Multiple choiceQuestion 1 · Recall
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By the end you can use a Boltzmann distribution to explain why a catalyst increases the rate, and say precisely what does and does not change when one is added.
Picture the molecules in a gas as a crowd of climbers facing a wall of height . Only those with enough energy get over.
You can get more climbers over in two completely different ways.
The syllabus asks you to explain catalysis the second way. Heating and catalysing both speed a reaction up, but they do it by changing opposite things.
Boltzmann distribution
A graph of the number of molecules with a given energy against that energy, at a fixed temperature. The area under the curve to the right of is the fraction of molecules with energy greater than or equal to — the only ones that can react on collision.
The shape of the distribution is set by the temperature — how the available thermal energy is shared out among the molecules. A catalyst does not heat anything and does not change the total energy, so the distribution of molecular energies is unchanged.
What the catalyst changes is the requirement: the reaction now proceeds through a mechanism whose highest point is lower.
This is why the explanation sounds almost backwards compared with heating. Heating makes molecules faster; a catalyst makes the reaction easier.
Worked example: Counting the molecules that qualify
At a certain temperature, only about 1 molecule in has enough energy to clear the uncatalysed barrier. With a catalyst, the barrier is low enough that about 1 molecule in qualifies (illustrative numbers).
Explain what this does to the rate, using the Boltzmann distribution.
No. The Maxwell–Boltzmann distribution depends on temperature and on the mass of the molecules, not on the reaction pathway. At the same temperature the curve is identical with and without a catalyst.
On an exam sketch, draw one curve, then two vertical lines at and , with . Do not draw a second curve shifted to the right — that is the answer for increasing the temperature, and it will lose the mark.
A catalyst shifts the Boltzmann curve to the right.
Shifting the curve to the right means the molecules have more energy, which is what heating does. A catalyst leaves the curve where it is and lowers the threshold.
A catalyst increases the number of collisions.
It increases the fraction of collisions that are successful, by lowering the energy needed. The collision frequency itself is essentially unchanged.
A catalyst increases the activation energy so the reaction can proceed.
The opposite: the activation energy is lowered, which is why more molecules clear it.
Because the catalyst lowers Ea, ΔH must be smaller too.
A typical 2-mark answer: "The catalyst provides an alternative route with lower activation energy, so a greater proportion of molecules have energy greater than or equal to the activation energy (shown by the larger area under the curve to the right of ), so more successful collisions per second."
If the question says "use the Boltzmann distribution", you must mention the area or the proportion of molecules with . Just saying "lower activation energy" repeats the definition and may not earn the distribution mark.
is the gap between reactants and products. Lowering the top of the hill does not change the heights of the two ends.