Effect of temperature on reaction rates and activation energy

· Activation energy, EA = minimum energy required for a collision to be effective.
· A collision is effective only if particles collide with energy ≥ EA and with a suitable orientation.
· Particles with energy below EA collide but do not react: these are non-effective collisions.
· A lower EA means a greater proportion of particles have enough energy to react, so the rate increases.
· In this topic, explanations should focus on temperature, Boltzmann distribution, and frequency of effective collisions.

This diagram shows that reactant particles must gain enough energy to reach the top of the energy barrier. The height of this barrier represents activation energy, EA. Only collisions with energy at least equal to EA can lead to reaction. Source

Boltzmann distribution

· A Boltzmann distribution shows the spread of molecular energies in a sample at a fixed temperature.
· The x-axis = energy of particles.
· The y-axis = number / fraction of particles with that energy.
· The curve starts at the origin because no particles have zero energy.
· The curve has a peak: this is the most probable energy.
· The curve then falls away because fewer particles have very high energy.
· The area under the curve represents the total number of particles, so it stays the same if the amount of substance is unchanged.
· EA is shown as a vertical line; the area to the right of EA represents particles with enough energy to react.

Increasing temperature

· Increasing temperature gives particles a higher mean kinetic energy.
· Particles move faster, so the frequency of collisions increases slightly.
· More importantly, a much larger proportion of particles now have energy ≥ EA.
· Therefore, the frequency of effective collisions increases significantly.
· This causes the rate of reaction to increase.
· In exam answers, do not only say “particles move faster”; always link to more particles exceeding EA and more effective collisions.

At higher temperature, the Boltzmann curve becomes lower and broader, with the peak shifted to higher energy. The area beyond EA increases greatly, so many more particles have enough energy for effective collisions. Source

Explaining temperature using the Boltzmann distribution

· When temperature increases, the Boltzmann curve becomes lower and broader.
· The peak shifts to the right, meaning the most probable energy increases.
· The total area under the curve remains constant if the number of particles is unchanged.
· The area to the right of EA increases greatly.
· This means a greater fraction of particles have energy ≥ EA.
· Therefore, there are more effective collisions per unit time, so the reaction rate increases.
· Small increases in temperature can cause a large increase in rate because the high-energy tail beyond EA increases disproportionately.

The vertical EA line separates non-reacting collisions from potentially effective collisions. The shaded or labelled region beyond EA represents particles that can react when they collide. Source

Decreasing temperature

· Decreasing temperature lowers the particles’ mean kinetic energy.
· Particles move more slowly, so the frequency of collisions decreases.
· A smaller proportion of particles have energy ≥ EA.
· The area to the right of EA on the Boltzmann distribution becomes smaller.
· Therefore, the frequency of effective collisions decreases, so the rate of reaction decreases.

Exam wording: high-scoring phrases

· “At higher temperature, particles have greater mean kinetic energy.”
· “The Boltzmann distribution becomes broader and lower, with the peak shifted to higher energy.”
· “A greater proportion of particles have energy equal to or greater than EA.”
· “The frequency of effective collisions increases.”
· “Therefore, the rate of reaction increases.”
· Avoid vague answers such as “particles are more reactive” or “collisions are harder” unless linked clearly to EA and effective collisions.

Collision theory explains why not every collision leads to reaction. For a reaction to occur, particles must collide with enough energy to overcome activation energy and with a suitable orientation. Source