6.1.1 Rates of Reaction - Titrimetric Method

• 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 line 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) 

Steps in the procedure 

• Mix 25 cm³ of 1.0 mol dm^-3 aqueous propanone with 25 cm³ of 1.0 mol dm^-3 sulfuric acid in a beaker 
• Add 50 cm³ of 0.02 mol dm^-3 iodine solution
• Start the stop clock as soon this is added to the beaker
• Using a pipette, withdraw 10 cm³ portion of the reaction mixture and transfer this to a conical flask 
  • Stop this portion of the reaction by adding a spatula of sodium hydrogen carbonate
• Titrate this sample against 0.01 mol dm^-3 sodium thiosulfate(VI) solution using starch as an indicator 
  • Record your result
• Withdraw 10 cm³ of the reaction mixture every 3 minutes and repeat the process of stopping the reaction and titrating against the sodium thiosulfate(VI) solution 

Practical tips

• Note the time at which the sodium hydrogencarbonate is added to the withdrawn portions of reaction mixture as this is when the reaction will be stopped and the concentration of iodine can be calculated accurately
• Keep the clock running and note the time throughout the practical rather than stopping the clock 
• Have everything ready and set up before beginning the procedure

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