Concentration, Pressure, and Reaction Rates Revision Notes | A Level AQA Chemistry

1. Why Concentration and Pressure Control How Fast Reactions Go

Picture a crowded dance floor versus an empty one. On the crowded floor, people bump into each other constantly. On the empty one, collisions are rare. Reactions work the same way: the more particles packed into a given space, the more often they collide, and the faster the reaction proceeds.

This lesson focuses on two variables that change how crowded that dance floor is: concentration (for solutions) and pressure (for gases). Both act through exactly the same underlying mechanism — altering collision frequency — and AQA expects you to explain that mechanism precisely.

2. Collision Theory: The Foundation

Before unpacking concentration and pressure, you need the core idea firmly in place. According to collision theory, a chemical reaction can only occur when reactant particles collide with sufficient energy (at least the activation energy, $E_a$) and with the correct orientation. Two things therefore determine rate: how often collisions happen, and how many of those collisions are successful.

This lesson is concerned with collision frequency — the number of collisions per unit time. Concentration and pressure changes do not alter the energy of individual particles (that is temperature’s job), but they do change how often particles meet. More frequent collisions mean more chances for a successful collision, so the rate increases.

Keep that distinction clear: concentration and pressure affect how often particles collide, not how energetically they collide. Confusing these two ideas is one of the most common sources of lost marks at A Level.

3. The Effect of Concentration on Reaction Rate

Concentration tells you how many solute particles are dissolved in a given volume of solution. When you increase the concentration of a reactant, you are placing more particles into the same volume of solvent. The particles are therefore closer together on average, and they encounter one another more frequently.

The result is a higher collision frequency. Because more collisions occur per second, the number of successful collisions per second also rises — assuming temperature (and therefore the fraction of particles with energy $\geq E_a$) stays constant. The reaction rate increases.

The reverse is equally important for exam questions. Decreasing concentration reduces the number of particles per unit volume, lowers collision frequency, and therefore decreases the reaction rate. This is why reactions slow down as reactants are consumed over time.

As concentration increases, collision frequency and reaction rate both increase, although the exact mathematical relationship depends on the reaction order studied later in rate equations.

4. The Effect of Pressure on Reaction Rate (Gases Only)

Pressure applies to reactions involving gases. When you increase the pressure on a gas mixture, you compress the gas into a smaller volume. The same number of gas particles now occupies less space, so the particle density — the number of particles per unit volume — increases. This is directly analogous to increasing the concentration of a solution.

With particles packed more tightly, they collide more frequently. Collision frequency rises, the number of successful collisions per second increases, and the reaction rate goes up. Decreasing pressure has the opposite effect: the gas expands, particle density falls, collision frequency drops, and the rate decreases.

This is why industrial processes involving gaseous reactants — such as the Haber process for ammonia synthesis — operate at high pressures. Higher pressure directly increases the rate of the forward reaction by increasing collision frequency between nitrogen and hydrogen molecules.

5. Comparing Concentration and Pressure: The Same Mechanism, Different Contexts

At their core, concentration and pressure are two ways of expressing the same physical idea: particle density. Increasing the concentration of a dissolved reactant and increasing the pressure of a gaseous reactant both raise the number of particles per unit volume. The downstream effect on collision frequency, and therefore on reaction rate, is identical in nature.

The table makes the parallel explicit. When an exam question asks you to compare the effects of concentration and pressure, you should identify this shared mechanism — both increase particle density and therefore collision frequency — while noting the different physical contexts in which each applies.

6. Worked Exam-Style Explanation: Applying the Reasoning

AQA ‘explain’ questions on this topic typically award 2–3 marks and require a logical chain: physical change → particle-level consequence → effect on rate. Let’s walk through a model answer for each variable so you can see exactly what the mark scheme rewards.

Worked example: Constructing a Mark-Winning Explanation: Concentration

Explain why increasing the concentration of a reactant in solution increases the rate of reaction. [3 marks]

1. Increasing concentration means there are more reactant particles dissolved in the same volume of solution. — [B1] This is the starting point. Name what has changed at the particle level, not just that 'there is more reactant'.
2. Because the particles are more densely packed, they encounter one another more frequently, so the collision frequency increases. — [B1] This is the key mechanistic step. Use the phrase 'collision frequency increases' — it is the AQA-expected language.
3. A greater collision frequency means more collisions occur per second that have sufficient energy to overcome the activation energy, so the rate of reaction increases. — [B1] Complete the chain by connecting collision frequency to successful collisions and then to rate. Do not stop at 'more collisions' without this final step.

Answer: There are more reactant particles per unit volume, so collision frequency increases. More collisions per second have sufficient energy to overcome the activation energy, so the rate of reaction increases.

Worked example: Constructing a Mark-Winning Explanation: Pressure

Explain why increasing the pressure of a gaseous reaction mixture increases the rate of reaction. [3 marks]

1. Increasing pressure compresses the gas, reducing the volume it occupies. The same number of gas particles is now contained in a smaller space. — [B1] Always mention the volume decrease explicitly. Examiners look for this as evidence that you understand the mechanism, not just the outcome.
2. The particle density increases, so gas molecules collide with one another more frequently. Collision frequency increases. — [B1] 'Particle density increases' is the bridge between the physical change and the collision argument. Include it.
3. More frequent collisions mean more collisions per second with energy greater than or equal to the activation energy, so the rate of reaction increases. — [B1] Mirror the same final step used for concentration. The logic is identical because the underlying mechanism is the same.

Answer: Increasing pressure reduces the volume, increasing particle density. Collision frequency increases, so more collisions per second are successful, and the rate of reaction increases.

7. Exam Technique and Common Pitfalls

The marking points for concentration and pressure questions follow a predictable three-step pattern: physical change → collision frequency → rate. Examiners report that the most common reason students lose marks is stopping after step one — stating that there are ‘more particles’ without explaining what that means for collisions or rate.

A second frequent error is importing temperature-based language into concentration or pressure answers. Phrases such as ‘particles have more energy’ or ‘particles move faster’ are only correct when temperature has changed. If the question specifies that only concentration or pressure has changed, those phrases will not earn marks and may actively contradict the correct explanation.

For ‘compare’ questions, structure your answer to address both variables explicitly and then identify the shared mechanism. A strong comparative answer might read: ‘Both increasing concentration and increasing pressure raise the number of particles per unit volume, increasing collision frequency and therefore reaction rate. Concentration applies to dissolved reactants, whereas pressure applies to gaseous reactants.’

Every spec point in this lesson — the qualitative effect of concentration on collision frequency, and the qualitative effect of pressure on collision frequency — maps onto the same three-step explanatory chain. Master that chain, apply it consistently, and you will secure the available marks regardless of how the question is phrased.