Rates – Iodine Clock (OCR A Level Chemistry A): Revision Note

PAG 10.1: Rates – Iodine clock

• Clock reactions 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

2S₂O₃^2- (aq) + I₂ (aq) → 2I^- (aq) + S₄O₆^2- (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

Method

• Measure the required solutions using burettes

  • Place them into small labelled beakers

• Measure the excess sulfuric acid

  • Use a measuring cylinder

  • The precision of a burette is not required

• Add 1 cm³ of 0.25 mol dm^-3 hydrogen peroxide to start the reaction and immediately start the timer

• Stop the timer when the blue-black colour appears

• Suitable volume compositions to use could be as follows:

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:

Analysis

• The time of reaction is converted to rate of reaction by calculating the reciprocal value

• A graph is plotted of rate versus concentration:

• 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

 Practical skills reminder

• This practical develops essential skills in investigating reaction rates using clock reactions.

• It also supports:

  • Measuring precise volumes using burettes and measuring cylinders

  • Using a visible end-point to record reaction time

  • Calculating rate as the reciprocal of time and interpreting rate–concentration graphs to determine reaction order