Effect of temperature on reaction rates and the concept of activation energy
Cambridge A-Level Chemistry (9701) · Unit 8: Reaction kinetics · 6 flashcards
Effect of temperature on reaction rates and the concept of activation energy is topic 8.2 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 8 — Reaction kinetics , alongside Rate of reaction and Homogeneous and heterogeneous catalysts. In one line: Activation energy (Eₐ) is the minimum amount of energy required for a collision between reactant particles to be effective, leading to a chemical reaction.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 6 flashcards — 1 definition and 5 key concepts — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
'activation energy' (Eₐ) in the context of chemical reactions
Activation energy (Eₐ) is the minimum amount of energy required for a collision between reactant particles to be effective, leading to a chemical reaction.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define activation energy, Eₐ, as the least energy a collision must have to be effective.
- Sketch the Boltzmann distribution and use it to explain why activation energy is significant.
- Explain qualitatively how a change in temperature affects reaction rate, using both the Boltzmann distribution and the frequency of effective collisions.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Define 'activation energy' (Eₐ) in the context of chemical reactions.
Activation energy (Eₐ) is the minimum amount of energy required for a collision between reactant particles to be effective, leading to a chemical reaction.
Sketch a Boltzmann distribution curve. Label the axes and indicate the position of the activation energy (Eₐ).
The Boltzmann distribution curve plots the number of molecules (y-axis) against kinetic energy (x-axis). The activation energy (Eₐ) is marked as a vertical line on the x-axis, representing the minimum energy required for reaction. The area to the right of Eₐ represents the number of molecules with sufficient energy to react.
Explain how increasing the temperature affects the shape of the Boltzmann distribution curve.
Increasing the temperature flattens and broadens the Boltzmann distribution curve. This shifts the peak to the right (higher average kinetic energy) and increases the area under the curve to the right of the activation energy, indicating more molecules possess sufficient energy to react.
Explain qualitatively how increasing temperature affects reaction rate, referencing both the Boltzmann distribution and the frequency of effective collisions.
Increasing temperature increases the average kinetic energy of molecules. According to the Boltzmann distribution, a larger proportion of molecules now have energy exceeding the activation energy. This leads to a higher frequency of effective collisions, increasing the reaction rate.
How does a higher activation energy (Eₐ) affect the rate of a reaction at a given temperature?
A higher activation energy means that a smaller proportion of molecules will possess sufficient energy to overcome the energy barrier and react. Consequently, the reaction rate will be slower at a given temperature.
Explain why even a small increase in temperature can lead to a significant increase in reaction rate.
Due to the shape of the Boltzmann distribution, a small increase in temperature causes a large increase in the proportion of molecules with energy greater than or equal to the activation energy. This gives a much greater frequency of effective collisions and therefore a significantly faster reaction rate.
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Effect of temperature on reaction rates and the concept of activation energy sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Effect of temperature on reaction rates and the concept of activation energy deck
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