Factors which affect the rates of chemical reactions — AQA GCSE Chemistry
Verified courseTeaching note checked against the official specification.
Factors which affect the rates of chemical reactions explained
Factors which affect the rates of chemical reactions covers 4 specification points, including The concentrations of reactants in solution,… and Students should be able to recall how changing….
Read the full explanation
Open a point below for its explanation and revision guidance.
Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts.
Rate means how quickly reactants are used or products formed. For a reaction to happen, particles must collide with at least the activation energy. Increasing the concentration of reactants in solution means more particles in the same volume, so collisions are more frequent and the rate rises. Increasing the pressure of reacting gases squeezes the same particles into a smaller volume, again increasing collision frequency. Increasing the surface area of a solid, for example using powder instead of a large lump, exposes more particles to collision. Raising temperature increases both collision frequency and the proportion of collisions with enough energy, so the effect is usually large. A catalyst provides an alternative pathway with lower activation energy, increasing the rate without being used up.
Students should be able to recall how changing these factors affects the rate of chemical reactions.
This statement requires recall of the direction of rate changes when specific factors are altered. Increasing the concentration of a solution, increasing the pressure of a gas, increasing the surface area of a solid, increasing the temperature, and adding a catalyst all increase the rate of a chemical reaction. Conversely, decreasing these factors decreases the rate. You must be able to state these effects clearly. For example, if asked how using a powder instead of a lump affects the rate, you should recall that the powder has a larger surface area and therefore increases the rate. Note that while these factors change the rate, changing concentration, pressure, or temperature can also shift the position of equilibrium in reversible reactions, which affects the yield. The focus here is strictly on recalling the effect on the rate of reaction.
Required practical 5: investigate how changes in concentration affect the rates of reactions by a method involving measuring the volume of a gas produced and a method involving a change in colour or turbidity.
In this practical you change the concentration of a reactant and measure how fast the reaction proceeds. In the gas-volume method, a suitable reaction such as magnesium with dilute hydrochloric acid produces hydrogen; you collect the gas in a syringe or inverted measuring cylinder and record the volume at regular time intervals. The initial gradient of a volume-time graph gives a measure of rate. In the colour or turbidity method, a reaction such as sodium thiosulfate with dilute hydrochloric acid produces a pale yellow precipitate of sulfur; you time how long a black cross viewed through the mixture takes to disappear. A shorter time means a faster rate. In both methods, control variables such as temperature, volume and concentration of the other reactant must be kept constant for a fair comparison.
This should be an investigation involving developing a hypothesis.
A hypothesis is a testable prediction linked to scientific reasoning. For this practical, you might predict that as the concentration of a reactant increases, the time taken for a cross to disappear decreases, because more reactant particles in the same volume collide more frequently. The hypothesis must identify the independent variable, the dependent variable and the expected direction of change, and it should be justified using collision theory. You then design a method that produces evidence capable of supporting or refuting the prediction. This means choosing suitable concentrations, controlling variables such as temperature and volume, deciding how many readings and repeats to take, and selecting a measurement that genuinely reflects rate. Finally, you compare your results with the hypothesis and explain any anomalies.
Your focus
- Identify the factors that affect the rate of a chemical reaction from a description of an experiment.
- Explain how changing concentration, pressure, surface area, temperature or adding a catalyst affects collision frequency or collision energy.
- Apply collision theory to predict whether a reaction will be faster or slower when a named factor is changed.
Show all 12 objectives
- State how changing concentration, pressure, or surface area affects the rate of a chemical reaction.
- State how changing temperature affects the rate of a chemical reaction.
- State the effect of adding a catalyst on the rate of a chemical reaction.
- Carry out a rate experiment in which concentration is changed and either gas volume or turbidity is measured.
- Record and process data appropriately, including plotting a graph and using its initial gradient or a timed endpoint to compare rates.
- Evaluate the method by identifying control variables, sources of error and ways to improve reliability.
- Write a testable hypothesis that links a change in concentration to a measurable change in rate.
- Plan a fair investigation that controls relevant variables and produces reliable data.
- Use collected evidence to judge whether the hypothesis is supported and to suggest improvements to the method.
Factors which affect the rates of chemical reactions exam tips
Marking Points
- Rate is the change in amount of reactant or product per unit time, and it depends on the frequency and energy of collisions between particles.
- Increasing the concentration of reactants in solution increases the number of particles per unit volume, so successful collisions become more frequent and the rate increases.
- Increasing the pressure of reacting gases reduces the volume occupied by the same number of particles, increasing collision frequency and therefore the rate.
- Increasing the surface area of a solid reactant exposes more particles at the surface, so collisions with other reactants become more frequent and the rate increases.
- Increasing temperature raises the average kinetic energy of particles, so collisions are more frequent and a greater proportion exceed the activation energy, increasing the rate.
