OCR AS Chemistry F332: The Effect of Temperature on Rate

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The Effect of Temperature on Rate
Collision theory says that reactions occur when molecules collide with a certain minimum
kinetic energy.
The more frequent these collisions, the faster the reaction.
This energy is needed to overcome the energy barrier, the activation enthalpy, for the
reaction ­ this is the energy needed to start breaking the bonds in the colliding molecules so
that the collision can lead to a reaction.
The Distribution of Energies
At any temperature, the speeds ­ and therefore the kinetic energies ­ of the
molecules in a substance are spread over a wide range.
Some molecules have high kinetic energies, many have medium energies and some have
low energies.
This distribution of kinetic energies in a gas at a given temperature is shown below.
It is called the Maxwell-Boltzmann distribution.

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As the temperature increases, more molecules move at higher speeds and have
higher kinetic energies.
The graph below shows how the distribution of energies changes when you increase the
temperature by 10c, from 300K to 310K.
You can see that there is still a spread of energies, but now a greater proportion of
molecules have higher energies.
We need to think about how many collisions have energy greater than 50kJ mol 1, because
these are the collisions that can lead to a reaction.…read more

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The graph below shows the curve for both 300K and 310K.
You can see that, at the higher temperature, a significantly higher proportion of molecules
have energies above 50kJ mol 1 ­ about twice as many.
This means that twice as many molecules have enough energy to react ­ so the
reaction goes twice as fast.
Overall, reactions go faster at higher temperatures because a larger proportion
of the colliding molecules have the minimum activation enthalpy needed to
react.…read more


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