4.2 Temperature

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The rate of almost all chemical reactions increases if the temperature of the system is increased. A general rule of thumb is that the rate will double with an increase of 10°C (but "rules of thumb" are not iron-clad rule; exceptions exist).

The collision theory helps us understand why this occurs. Recall the kinetic energy curves from Thermodynamics Section 1.3 and earlier this unit (Section 2.3):

threshold energy '

It is important to remember what this curve represents - all particles in a sample of matter are in constant motion because they possess kinetic energy, but not all particles have the same amount of KE - some have only a little energy and are slow moving, some have a lot of energy and move fast, but most are somewhere in the middle.

In this unit we have added the idea of threshold energy - only particles having at least as much energy as the threshold energy have sufficient energy for a successful collision leading to a reaction.

Why does increasing temperature increase reaction rate?

Students often predict that increasing temperature will cause the threshold energy to be lowered:

but that is NOT the case! The amount of energy required for successful collisions for a particular reaction is not changed when temperature increases.

kinetic energy curve effect of temperature - NOT!

Instead . . .

When temperature is increased, all of the particles in the sample of matter now have more kinetic energy, and hence are moving faster. There are still some that are slower than the rest, others that are really fast, but most still in the middle. But the average kinetic energy of the entire sample increases.

kinetic energy curve effect of temperature

T1 represents the energy distribution at the lower temperature; T2 is the distribution at the higher temperature. The threshold energy is unchanged, but for the new curve (T2) more particles will be above the threshold energy, and thus will have successful reactions.

This will increase the rate for two reasons:

Because all particles are moving faster, there will be more frequent collisions, and thus an increase in rate. Remember our thought experiment, with blindfolded students moving around in a room? Increasing the temperature is like having the students start running around the room - obviously there will be more collisions.
Since all of the particles have more energy, more particles will meet the minimum energy threshold (or activation energy) required for a successful collision. The graph below illustrates this:

Potential Energy CurveThe potential energy curve will be unchanged when we illustrate a reaction occurring at a higher temperature.

But at a higher temperature more particles will have enough energy to get over the activation energy "hump":

potential energy curve temperature effect