Effect of temperature on reaction rates and the concept of activation energy

AS Level Chemistry · Reaction kinetics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll AS Level Chemistry topics

The idea

The activation energy, EA, is the minimum energy that colliding particles need for the collision to be effective. Particles in a sample do not all have the same energy. The Boltzmann distribution is a curve that shows how many molecules have each energy.

Only the molecules to the right of EA on the curve have enough energy to react. At room temperature this is usually a small part of the total.

When the temperature rises, the curve flattens and spreads to the right. Collisions are slightly more frequent, but the main effect is that a much larger proportion of molecules now have energy of at least EA, so effective collisions are far more frequent.

Rules to remember

Common mistake

Students draw the higher-temperature curve with a higher peak, or say that heating lowers the activation energy. The peak drops and moves right, and EA stays where it is.

Worked example

Reactions X and Y are carried out at the same temperature with the same concentrations. X has EA = 50 kJ mol−1 and Y has EA = 100 kJ mol−1. Which is likely to be faster, and why?

  1. Same temperature means both reactions have the same Boltzmann distribution of energies.
  2. Mark EA for X and for Y on that curve: the line for X is further to the left.
  3. The area under the curve to the right of 50 kJ mol−1 is much bigger than the area to the right of 100 kJ mol−1.
  4. So a larger proportion of collisions in X have enough energy to be effective.

Answer: X is likely to be faster, because more molecules have energy of at least its lower activation energy.

Practice questions

Try each one, then open the answer.

1. Activation energy, EA, is defined as

  1. A
    the minimum energy that colliding particles must have for a collision to be effective
  2. B
    the energy released when the products form
  3. C
    the average kinetic energy of the particles
  4. D
    the enthalpy change of the reaction
Show answer

Answer: A. EA is an energy barrier: colliding particles need at least this much energy to break the necessary bonds and react. It is unrelated to the size or sign of ΔH.

2. How does the Boltzmann distribution curve change when the temperature of a gas is raised?

  1. A
    the peak moves to higher energy and becomes lower; the area under the curve is unchanged
  2. B
    the peak moves to higher energy and becomes taller
  3. C
    the curve does not change but EA moves to the left
  4. D
    the area under the curve increases because the molecules gain energy
Show answer

Answer: A. The number of molecules is fixed, so the area cannot change. The curve spreads to higher energies, which lowers and flattens the peak and pushes more molecules past EA.

3. What is plotted on each axis of a Boltzmann distribution curve?

  1. A
    x-axis: time; y-axis: concentration
  2. B
    x-axis: temperature; y-axis: rate of reaction
  3. C
    x-axis: number of molecules; y-axis: energy
  4. D
    x-axis: energy; y-axis: number of molecules with that energy
Show answer

Answer: D. The Boltzmann distribution shows how many molecules have each value of kinetic energy, so energy is on the x-axis and the number (or proportion) of molecules is on the y-axis.

More questions on this topic

Read the full notes

Keep going

← Rate of reactionHomogeneous and heterogeneous catalysts →All rules on one pageStuck? Ask a question