Temperature, activation energy and Boltzmann distributions

By the end of this lesson

By the end, you can define activation energy, sketch and interpret a Boltzmann distribution, and explain the effect of temperature on reaction rate.

Definition

Activation energy

The activation energy, $E_\text{A}$ , is the minimum energy required for a collision to be effective.

It is not the energy released by the reaction. It is the energy barrier that reacting particles must get over before bonds can rearrange.

A hill on the reaction route

Think of reactant particles as cyclists approaching a hill. Only cyclists with enough energy to reach the top can cross to the products' side.

$E_\text{A}$ is the height of that hill. A collision with less energy may still be a collision, but it cannot be effective.

The Boltzmann distribution

A Boltzmann distribution is a graph of number of particles against energy.

To sketch it:

Put energy on the horizontal axis and number of particles on the vertical axis.
Start at the origin and draw a curve that rises to one broad peak.
Make the curve fall gradually towards, but never quite reach, the horizontal axis.
Mark $E_\text{A}$ as a vertical line. The area to the right represents particles with energy at least $E_\text{A}$ .

The total area under the curve represents the total number of particles.

The energy barrier

DefinitionQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

Answer the 2 questions above to continue.