Question 1
Paper 2Hard11 marksDuring the gas-phase oxidation of nitric oxide to nitrogen dioxide, the overall reaction is:
2NO(g) + O₂(g) → 2NO₂(g)
Initial-rate data were collected at constant temperature.
| Experiment | [NO] / mol dm⁻³ | [O₂] / mol dm⁻³ | Initial rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|
| 1 | 1.00 × 10⁻² | 2.00 × 10⁻² | 1.20 × 10⁻⁵ |
| 2 | 2.00 × 10⁻² | 2.00 × 10⁻² | 4.80 × 10⁻⁵ |
| 3 | 2.00 × 10⁻² | 4.00 × 10⁻² | 9.60 × 10⁻⁵ |
Two mechanisms have been proposed. Each step is elementary.
Mechanism I:
NO(g) + O₂(g) → NO₃(g) (slow)
NO₃(g) + NO(g) → 2NO₂(g) (fast)
Mechanism II:
2NO(g) ⇌ N₂O₂(g) (fast equilibrium)
N₂O₂(g) + O₂(g) → 2NO₂(g) (slow)
For a proposed energy profile for mechanism II, the relative energies of successive stages are:
| Stage along the reaction pathway | Relative energy / kJ mol⁻¹ |
|---|---|
| 2NO(g) + O₂(g) | 0 |
| During elementary step 1 | +35 |
| Between the two steps | +12 |
| During elementary step 2 | +60 |
| 2NO₂(g) | −114 |
Determine the order of reaction with respect to each reactant and write the rate equation.
Assess both proposed mechanisms using the overall equation and the initial-rate data. State which mechanism is consistent with both.
Sketch an energy profile for mechanism II using the values given. Label the reactants, products, intermediate and both transition states. Mark the activation energy of the rate-determining step, and state which step the profile indicates is rate-determining.