How temperature changes the rate

6 minAS Level8.2.3

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can explain, using both the Boltzmann distribution and collision frequency, why raising the temperature increases the rate of a reaction.

Two things change at once

Raising the temperature does two separate things to the particles.

1. They move faster. Higher temperature means higher average kinetic energy, so particles travel faster and collide more often. The frequency of collisions increases.

2. The energy distribution shifts. The Boltzmann curve becomes broader and its peak moves to higher energy. Far more molecules now have energy at least equal to $E_a$ .

Both effects increase the rate, but they are not equally important. The second one dominates, because the high-energy tail of the distribution grows very rapidly with temperature.

At the higher temperature, the curve is flatter and shifted right. The area beyond

E_a

, which represents the molecules that can react, grows substantially.

Why the tail matters so much

The Boltzmann tail is not a straight line; it falls away steeply. A small shift to the right can multiply the small number of molecules in the tail.

Suppose, for illustration, that at 300 K about 1 in 100 molecules has enough energy to react. At 310 K, the fraction might be around 2 or 3 in 100. The number of collisions has increased only slightly, because the particles are only a little faster. But the number of effective collisions has roughly doubled or tripled.

This is why a modest temperature rise, such as 10 K, can produce a large increase in rate. It is not mainly because particles collide more often; it is because a much larger fraction of collisions now has enough energy.

The activation energy itself does not change with temperature. What changes is how many molecules can clear that barrier.

Worked example: Explaining a classroom observation

Two identical mixtures of marble chips and hydrochloric acid are prepared. One is kept at 20 °C and the other at 30 °C. The 30 °C mixture reacts noticeably faster. Explain fully.

Have a go first?

Common mistakes

Raising the temperature increases the rate only because particles collide more often.

Collision frequency does increase, but that is a small effect. The dominant effect is the much larger fraction of molecules with energy at least $E_a$ .

Raising the temperature lowers the activation energy.

$E_a$ is a property of the reaction and does not change with temperature. The distribution of molecular energies changes instead, so more molecules exceed the same barrier.

In the exam

A complete answer mentions both effects: more frequent collisions and a larger fraction of molecules with energy at least $E_a$ .
Then say which matters more: the increased fraction of effective collisions, because the Boltzmann tail grows rapidly.
Refer to the Boltzmann distribution by name and describe the change: broader curve, peak at higher energy, larger area beyond $E_a$ .
Do not say the activation energy decreases. Only a catalyst does that.
If the question gives a small temperature rise with a large rate increase, use the tail argument to explain why.

Key takeaways

Higher temperature increases the average kinetic energy of the particles.
Particles collide more frequently, which increases the rate.
The Boltzmann curve broadens and shifts right, so a much larger fraction has energy at least Ea.
The increased fraction of effective collisions is the dominant reason for the rate increase.
Activation energy does not change with temperature.

The two effects

Multiple choiceQuestion 1 · Apply

Multiple choiceQuestion 2 · Apply

Multiple choiceQuestion 3 · Recall

Answer the 3 questions above to continue.