Investigating Rate of ReactionsOxford AQA IGCSE Chemistry: Revision notes

This required practical investigates how changing a factor — such as temperature, concentration, surface area, or the presence of a catalyst — affects the rate of a chemical reaction. Students design and carry out an experiment, changing one variable at a time while keeping other variables constant (a fair test), and measure how quickly the reaction proceeds.

For a reaction that produces a gas (such as marble chips reacting with hydrochloric acid), rate can be measured in two common ways:

1. Measuring mass loss — the reaction flask sits on a balance; as gas escapes, the total mass decreases, and the mass is recorded at regular time intervals (e.g. every 10 seconds)
2. Collecting gas volume — the gas produced is collected and its volume measured at regular time intervals using a gas syringe or by displacement of water into an inverted measuring cylinder

In both methods, a graph of mass lost (or gas volume) against time can be plotted, and the gradient at any point gives the rate of reaction at that time.

For a reaction that produces a precipitate that clouds a solution (such as sodium thiosulfate reacting with hydrochloric acid to produce a cloudy sulfur precipitate), the "disappearing cross" method can be used: a mark is placed underneath the reaction flask, and the time taken for the mark to become no longer visible from above (as the solution turns cloudy) is recorded with a stopwatch. A faster reaction produces cloudiness, and so obscures the mark, more quickly.

To ensure the investigation is a fair test when testing one factor (e.g. temperature), all other factors must be kept constant, for example: the same concentration and volume of acid, the same mass or surface area of solid reactant, and the same volume of solution. Only the independent variable being tested should change between repeats.

Risks (e.g. from acids, hot equipment or produced gases) should be identified and appropriate precautions taken, such as wearing eye protection and working in a well-ventilated area.

Results are usually recorded in a table and then plotted as a graph, with time on the x-axis and mass lost, gas volume, or another measured quantity on the y-axis. Repeating the experiment and calculating a mean improves reliability and helps identify anomalous results, which should be checked and, if necessary, repeated.