A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Effect of Concentration and Pressure on Reaction Rate — mark scheme explained
The short answer
In A-Level Chemistry, understanding how changes in concentration and pressure affect the rate of a reaction is crucial. This section delves into the qualitative effects of these changes on collision frequency and, consequently, the rate of reaction. Collision Theory The rate of a chemical reaction depends on the frequency of collisions between reactant particles.
The question
Explain how increasing the concentration of a reactant in a solution affects the rate of reaction.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For explanation questions, marks are typically awarded for clear and detailed descriptions that demonstrate a deep understanding of the topic. For comparison questions, marks are given for identifying both similarities and differences accurately. Practical application questions require students to relate theoretical concepts to real-world scenarios.
What the command words demand
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Compare
- Identify similarities and differences between two or more concepts or processes.
- Describe
- Give a clear and concise account of a process or phenomenon.
- Apply
- Use theoretical principles to explain practical applications or solve problems.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed explanations, and check your work.
- Understand the basic principle of collision theory.1 markCollision theory states that for a reaction to occur, particles must collide with sufficient energy and in the correct orientation.
- Consider the effect of increasing concentration.1 markIncreasing the concentration of a reactant increases the number of particles per unit volume.
- Relate increased particle density to collision frequency.1 markMore particles in a given volume lead to more frequent collisions between reactant particles.
- Explain the impact on reaction rate.1 markThe higher frequency of collisions means more collisions occur per unit time, so the number of successful collisions per second increases (the proportion of successful collisions is unchanged), thereby increasing the overall rate of reaction.
Final answer: Increasing the concentration of a reactant increases the number of particles per unit volume, leading to more frequent collisions between reactant particles. This increases the number of successful collisions per unit time (the proportion of successful collisions is unchanged), resulting in an increased rate of reaction.
Work through every step correctly and you earn all 4 marks.
Another worked example
Describe how increasing the pressure of a gas affects the rate of a gaseous reaction.
- Understand the basic principle of collision theory for gases.0 marksFor a gaseous reaction, particles must collide with sufficient energy and in the correct orientation to react.
- Consider the effect of increasing pressure.1 markIncreasing the pressure of a gas decreases its volume, which increases the concentration of gas particles.
- Relate increased particle density to collision frequency.1 markMore gas particles in a smaller volume lead to more frequent collisions between the molecules.
- Explain the impact on reaction rate.2 marksThe higher frequency of collisions means more collisions occur per unit time, so the number of successful collisions per second increases (the proportion of successful collisions is unchanged), thereby increasing the overall rate of reaction.
Final answer: Increasing the pressure of a gas decreases its volume and increases the concentration of gas particles, leading to more frequent collisions between the molecules. This results in an increased rate of reaction.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing the effect of concentration with the effect of temperature on reaction rate.
Why it happens: Students sometimes mix up the effects of different factors, leading to incorrect explanations.
Fix: Always clearly distinguish between the effects of concentration and other factors like temperature. Concentration affects collision frequency by increasing the number of particles per unit volume, while temperature affects the energy of collisions.
Failing to explain why increased concentration leads to more frequent collisions.
Why it happens: Students may provide a superficial explanation without delving into the underlying reasoning.
Fix: Explain that increasing the concentration increases the number of particles per unit volume, which directly leads to more frequent collisions between reactant particles.
Not considering the effect of pressure on gas reactions specifically.
Why it happens: Students may generalize the effect of concentration to all types of reactions without recognizing the specific impact of pressure on gases.
Fix: Emphasize that increasing the pressure of a gas decreases its volume and increases the concentration of gas particles, leading to more frequent collisions. This is specific to gaseous reactions.
Forgetting to mention the importance of successful collisions in explaining reaction rate changes.
Why it happens: Students may focus solely on collision frequency without considering the need for successful collisions (those with sufficient energy and correct orientation).
Fix: Always include the requirement for successful collisions in your explanations. Increasing concentration or pressure leads to more frequent collisions, but only those with sufficient energy and correct orientation will result in a reaction.
Not providing a clear qualitative explanation of how changes in concentration and pressure affect collision frequency.
Why it happens: Students may provide vague or incomplete explanations, leading to a lack of clarity.
Fix: Practice explaining the effects qualitatively. For concentration, explain that more particles per unit volume lead to more frequent collisions. For pressure, explain that decreasing the volume increases particle density and collision frequency.
Failing to relate practical applications to theoretical principles.
Why it happens: Students may struggle to connect theory with real-world examples, leading to incomplete answers.
Fix: Practice relating theoretical concepts to practical applications. For example, explain how the Haber process uses high pressures to increase the rate of ammonia production by increasing collision frequency.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Collision Theory Explanation | Explain how raising reactant concentration increases the rate of reaction using collision theory. | 4 |
| Collision Theory Explanation | Explain how raising gas pressure increases reaction rate using collision theory. | 4 |
| Compare Rate Factors | Compare how concentration and pressure changes affect reaction rate, giving similarities and differences. | 6 |
| Apply Collision Theory | Relate collision theory principles to optimising reaction rate in an industrial process. | 5 |
| Total across these question types | 19 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.