Explain temperature effects using particle energies
Use both the Boltzmann distribution and collision frequency to explain how temperature changes rate.
Section 8 · Reaction kinetics
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Rate data and diagrams are illustrative unless a question supplies measurements. A diagram is qualitative unless labelled values are provided. Written practice is self-checked
Before you begin
- Rearrange simple ratios and use consistent units for time, volume, mass and concentration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At higher temperature, what happens to the fraction of particles with energy ≥ Eₐ?
What you will learn
- Describe how temperature changes particle energies
- Interpret the increased fraction above Eₐ
- Link more frequent effective collisions to rate
Higher temperature changes the energy distribution
At higher temperature, particles have greater average kinetic energy and their energies are spread over a wider range. A Boltzmann curve becomes broader with a lower peak; a greater fraction of particles lies to the right of a fixed Eₐ threshold.
The total number of particles is unchanged for the same sample, so the total area under the curve stays the same. The curve changes shape; the activation energy of the uncatalysed reaction does not change just because temperature rises.
Rate rises through effective collisions per second
Higher temperature also means particles move faster and collide more often. More importantly, a much greater fraction of collisions now involves particles with enough energy to overcome Eₐ. The frequency of effective collisions per second therefore increases and the rate rises.
Give both parts of the syllabus explanation: the Boltzmann distribution shows a larger fraction above Eₐ, and the greater collision frequency contributes to more effective collisions per unit time. Do not say only that particles ‘move faster’.
Pause and explain
Which two changes explain a higher-temperature rate increase?
Worked example
Explain why a higher temperature usually increases reaction rate using both collision frequency and a Boltzmann distribution.
Show the worked solution
- Particles move faster, so collisions occur more frequently.
- The distribution shifts to a broader, higher-energy spread, increasing the fraction with energy ≥ Eₐ.
- More effective collisions occur per second, increasing rate.
Answer Higher temperature increases collision frequency and the fraction of particles with energy at least Eₐ, so effective collisions occur more frequently and the rate increases.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A fixed Eₐ line is placed on distributions at T₁ and T₂, with T₂ > T₁. What happens to the area beyond Eₐ?
Give me a hint
Compare the high-energy tails, not the total area.
Compare my reasoning
- At T₂ the distribution is broader and lower at its peak.
- A greater fraction lies to the right of the same Eₐ line.
- The total area for the same sample remains constant.
The area beyond Eₐ is a greater fraction at T₂, while the total area remains the same for the sample.
Look for these in your work
- Higher-temperature shape recognised
- Threshold area compared
- Total area conserved
Explain the rate change when a reaction is warmed, including both parts of the particle-collision explanation.
Give me a hint
Include a collision-frequency effect and an energy-distribution effect.
Compare my reasoning
- Faster particles collide more frequently.
- A greater fraction has energy ≥ Eₐ.
- Effective collisions per second increase, so rate rises.
Warming increases collision frequency and the fraction above Eₐ; both contribute to more frequent effective collisions and a faster reaction.
Look for these in your work
- Both mechanisms included
- Effective collisions identified
- Rate direction stated
A student writes, ‘Heating makes Eₐ smaller, so more particles react.’ Correct the mechanism.
Give me a hint
Separate the fixed barrier from the particle-energy distribution.
Compare my reasoning
- At a given reaction pathway, heating does not itself lower Eₐ.
- Instead, the distribution broadens and more particles have energy ≥ Eₐ.
- Particles also collide more frequently, increasing the frequency of effective collisions.
Heating leaves the reaction's Eₐ unchanged; it increases particle energies and collision frequency, so more particles exceed the barrier and rate rises.
Look for these in your work
- Activation energy corrected
- Distribution change stated
- Collision frequency connected
Common mistakes
- Mentioning speed but not the fraction above Eₐ
- Claiming the total area under the same-sample distribution increases
- Saying temperature lowers the activation energy
- Claiming all collisions become effective
What can you explain now?
At higher temperature, what happens to the fraction of particles with energy ≥ Eₐ?
Compare with the explanation
It increases
The higher-temperature distribution has more area beyond the same threshold.
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Make it stick
How do temperature, the area beyond Eₐ and collision frequency combine to affect rate?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.