Reaction Kinetics: Question 1

Worked solution

Step 1: Recall what a catalyst does (and does not do)

A catalyst speeds up a reaction by providing an alternative reaction pathway with a lower activation energy, $E_A$, the minimum energy that colliding particles need for a collision to be successful. A catalyst is not used up: it takes part in the reaction (for example, by forming an intermediate, or by particles adsorbing onto its surface), but it is regenerated by the end of the reaction, which is why its mass is unchanged.

Crucially, a catalyst does not change the temperature, so it does not change the average kinetic energy of the particles, nor the total number of collisions occurring per second.

Step 2: Use the Boltzmann distribution to explain the effect

At a fixed temperature, the Maxwell–Boltzmann distribution of particle energies is unchanged by the presence of a catalyst (the same fraction of particles has any given energy as before. However, because the catalyst’s pathway has a lower activation energy, $E_A(\text{catalysed}) < E_A(\text{uncatalysed})$, the area under the distribution curve to the right of this new, lower energy threshold is much larger than the area to the right of the original, higher threshold. A greater fraction of the existing collisions therefore now has enough energy to react, so the rate increases) without any change in how often particles collide or how much energy they have on average.

Step 3: Why the other options are wrong

• B: A catalyst does not raise the temperature or add energy to the particles. Average kinetic energy and collision frequency are governed by temperature and concentration/pressure, not by the presence of a catalyst.
• C: A solid catalyst does not change the amount or pressure of the gaseous reactants, so it cannot increase the total number of collisions taking place; the rate increase comes from a greater proportion of existing collisions being successful, not from more collisions occurring.
• D: Reacting particles generally still need a suitable orientation for a successful collision on the catalysed pathway too. The catalyst does not eliminate this requirement, it lowers the energy barrier.

Final answer

• $\boxed{\text{A}}$. The catalyst provides an alternative pathway with a lower activation energy, so a greater fraction of collisions are successful.