Factors Affecting Rate of Reaction

The rate of a chemical reaction can be increased or decreased by changing the conditions. Understanding the factors that affect the rate of reaction is central to the AQA GCSE Chemistry specification and links closely with collision theory, which you will study in the next lesson. This lesson covers each factor in detail, including concentration, temperature, surface area, pressure and catalysts.

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Overview of Factors

There are five main factors that affect the rate of a chemical reaction:

Factor	Effect on Rate When Increased
Concentration of reactants in solution	Increases the rate
Temperature	Increases the rate
Surface area of solid reactants	Increases the rate
Pressure of gaseous reactants	Increases the rate
Catalyst (adding one)	Increases the rate

Each of these factors works by changing the frequency or energy of collisions between reacting particles. The details of why are explained by collision theory (see next lesson), but this lesson focuses on what each factor does and how it is investigated.

Exam Tip: In AQA GCSE Chemistry, you are expected to be able to predict the effect of changing any one of these factors on the rate of reaction, and to explain your prediction using ideas about particles and collisions.

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Factor 1: Concentration

When the concentration of a reactant in solution is increased, there are more particles of that reactant in the same volume. This means particles are closer together and collisions between reactant particles happen more frequently. More frequent collisions means a higher rate of reaction.

Concentration	Number of Particles per Volume	Collision Frequency	Rate
Low	Few	Low	Slow
Medium	Moderate	Moderate	Moderate
High	Many	High	Fast

Investigating the Effect of Concentration

A classic experiment uses the reaction between sodium thiosulfate and hydrochloric acid:

Na2S2O3(aq) + 2HCl(aq) --> 2NaCl(aq) + H2O(l) + SO2(g) + S(s)

The sulfur produced makes the solution go cloudy. By placing the flask over a paper cross and timing how long the cross takes to disappear, you can compare rates at different concentrations.

graph TD
    A[Increase concentration of Na2S2O3] --> B[More particles in same volume]
    B --> C[Particles are closer together]
    C --> D[More frequent collisions]
    D --> E[Cross disappears faster]
    E --> F[Shorter time = higher rate]

Exam Tip: When describing the effect of concentration, always use the phrase "more frequent collisions" or "increased frequency of collisions." Do not say "more collisions" alone — the examiner wants to see that you understand it is the frequency (number per second) that matters.

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Factor 2: Temperature

When the temperature is increased, particles gain more kinetic energy and move faster. This has two effects:

1. Particles collide more frequently because they are moving faster
2. A greater proportion of collisions have energy equal to or greater than the activation energy, so more collisions are successful

The second effect is the more important one. Even a small increase in temperature causes a significant increase in the number of particles with enough energy to react.

Temperature	Particle Speed	Collision Frequency	Proportion with Ea	Rate
Low (e.g. 20 C)	Slow	Low	Small	Slow
Medium (e.g. 40 C)	Moderate	Moderate	Moderate	Moderate
High (e.g. 60 C)	Fast	High	Large	Fast

Rule of Thumb

For many reactions, increasing the temperature by 10 degrees Celsius roughly doubles the rate of reaction. This is not an exact rule but gives a useful approximation.

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Factor 3: Surface Area

When a solid reactant is broken into smaller pieces, its surface area increases. A larger surface area exposes more particles at the surface to the other reactant, so there are more collisions per second.

Form of Solid	Relative Surface Area	Rate
Large lump	Small	Slow
Small chips	Medium	Moderate
Powder	Very large	Fast

Investigating the Effect of Surface Area

A common experiment uses the reaction between calcium carbonate (marble chips) and dilute hydrochloric acid:

CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + H2O(l) + CO2(g)

By using the same mass of marble chips in different sizes (large lumps, small chips, powder) and measuring the volume of CO2 produced over time, you can compare rates.

graph LR
    A[Same mass of CaCO3] --> B[Large lumps]
    A --> C[Small chips]
    A --> D[Powder]
    B --> B1[Small surface area = slow rate]
    C --> C1[Medium surface area = moderate rate]
    D --> D1[Large surface area = fast rate]

Exam Tip: When asked about surface area, always specify that you are breaking the solid into smaller pieces to increase the surface area exposed to the other reactant. A common error is to say "the surface area of the acid increases" — liquids and solutions do not have a meaningful surface area in this context.

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Factor 4: Pressure (Gases Only)

For reactions involving gases, increasing the pressure squeezes the gas particles into a smaller volume. This has the same effect as increasing concentration for solutions — the particles are closer together, so they collide more frequently.

Pressure	Volume	Particle Spacing	Collision Frequency	Rate
Low	Large	Far apart	Low	Slow
High	Small	Close together	High	Fast

This factor only applies to reactions involving gaseous reactants. It has no effect on reactions between solids or solutions.

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Factor 5: Catalysts

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. Catalysts work by providing an alternative reaction pathway with a lower activation energy.

We will study catalysts in detail in Lesson 4, but the key point for now is:

• A catalyst lowers the activation energy, so a greater proportion of collisions are successful
• A catalyst is not used up — it can be recovered chemically unchanged at the end of the reaction
• A catalyst does not change the products of the reaction

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Comparing Experiments on a Graph

When the effect of a factor is tested, the results are often shown on a single graph for comparison.