Effect of Temperature on Reaction Rates and the Concept of Activation Energy

Full Notes

Collision theory and activation energy has been outlined with more background theory and detail here.
This page is just what you need to know for CIE A-level Chemistry :)

What is Activation Energy?

Activation energy (E_a) is the minimum amount of energy that reacting particles must collide with for a chemical reaction to occur.

If a collision has less than the required activation energy , it is not effective — the particles simply bounce off each other.

If a collision has enough energy, it becomes an effective collision, and a reaction can happen.

Think of activation energy as an energy barrier that must be overcome for bonds to break and new bonds to form.

The Boltzmann Distribution

The Boltzmann distribution is a curve that shows the range and likely distribution of kinetic energies that particles have in a sample of a gas.

Features of the curve:

• Starts at the origin (0,0) because no particles have zero energy.
• Peaks at the most probable energy – the energy that most particles have.
• Has a long tail to the right because a few molecules have very high energy (never crosses the x axis again).
• Area under the curve = Total number of molecules (or is proportional to number of particles).
• Only molecules with energy ≥ activation energy (E_a) can react.

This graph helps us understand why not all collisions lead to a reaction as only a small fraction of particles have energy ≥ E_a.

Temperature and the Rate of Reaction

When temperature is increased:

• Particles move faster and have more kinetic energy.
• The entire Boltzmann distribution shifts right and the peak lowers.

The number of particles with E ≥ E_a increases significantly.

This means:

• There are more effective collisions per second.
• The rate of reaction increases, often dramatically.

Important:

• The total number of particles stays the same (area under the curve does not change).
• The activation energy line stays in the same place, but more particles now exceed it.
• Even a small temperature increase can have a large effect on reaction rate, because the number of particles with enough energy to react increases exponentially (this is covered in more detail with the Arrhenius Equation here (covered in much more detail here).

Summary

• Activation energy (E_a) is the minimum energy needed for a reaction to occur.
• The Boltzmann distribution shows the spread of molecular energies in a sample; only some exceed E_a.
• Increasing temperature shifts the distribution so more particles exceed E_a.
• Higher temperature → more effective collisions per second → faster rate of reaction.