2019 H2 Chem P1 Q15 — Kinetics Solution

The answer is D (3 and 4 only).

• Statement 1 is false: by the Arrhenius equation, \(k = A\text{e}^{-E_\text{a}/RT}\), the rate constant depends on temperature and \(E_\text{a}\), not on concentration. Higher concentration raises the rate (rate = k[reactants]) with k unchanged.
• Statement 2 is false: higher concentration gives more particles per unit volume, so more collisions per unit time, and the number of particles with energy \(\geq E_\text{a}\) rises; but the energy distribution is unchanged, so the proportion with energy \(\geq E_\text{a}\) stays the same.
• Statement 3 is true: when T increases, \(\dfrac{E_\text{a}}{RT}\) decreases, so \(\text{e}^{-E_\text{a}/RT}\) increases and k increases.
• Statement 4 is true: on the Maxwell-Boltzmann distribution, at a higher temperature the peak moves lower and to the right, so a larger proportion of particles have energy greater than the (unchanged) activation energy.

• A is wrong: it includes statement 1; concentration does not change k.
• B is wrong: statements 1 and 2 are both false.
• C is wrong: it includes statement 2; concentration changes the number, not the proportion, of particles with energy \(\geq E_\text{a}\).