Required practical 5: investigate how changes in concentration affect the…

The idea

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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Using the total volume of gas collected as the rate.

  Put it right: 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.

  Put it right: 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.

  Put it right: Use the same observer, the same cross and the same lighting, and to repeat readings to improve consistency.

Exam tips

• 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.