Chemguide: Support for CIE A level Chemistry

Learning outcome 8.2

Reaction kinetics

Effect of temperature

These statements are about the Boltzmann distribution and activation energy, and how these relate to the effect of temperature on reaction rates.

Before you go on, you should find and read the statements in your copy of the syllabus.

Statement 8.2.1 and 8.2.2

Start by re-reading the page about the collision theory. This time concentrate entirely on the sections about activation energy and the Maxwell-Boltzmann distribution.

You should know how activation energy relates to a simple energy diagram, and to the Boltzmann distribution.

And, of course, you should be able to define activation energy as the minimum energy that reactants need in order for a reaction to occur - or, as CIE defines it, as the minimum energy required for a collision to be effective.

Be very careful if you are asked to draw one of these Boltzmann distribution curves. Obviously, you will notice that it is lop-sided, but you should also notice that it starts steeply.

Note:  You will find many incorrect versions of these graphs (including occasionally on CIE exam papers!) where the curve starts shallowly and then gets steeper. The confusion is that this other shape of graph is produced if you plot number of molecules against molecular speed rather than molecular energy. As far as CIE is concerned, you will only be asked about the distribution of energies. If the horizontal axis is labelled "energy", then the graph shapes should be similar to those shown on Chemguide.

Statement 8.2.3

You will find this discussed on the page about the effect of temperature on rates of reaction.

Notice that although I have played down the effect of increases in collision rate at higher temperatures, CIE will expect you to know about the effect on collision rate as well as the much more important effect of the large increase in the number of molecules with energies equal to or greater than the activation energy.

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© Jim Clark 2020