Rates of reaction

This is a good opportunity to ask students about control variables by getting them to consider what other factors may affect the rate of reaction, such as temperature or surface area to volume ratio of reactants.

There are two suggested ways to measure rate of reaction when changing concentration. The first is to measure how quickly a precipitate is formed by observing the ‘cloudiness’ of a liquid. The second is to measure the volume of gas produced using a gas syringe or a measuring cylinder over water. 

Sodium thiosulfate disappearing cross experiment

This video from Malmesbury Education demonstrates the precipitate-forming reaction between sodium thiosulfate and hydrochloric acid.  Mrs Peers Dent draws a pencil cross on a piece of paper and measures the time taken for the cloudy precipitate to cause the cross to disappear. This is repeated at varying concentrations of the sodium thiosulfate to illustrate the effect that concentration has on the rate of reaction.

As a comparison, students could be asked to consider how the measured rate of reaction may vary in this video from Eggar’s Science. Rather than drawing the cross in pencil the teacher draws the cross in a black marker which should take longer to ’disappear’.

If conducting this practical in a classroom, students could be asked to draw their crosses in a variety of pens or pencils which will give a spread of results. Use this as an opportunity to discuss reliability and uncertainty of results. You could ask students to make recommendations on how to improve the reliability of the practical such as using identical printed crosses. Some students may also raise the question of variation in judgement or eyesight between different observers. Encourage students to think of changes that could be made to help quantify the measurement, such as placing a light meter underneath the flask to measure the change in opacity.

Volume of Gas

The Eggar’s Science video (above) also introduces the second method of measuring the rate of reaction: measuring the volume of gas produced. 

In this video from Malmesbury Education Mr Mitchell demonstrates two methods for collecting the gas from a reaction of magnesium with hydrochloric acid. The first method uses a glass measuring cylinder overturned in a glass trough. In the second method a gas syringe is used to collect the gas from the same experiment. In both examples the concentration of hydrochloric acid is changed to observe the effect on rate of reaction.

The video shares the results from the experiment and shows how these can be plotted on a graph. Students who are learning remotely could be asked to do this and to label the graphs to show that the steeper the gradient the greater the rate of reaction. The commentary introduces the concept of limiting reactants which students may be familiar with from photosynthesis. Encourage students to explain why the reaction has stopped and how they could change the method to produce more data points (using more of the limiting reactant or recording data at smaller intervals).

Alternative practical work on rates of reaction can be carried out with different reactants using these methods, provided that a gas is produced or a precipitate is formed. A common alternative is to react hydrochloric acid with marble chips, Cambridge Assessment International Education offer a virtual practical using this method (accessed from the page link).