominus}_{\text{cell}} = E^{\ominus}(\text{Fe}^{3+}/\text{Fe}^{2+}) - E^{\ominus}(\text{Ag}^+/\text{Ag})\) [1]
- \(E^{\ominus}_{\text{cell}} = +0.77\text{ V} - (+0.80\text{ V}) =\) −0.03 V [1]
- Since \(E^{\ominus}_{\text{cell}}\) 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]