Factors which affect the rates of chemical reactions: Required practical activity 11: 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.

The idea

This required practical uses two complementary methods. In the gas-volume method, a known mass of marble chips (calcium carbonate) reacts with hydrochloric acid of different concentrations; the gas syringe or inverted measuring cylinder collects carbon dioxide, and you record the volume at regular time intervals, such as every 10 s. A steeper initial gradient on a volume–time graph means a faster rate. In the turbidity method, sodium thiosulfate solution reacts with dilute hydrochloric acid to form a pale yellow precipitate of sulfur; the mixture becomes cloudy. You place the flask over a printed cross and time how long the cross takes to disappear. A shorter time means a faster rate. For both methods, change only the concentration of one reactant, keep temperature, volume and other conditions constant, and repeat to improve reliability.

What earns the marks

• Describe measuring gas volume at regular time intervals using a gas syringe or inverted measuring cylinder, and plotting volume against time.
• Describe measuring the time for a cross to disappear as sulfur precipitate forms, and using 1 ÷ time as a measure of rate.
• Identify the independent variable as concentration of one reactant and the dependent variable as gas volume or time.
• State control variables such as temperature, volume of solution, mass or size of solid, and same cross or light intensity.
• Explain that a steeper initial gradient on a volume–time graph indicates a faster rate.
• Explain that a shorter time for the cross to disappear indicates a faster rate.

Common mistakes and how to put them right

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

  Put it right: Rate is related to the initial gradient or to 1 ÷ time, not the final total volume.

• Changing temperature or volume of acid while changing concentration.

  Put it right: Keep all variables except concentration constant.

• Reading the cross through the side of the flask or using different lighting.

  Put it right: View from directly above and keep lighting and cross identical.

Exam tips

• State clearly what is measured and how it is used to compare rates, for example initial gradient or 1 ÷ time.
• Name the control variables explicitly; examiners look for temperature, volume and solid mass or particle size.
• For the turbidity method, explain that the cross disappears when enough sulfur precipitate has formed to block the view.