Learn to monitor the rate of reaction and identify the effects of changing temperature and concentration, using both initial rate and continuous monitoring methods

The rate of reaction is simply a measure of how fast a reaction proceeds, which we can determine through monitoring how fast a reactant is used up or how fast a product is formed. An initial rate method measures how long it takes for a reaction to go to completion or for a specified volume/mass of product to form.  A continuous rate method measures the rate of reaction of the course of a reaction, usually by making a measurement at regular time intervals.

In most instances, students will be expected to understand how changing the temperature or concentration effects the rate of reaction and how, given the reactants/products of the reaction, a method might be developed to monitor the rate.

The following two videos explore the effect of changing the temperature and changing concentration on the same reaction between sodium thiosulfate and hydrochloric acid.

Sodium thiosulfate and hydrochloric acid (concentration)

Sodium thiosulfate and hydrochloric acid (temperature)

Questions you can ask your students:

  • In this method, what are the dependent, independent and control variables?
  • Is the instructor measuring how fast a reactant is used up or how fast a product is formed?
  • What sources of error can you identify that would make your results less accurate?
  • How could you make the mixing consistent between experiments?
  • How might you mitigate some of these sources of error?
  • To what degree of accuracy should you record the initial temperature and the time shown on the stopwatch?
  • How might you improve the graph shown in the second video? (Hint: Is the line of best fit ‘correct’?
  • How might calorimetry make this experiment more accurate?

It’s important for students to consider sources of error, from misreading measuring cylinder or thermometer values or stopping the stopwatch early or late. Remind them to read liquid levels from the bottom of the meniscus at eye level. Students will also need to develop an appreciation that there is an unavoidable time lag between the cross disappearing and stopping the clock. You may want to use these experiments to introduce the idea of robotics in chemistry to ensure a completely reproducible technique from experiment to experiment.

One experiment to demonstrate the difference between an initial rate method and continuous monitoring method is the reaction between hydrochloric acid and calcium carbonate or magnesium. In both these reactions, the gas evolved (carbon dioxide for calcium carbonate, hydrogen for magnesium) can be collected in a gas syringe or using an inverted measuring cylinder in water.

The initial rate method monitors the time taken for a specified volume of gas, (100 cm3 for example) to be released as demonstrated in this video from SpaceyScience.

Capturing the results for differing temperatures, concentrations or mass of solid will enable a student to understand how these variables affect the rate of reaction. Care needs to be taken when working with gas syringes as the inner part can ‘fly out’ and break if the gas is released too quickly. Encourage students to keep one hand ‘just ahead’ of the gas syringe to act as a barrier in such an event.

Collecting a gas can also be used to continuously monitor a reaction over time. This video from Malmesbury Education demonstrates continuous monitoring via gas collection using an inverted measuring cylinder approach and a gas syringe (starts at 2:38). 

Questions you can ask your students:

  • What sources of error can you identify that might lead to a less accurate result being recorded?
  • How can you determine the initial rate of reaction from your continuous monitoring results?

 Students will need to feel confident plotting the results from their experiment, drawing a line of best fit and interpreting the graph. Students need to take care if they decide to plot a graph using a programme such as Excel® rather than by hand. Some programmes will simply ‘join the dots’ rather than plotting a line of best fit and you might want to encourage your students to print off the graph and then draw the line of best fit by hand. A final point worth considering; you can either plot the dependent variable (time/volume of gas) against the time taken in seconds or against the rate (1/time taken).

A famous and not-to-be-missed rate of reaction experiment is the iodine clock experiment in which iodide ions are oxidised by hydrogen peroxide and then reduced by thiosulfate ions in the presence of starch. The dramatic colour change from colourless to inky purple indicates the end of the reaction. Another video from SpaceyScience helpfully illustrates the experimental set-up.

This final video from Chemistry Channel offers a continuous monitoring method of the acid catalysed reaction of propanone and iodine, linking rates of reaction, redox chemistry and titration. This is a great practical to stretch more able students and to diagnose understanding within an unfamiliar but accessible experimental technique.

And sometimes it’s just fun to enjoy a little bit of chemistry thanks to TheAzmanam

Also check out:

  • Rates of reaction – this CPD article explains how the concept of reaction rates could be introduced.
  • Gradients and rates of change – this article highlights the maths skills students need to correctly interpret graphs.
  • Burning milk powder – some experiments are best left to teachers! This visual demonstration highlights the true impact that increasing the surface area to volume ratio can have on the rate of reaction.
  • Rating rate laws – not all students have developed the mathematical understanding alongside their experimental knowledge. Using recent research, this article shows how to help students develop a deeper understanding of mathematical model-building.