Rates of Reaction: Question 10

Worked solution

Step 1: Rule out what temperature does not change

Raising the temperature of a reaction mixture in a fixed volume does not change the number of particles present in each unit of volume. That is the effect of increasing concentration or pressure, not temperature, so option A is wrong.

Temperature also does not change the activation energy, $E_a$, of a reaction. Activation energy is a fixed property of a particular reaction pathway, and only a catalyst (by providing an alternative pathway) can lower it, so option C is wrong.

Even at a higher temperature, only a proportion of collisions have enough combined energy to react successfully, not every collision succeeds, or the rate would no longer depend on further increases in temperature or concentration at all, so option D is wrong.

Step 2: The real explanation. The spread of particle energies

At any temperature, the particles in a reaction mixture do not all have the same kinetic energy: there is a wide spread, from slow-moving particles with little energy to a small number of fast-moving particles with much more energy than average. Only particles whose energy is greater than or equal to the activation energy, $E_a$, can react successfully when they collide, and this threshold usually lies well out towards the high-energy end of the spread, where relatively few particles are found.

Because so few particles are normally found in that high-energy region, even a small rise in the average kinetic energy of all the particles (here, only about $3\%$) shifts a disproportionately large extra number of particles above the fixed threshold $E_a$. This is why the proportion of successful collisions, and so the rate of reaction, can roughly double for only a $10\,^{\circ}\text{C}$ rise, even though the average kinetic energy itself has barely changed.

Step 3: Select the answer

Only option B correctly attributes the large increase in rate to this disproportionate increase in the proportion of particles with energy $\geq E_a$, without wrongly claiming a change in particle density, activation energy, or the certainty of every collision succeeding.

Final answers

• $\boxed{\text{option B}}$. A small rise in temperature shifts a much larger proportion of particles above the fixed activation energy threshold, producing a much larger increase in the rate of reaction.