OCR A Level Chemistry

Revision Notes

1.4.4 Rates – Iodine Clock

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PAG 10.1: Rates – Iodine Clock

The Iodine Clock Reaction

  • Clock reactions are so called because they show a sharp dramatic colour change after a period of time has elapsed
  • They make ideal reactions for studying kinetics
  • Iodine clock reactions come in a number of variations, but they generally all use starch to show a sudden purple-black colour at the end of the reaction
  • A common iodine clock reaction uses the reaction between hydrogen peroxide and iodine

H2O2 (aq) + 2I- (aq) + 2H+(aq) → I2 (aq) + 2H2O (l)

  • Adding sodium thiosulfate to the reaction mixture uses up the iodine and acts as the reaction timer

2S2O32- (aq) + I2 (aq) → 2I- (aq) + S4O62- (aq)

  • The amounts chosen are such that the iodine produced is in excess compared to the other reagents
    • Therefore, as soon as the iodine is in excess the blue-black colour of iodine in starch is seen

Iodine clock reaction experiment, downloadable AS & A Level Chemistry revision notes

The iodine clock reaction provides a good way to study reaction kinetics

Steps in the procedure

  • The solutions are measured in burettes and placed in a small beaker
  • The sulfuric acid is in excess so can be measured in a measuring cylinder rather than burette
  • The reaction is started by adding 1cm3 of 0.25 mol dm-3 hydrogen peroxide and starting a timer
  • The timer is stopped when the blue black colour appears
  • Suitable volume compositions to use could be as follows:

Iodine clock volume compositions table

Iodine clock reactions volumes table, downloadable AS & A Level Chemistry revision notes

 

Practical tips

  • Hydrogen peroxide is typically found in 'volume' concentrations, based on the volume of oxygen given of when it decomposes:

2H2O2 (aq)  → O2 (g) +  2H2O (l)

  • For example in school laboratories, a suitable concentration of hydrogen peroxide may be listed as 3% or '10 vol'
    • '10 vol' means that when 1cm3 of hydrogen peroxide decomposes it generates 10 cm3 of oxygen
    • '10 vol' or 3% hydrogen peroxide has a concentration of 0.979 mol dm3

Specimen Results

  • Here is a set of typical results for the iodine clock reaction

Specimen results for the iodine clock reaction table

 

Analysis

  • The time of reaction is converted to rate of reaction by calculating the reciprocal value
  • A graph is plotted of rate versus concentration

 

A rate- concentration graph for the iodine clock reaction

  • From this graph we can see that the rate of reaction is directly proportional to the concentration of potassium iodide:
    • As concentration doubles; the rate of reaction also doubles

  • This tells us that the reaction is first order with respect to potassium iodide

 

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