Acid-Catalysed Iodination of Propanone (Edexcel International A Level Chemistry)

• The iodination of propanone is carried out using a catalyst of dilute sulfuric acid
• The reaction between iodine in an acidic solution and propanone is

CH₃COCH₃ (aq) + I₂ (aq) → CH₃COCH₂I (aq) + H⁺ (aq) + I^- (aq) 

• We can follow the rate of this reaction by using a continuous monitoring method and therefore we will be able to deduce the order of reaction with respect to a substance using a concentration time graph 
• We can calculate the order of reaction with respect to iodine as the propanone and acid are in a large excess so their concentration do not change during the reaction 
• Once a portion of the reaction mixture has been removed, we stop the reaction by adding sodium hydrogencarbonate
• Performing a titration again sodium thiosulfate(VI) solution and using starch as an indicator is used to determine the concentration of iodine

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

Results

Sample Data from the Reaction

• This graph is a straight line graph
  • As the gradient is constant, the rate of reaction is constant, and therefore independent of the concentration of the iodine solution
  • This means the reaction is zero order with respect to the iodine
  • The rate equation for the reaction is
    • Rate = k[CH₃COCH₃ (aq)] [H⁺ (aq)]

Concentration-time graphs of a zero-order reaction