- A catalyst increases the rate by providing an alternative reaction pathway of lower activation energy, and it is not used up during the reaction.
- Recall that increasing the concentration of reactants in solution increases the rate of reaction.
- Recall that increasing the pressure of reacting gases increases the rate of reaction.
- Recall that increasing the surface area of a solid reactant (e.g., by using a powder) increases the rate of reaction.
- Recall that increasing the temperature increases the rate of reaction.
- Recall that adding a catalyst increases the rate of reaction.
- Recognise that while these factors affect reaction rate, changing temperature, pressure, or concentration can also shift equilibrium and alter yield in reversible reactions.
- Change only the concentration of the chosen reactant while keeping temperature, volumes and other reactant amounts constant to make the comparison fair.
- In the gas-volume method, measure the volume of gas produced at regular time intervals using a gas syringe or inverted measuring cylinder, and use the initial rate from the graph.
- In the turbidity method, measure the time taken for a cross or mark to become obscured by the precipitate, recognising that a shorter time indicates a faster reaction.
- Plot appropriate graphs, such as volume of gas against time or time against concentration, and interpret the gradient or the time taken as a measure of rate.
- Repeat each concentration and calculate a mean to reduce the effect of random errors and improve the reliability of the conclusion.
- Identify and control relevant variables, including temperature, the volume of each solution and the mass or particle size of any solid reactant.
- Write a hypothesis that predicts a relationship between the independent variable, such as concentration, and the dependent variable, such as time or volume of gas.
- Justify the hypothesis using collision theory, for example more particles per unit volume lead to more frequent successful collisions.
- Design a method that changes only the independent variable and controls other variables such as temperature, volumes and reactant amounts.
- Select a suitable range and number of concentrations, and plan repeats so that a pattern can be identified and anomalies recognised.
- Collect and present data in a table and graph, then use the evidence to state whether the hypothesis is supported.
- Evaluate the investigation by discussing sources of error, such as timing reactions by eye, and suggest improvements.
Examiner Tips
- 💡Link every factor back to collision frequency or collision energy rather than simply saying the reaction is faster.
- 💡When comparing two experiments, state which factor changed, which factors were controlled, and how the changed factor alters collisions.
- 💡Use the phrase activation energy correctly: particles must collide with at least this minimum energy for a reaction to occur.
- 💡For recall questions, clearly state the direction of change (e.g., 'increases the rate') rather than just saying 'changes the rate'.
- 💡Use comparative language such as 'faster' or 'slower' when comparing two sets of conditions.
- 💡Describe the independent variable, dependent variable and at least two control variables when writing a method.
- 💡Explain why a graph is plotted rather than relying on a single reading, linking the gradient to the rate of reaction.
- 💡When comparing methods, state one advantage of measuring gas volume and one advantage of the turbidity method, such as continuous data versus a simple timed endpoint.
- 💡Use the structure if concentration increases, then rate increases because collision frequency increases when writing a hypothesis.
- 💡Name the independent variable, dependent variable and control variables explicitly in your plan.
- 💡Refer back to the hypothesis in your conclusion and state whether the evidence supports it, using data values as evidence.
Common Mistakes
- Saying that a catalyst increases the rate by giving particles more energy; the correction is that a catalyst lowers the activation energy by providing an alternative pathway.
- Confusing surface area with concentration when a solid is powdered; the correction is that powdering changes the exposed surface area of the solid, not the concentration of a solution.
- Believing that increasing temperature only makes particles collide more often; the correction is that it also greatly increases the proportion of collisions with energy at least equal to the activation energy.
- Confusing rate with yield; correct this by remembering that rate is about how fast the reaction happens, while yield is about how much product is formed.
- Stating that a larger lump reacts faster because it has more mass; correct this by recalling that a larger lump has less exposed surface area per unit mass, so it reacts more slowly.
- Thinking that a catalyst is used up; correct this by recalling that a catalyst increases the rate but is not consumed.
- Using the total volume of gas collected as the rate; the correction is that the initial rate is found from the steepness of the graph at the start, not from the final volume.
- Failing to keep the temperature constant when comparing concentrations; the correction is to use a water bath or record temperature and keep it the same, because temperature also changes rate.
- Judging the endpoint of the turbidity method inconsistently; the correction is to use the same observer, the same cross and the same lighting, and to repeat readings to improve consistency.
- Writing a hypothesis as a question or a vague statement; the correction is to write a clear prediction such as increasing concentration will decrease the time taken for the cross to disappear.
- Changing more than one variable at a time; the correction is to identify the independent variable and keep all other relevant variables constant.
- Ignoring anomalous results when drawing a conclusion; the correction is to identify anomalies, consider repeating them and base the conclusion on the overall pattern.