Factors which affect the rates of chemical reactions: This should be an investigation involving developing a hypothesis.

The idea

Rates investigations begin with a testable hypothesis, not a guess. You identify one factor, such as concentration, temperature, surface area or a catalyst, and predict its effect using collision theory. For example: 'Increasing the concentration of hydrochloric acid increases the rate of reaction with marble chips because there are more acid particles per unit volume, so more frequent successful collisions occur.' A good hypothesis names the independent variable, the dependent variable and the direction of change, and links to scientific reasoning. You then plan a method that controls other variables, choose measurements such as gas volume or mass loss over time, and decide how results will test the prediction.

What earns the marks

• State a clear hypothesis that links the chosen factor to the rate of reaction and predicts the direction of change.
• Identify the independent variable, dependent variable and at least two control variables in the investigation.
• Justify the hypothesis using collision theory, referring to frequency or energy of collisions between reacting particles.
• Describe a suitable method for measuring rate, such as volume of gas collected at timed intervals or loss in mass over time.
• Explain how the results will be used to judge whether the hypothesis is supported, for example by comparing gradients or times to completion.

Common mistakes and how to put them right

• Writing a hypothesis as a question or a vague statement such as 'I think it will be faster'.

  Put it right by making a precise prediction that names the factor and the expected effect on rate.

• Changing several variables at once.

  Put it right by keeping all variables except the independent variable constant so the test is fair.

• Failing to link the prediction to collision theory.

  Put it right by explaining that the factor changes either the frequency of collisions or the proportion of particles with energy greater than or equal to the activation energy.

Exam tips

• Write the hypothesis as a single sentence containing the independent variable, the dependent variable and the predicted direction of change.
• When describing the method, state exactly what is measured and how often, for example gas volume every 10 seconds.
• Use collision theory vocabulary such as frequency of collisions and activation energy to justify your prediction.