Rate of reaction
Cambridge A-Level Chemistry (9701) · Unit 8: Reaction kinetics · 8 flashcards
Rate of reaction is topic 8.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 8 — Reaction kinetics , alongside Effect of temperature on reaction rates and the concept of activation energy and Homogeneous and heterogeneous catalysts. In one line: Rate of reaction is the change in concentration of a reactant or product per unit time. It's usually expressed in mol dm⁻³ s⁻¹.
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 8 flashcards — 3 definitions, 4 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 3 definition cards 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.
Some cards in this deck are on other syllabus topics: 8.2 Effect of temperature on reaction rates and the concept of activation energy. Each card is labelled with its topic.
'rate of reaction'
Rate of reaction is the change in concentration of a reactant or product per unit time. It's usually expressed in mol dm⁻³ s⁻¹.
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).
- Explain the terms rate of reaction, collision frequency, and effective and non-effective collisions, and use them correctly.
- Explain qualitatively, using the frequency of effective collisions, how changing concentration or pressure affects reaction rate.
- Calculate a reaction rate from experimental data.
Also in this deck: 8.2 Effect of temperature on reaction rates and the concept of activation energy
- 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.
Tips to avoid common mistakes in these topics
- › When explaining rate using collision theory, you must use the phrase 'frequency of collisions' rather than just 'number of collisions'.
- › To find the rate 'at' a specific time, draw a tangent to the curve at that point and calculate its gradient.
Topic 8.1 · Rate of reaction
Define 'rate of reaction'.
Rate of reaction is the change in concentration of a reactant or product per unit time. It's usually expressed in mol dm⁻³ s⁻¹.
Topic 8.1 · Rate of reaction
Explain the relationship between frequency of collisions and the rate of reaction.
The rate of reaction is directly proportional to the frequency of collisions. A higher frequency of collisions (more collisions per unit time) generally leads to a faster rate of reaction, assuming the collisions also have sufficient energy.
Topic 8.1 · Rate of reaction
What is the difference between an 'effective collision' and a 'non-effective collision'?
An effective collision results in a chemical reaction because the colliding particles have sufficient energy (activation energy) and proper orientation. A non-effective collision does not result in a reaction due to insufficient energy or incorrect orientation.
Topic 8.1 · Rate of reaction
How does increasing the concentration of reactants affect the rate of reaction, in terms of collisions?
Increasing the concentration increases the number of reactant particles per unit volume. This leads to a higher frequency of collisions, and thus, a faster rate of reaction because there are more opportunities for effective collisions to occur.
Topic 8.1 · Rate of reaction
Explain the effect of increasing pressure on the rate of reaction for gaseous reactants.
Increasing the pressure of gaseous reactants increases the concentration of the gases. This leads to a higher frequency of collisions, resulting in an increased rate of reaction.
Topic 8.1 · Rate of reaction
Describe how to calculate the rate of reaction from experimental data measuring the change in concentration of a reactant over time.
The rate of reaction can be calculated as the change in concentration (Δ[reactant]) divided by the change in time (Δt): Rate = -Δ[reactant]/Δt. The negative sign indicates the reactant concentration is decreasing.
Topic 8.2 · Effect of temperature on reaction rates and the concept of activation energy
Define 'activation energy'.
Activation energy (Eₐ) is the minimum amount of energy required for a reaction to occur between colliding particles. It is the energy needed to overcome the energy barrier to form the activated complex.
Topic 8.2 · Effect of temperature on reaction rates and the concept of activation energy
Explain qualitatively the effect of increasing temperature on the rate of reaction, in terms of effective collisions.
Increasing temperature increases the average kinetic energy of particles. This results in more collisions and a greater proportion of collisions having energy greater than or equal to the activation energy, leading to a significant increase in the frequency of effective collisions and the rate of reaction.
More Chemistry flashcards
Browse every 9701 flashcard topic by syllabus area.
All Chemistry FlashcardsMore topics in Unit 8 — Reaction kinetics
Rate of reaction 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 Rate of reaction deck
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