Rates – Iodine Clock (OCR A Level Chemistry)

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

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

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

 

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