Stop-watch at the ready, as students explore rate equation and clock reactions

A teacher asks the students to design a clock reaction to determine which is the rate determining step in the iodination of propanone.

Pre-test questions

(Remember to give full references for any information beyond A-level that you find out)

  1. Consider the reaction: 2 A + B → C + D
  • Sketch a graph to show how the reaction rate (taken here as the change in [C] per unit time) would vary with [A] if the reaction was;
    • I. zero order with respect to A
    • II. first order with respect to A
    • III. second order with respect to A
  • A detailed study of the kinetics of the reaction revealed that the mechanism involved the formation of an intermediate E;
    • Step one: A + A → C + E slow 
    • Step two: E + B → D fast

Which step is the rate determining step in the reaction?

  1. The rate of a reaction can be determined experimentally using an initial rate method. This involves measuring the time it takes for some easily recognisable event to occur very early on in a reaction (usually when less than 10% of the reaction has occurred).
  • For each of the following reactions, identify a visual change that you could measure the time taken to occur and hence used to determine the initial rate of the reaction;
    • I. Mg + 2 HCl → MgCl2 + H2
    • II. Na2 S2 O3 + 2 HCl → 2 NaCl + S + SO2 + H2 O
    • III. CH3 CH2 Cl + OH→ CH3 CH2 OH + Cl
  • In each of these reactions, the reaction time for the observed change is measured. Define the term “reaction rate” and hence explain how the reaction time can be converted into a reaction rate in any one of the reactions above.
  1. The reaction Q + R P was studied using the method of initial rates. The initial rate of formation of was measured in three different experiments. The data are provided below

Problem 7 question 3

  • Use the data above to determine the rate equation for the reaction.
  • What is the value of the rate constant, k?



  • Propanone solution [Highly flammable; Irritant], 100 cm3 of 2.0 M 
  • Hydrochloric acid solution [Low hazard], 100 cm3 of 1.0 M 
  • Iodine solution in aqueous potassium iodide [Low hazard], 30 cm3 of 0.005 M 
  • Distilled water


  • Burettes, burette stands and clamps and funnels (1 for each solution and water) (these can be shared between groups if needed), x 4
  • Conical flask, 100 cm3, x 2
  • Test tubes (for storage of acid and iodine solutions before addition)
  • Stopwatch
  • White tile or white paper x 2

Health, safety and technical notes

  • Read our standard health and safety guidance 
  • Wear eye protection
  • Wear clothing protection, if desired.
  • Propanone solution [Highly flammable; Irritant] see CLEAPSS Hazcard HC085a
  • Hydrochloric acid solution [Low hazard] see CLEAPSS Hazcard HC047a
  • Iodine solution in aqueous potassium iodide see CLEAPSS Hazcard HC047b
  • The product of the reaction, iodopropanone, is a lachrymator (strongly irritant to eyes). The reaction mixture must therefore be disposed of as soon as measurements have been taken by flushing down a fume cupboard sink with lots of running water.