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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define activation energy, EA, as the minimum energy required for a collision to be effective
- sketch and use the Boltzmann distribution to explain the significance of activation energy
- explain qualitatively, in terms both of the Boltzmann distribution and of frequency of effective collisions, the effect of temperature change on the rate of a reaction 8.3 Homogeneous and heterogeneous catalysts Learning outcomes
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.
Tips to avoid common mistakes in Effect of temperature on reaction rates and the concept of activation energy
- › Link the overall order and individual reactant orders to the mechanism; SN2 is suggested by first-order kinetics for both reactants.
- › A full description of heterogeneous catalysis must include adsorption of reactants, bond weakening, reaction, and finally desorption of products.
- › Use a sharp pencil or fine pen to plot points as small crosses (x) for better precision.
- › When explaining rate using collision theory, you must use the phrase 'frequency of collisions' rather than just 'number of collisions'.
- › Always check the units specified in the answer options and convert your time values accordingly before final calculations.
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 exponential nature of the Boltzmann distribution, even a small increase in temperature can significantly increase the *proportion* of molecules with energy exceeding the activation energy. This results in a disproportionately larger number of effective collisions and thus 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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