Kinetics experiments: choosing and analysing a method (Core Practical 9)

Continuous monitoring

Follow one reaction mixture over time.

• Colorimetry when one species is coloured (I2, MnO4-, Cu2+): choose a filter of the complementary colour, zero with water, and make a calibration curve of absorbance against concentration using standard solutions. Then read concentrations of the mixture from the curve.
• Gas volume or mass loss for reactions that give a gas.
• Sampling and quenching: withdraw portions at timed intervals, stop the reaction (CP9a adds the sample to sodium hydrogencarbonate to remove the acid catalyst) and titrate.
• pH or conductivity when the number or type of ions changes.

The result is a concentration-time graph, analysed with tangents and half-lives.

Initial rates and clock reactions

• Run several mixtures with different starting concentrations of one reactant, keeping the others and the total volume the same (add water).
• A clock reaction uses a small, fixed amount of a substance (such as thiosulfate) that removes a product until it runs out; then a colour appears. Only a small fraction of the reactants has been used, so the rate is close to the initial rate, and rate is proportional to 1/t. Too much thiosulfate would mean a large fraction reacts, so 1/t would no longer reflect the initial rate.
• Calculate each concentration in the mixture from the volumes: c(mixture) = c(stock) x V(stock) / V(total).

Errors and improvements

• Temperature must be constant: use a water bath and let solutions reach temperature before mixing.
• Start timing at the moment of mixing; judge the colour change consistently (a light sensor helps).
• Quenching must be quick and complete.
• 1/t gives an average rate over the time measured; a tangent at t = 0 on a concentration-time curve is a better initial rate.

Worked example

In a clock reaction, 7.50 cm³ of 0.100 mol dm^-3 KI is mixed with other solutions to a total volume of 25.0 cm³. Find [I-] in the mixture.

1. n(I-) = 0.00750 x 0.100 = 7.50 x 10^-4 mol
2. [I-] = 7.50 x 10^-4 / 0.0250

Answer: 0.0300 mol dm^-3

Check yourself

1. 12.5 cm³ of 0.200 mol dm^-3 sodium thiosulfate is made up to a total volume of 50.0 cm³ with acid and water. Calculate the concentration of thiosulfate in the mixture.
   Show answer

   0.0500 mol dm^-3. 0.200 x 12.5 / 50.0 = 0.0500 mol dm^-3.

2. In a clock reaction, doubling [S2O8²-] (all else constant) changes the time from 84 s to 42 s. What is the order with respect to S2O8²-?
   Show answer

   1. Rate is proportional to 1/t, so the rate doubles: 2ⁿ = 84 / 42 = 2, n = 1.

3. Why is only a small, fixed amount of thiosulfate used in an iodine clock reaction?
   • So that the colour change is slower
   • Because thiosulfate is a catalyst
   • To keep the total volume constant
   • So that only a small fraction of the reactants has reacted when the colour appears, giving the initial rate
   Show answer

   So that only a small fraction of the reactants has reacted when the colour appears, giving the initial rate. Many candidates missed this in October 2024.

4. Why is a calibration curve made when using a colorimeter to follow a reaction?
   • To convert absorbance readings into concentrations
   • To find the wavelength of the filter
   • To measure the temperature
   • To remove the need for a blank
   Show answer

   To convert absorbance readings into concentrations. Standard solutions of known concentration give absorbance values to plot.

5. In Core Practical 9a, how is each sample quenched?
   • By adding starch
   • By heating it
   • By adding it to sodium hydrogencarbonate solution, which removes the acid catalyst
   • By adding more iodine
   Show answer

   By adding it to sodium hydrogencarbonate solution, which removes the acid catalyst. Without H+ the reaction effectively stops.

6. Why is 1/t only an approximation to the initial rate?
   • It is an average rate over the time measured, not the rate at t = 0
   • The time is measured in seconds
   • It ignores the temperature
   • It is always too large by a factor of 10
   Show answer

   It is an average rate over the time measured, not the rate at t = 0. Pearson's guide notes a tangent at t = 0 gives a more reliable initial rate.

7. Which method would best follow the rate of a reaction in which purple MnO4- ions are used up, without disturbing the mixture?
   • Sampling and titration
   • Measuring gas volume
   • Colorimetry
   • Measuring mass loss
   Show answer

   Colorimetry. The colour fades, so absorbance can be read continuously.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH16 October 2024
• Pearson WCH16 January 2022 question paper (unused)

Last updated 8 November 2026. Spotted a mistake? Tell us.