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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Understand the basic principle of collision theory.
- S2
Collision theory states that for a reaction to occur, particles must collide with sufficient energy and in the correct orientation.
- S3
Step 2: Consider the effect of increasing concentration.
- S4
Increasing the concentration of a reactant increases the number of particles per unit volume.
- S5
Step 3: Relate increased particle density to collision frequency.
- S6
More particles in a given volume lead to more frequent collisions between reactant particles.
- S7
Step 4: Explain the impact on reaction rate.
- S8
The 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.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Understand the basic principle of collision theory.
- S2
Collision theory states that for a reaction to occur, particles must collide with sufficient energy and in the correct orientation.
- S3
Step 2: Consider the effect of increasing concentration.
- S4
Increasing the concentration of a reactant increases the number of particles per unit volume.
- S5
Step 3: Relate increased particle density to collision frequency.
- S6
More particles in a given volume lead to more frequent collisions between reactant particles.
- S7
Step 4: Explain the impact on reaction rate.
- S8
The 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.
Common mistakes
- Confusing the effect of concentration with the effect of temperature on reaction rate. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)4 marks