Continuous Monitoring Method (Edexcel A Level Chemistry): Revision Note

Continuous Monitoring Method

• Continuous monitoring involves collecting experimental data throughout the course of a reaction to plot a concentration-time graph 

• Two of the most common ways to collect this data are by:

  • Measuring the volume / amount of gas evolved over time

  • Measuring the mass of reactants lost over time

• Another alternative method involves the use of colorimetry:

The iodination of propanone

• The iodination of propanone provides a suitable experiment in which the rate of reaction can be measured throughout the reaction by using a colorimeter

• The reaction is carried out using a catalyst of dilute sulfuric acid

• The iodine decolourises during the reaction as it turns into iodopropanone and hydrogen iodide:

CH₃COCH₃   +   I₂  → CH₃COCH₂I   + HI

• The colorimeter measures colour absorbance which is proportional to the concentration of the coloured species

• Before the investigation begins it is necessary to  measure the absorbance of a set of standard solutions of iodine and obtain a calibration curve

• For example, here is a calibration curve for a transition metal ion that allows you to convert colorimeter readings into concentrations:

A calibration curve showing the relationship between colour absorbance and concentration

• The colorimeter uses very small volumes of solutions, so four burettes can be filled with solutions of 0.02 mol dm^-3 iodine, 1.0 mol dm^-3 propanone and 1.0  mol dm^-3 sulfuric acid and distilled water

• By varying the volumes of solutions while maintaining a constant total volume with the use of distilled water, you can obtain a number of different concentrations

• The solutions are measured into a small beaker, leaving the iodine in a separate beaker - this starts the reaction, so it can be added when you start a timer or stop watch

• The iodine is added to the other liquids, the contents mixed and then quickly transferred into the cuvette (small receptacle) and the colorimeter / data logger started

The set up for using a colorimeter and data logger to continuously measure the rate of reaction

• A typical set of volume compositions could be as follows:

Volume Compositions Table

 Practical tip

• Choose a filter that gives the strongest absorbance for the solution you are using - this will be the complementary colour to the colour of the solution under investigation

• The specimen results below are for run 5, so the initial concentration of iodine is 0.0080 mol dm^-3

Specimen results table for the iodination of propanone

Graph showing the change in concentration of iodine during the course of the reaction

Measuring rate

• The rate of reaction is the gradient of the concentration-time graph

• For a curve, a tangent is drawn at the time of interest and its gradient found

• For the iodination of propanone the graph is a straight line, so the gradient, and therefore the rate, is constant: the reaction is zero order with respect to iodine

• Gradient = Δy / Δx = (0.00620 - 0.00800) / 600 = -3.0 x 10^-6 mol dm^-3 s^-1, so the rate of reaction is 3.0 x 10^-6 mol dm^-3 s^-1