▶️ Answer/Explanation
(a)(i) Three possible rate equations:
1. rate = k[A][B]
2. rate = k[A]²
3. rate = k[B]²
Explanation: When both concentrations double, the rate quadruples (from 7.62×10⁻⁴ to 3.05×10⁻³), indicating a second-order reaction. The rate could depend on either [A]², [B]², or [A][B].
(a)(ii) Calculation of rate constant k:
Using rate = k[A][B]:
k = rate / ([A][B]) = 7.62 × 10⁻⁴ / (0.01 × 0.01) = 7.62 mol⁻¹ dm³ s⁻¹
Explanation: The units of k are derived from the second-order rate equation (mol⁻¹ dm³ s⁻¹).
(a)(iii) Half-life explanation:
The equation \(k = 0.693/t_{1/2}\) applies only to first-order reactions. Since this reaction is second-order, the half-life depends on initial concentrations and cannot be calculated using this formula.
(b)(i) Heterogeneous catalysts: Platinum (Pt) and Rhodium (Rh).
Explanation: These metals are used in catalytic converters to reduce NO₂ emissions from car exhausts.
(b)(ii) Mode of action of iron catalyst:
1. Adsorption of N₂ and H₂ onto the iron surface.
2. Weakening of N≡N and H-H bonds.
3. Formation of NH₃, which then desorbs from the surface.
Explanation: The iron catalyst provides a surface for reactant molecules to interact, lowering the activation energy.
(b)(iii) Homogeneous catalysis equations:
1. \(S_2O_8^{2-} + 2Fe^{2+} → 2SO_4^{2-} + 2Fe^{3+}\)
2. \(2Fe^{3+} + 2I^- → 2Fe^{2+} + I_2\)
Explanation: Fe²⁺ is oxidised to Fe³⁺ by S₂O₈²⁻, then Fe³⁺ oxidises I⁻ to I₂ while being reduced back to Fe²⁺.
(b)(iv) Catalyst difference:
• Homogeneous: Catalyst and reactants in the same phase (e.g., all in solution).
• Heterogeneous: Catalyst and reactants in different phases (e.g., solid catalyst with gaseous reactants).
(c)(i) Stable oxidation states:
Iron exhibits stable +2 and +3 states because its 3d and 4s orbitals have similar energies, allowing variable electron loss.
(c)(ii) Selected oxidising agents:
1. \(H_2O_2 + 2Fe^{2+} → 2Fe^{3+} + 2OH^-\) (E°cell = +1.71 V)
2. \(ClO^- + 2Fe^{2+} + H_2O → 2Fe^{3+} + Cl^- + 2OH^-\) (E°cell = +1.70 V)
Explanation: Both reactions have positive E°cell values, indicating spontaneous oxidation of Fe²⁺ under alkaline conditions.
