Rates of Reaction - Titrimetric Method (Edexcel International A Level (IAL) Chemistry): Revision Note

Core Practical 9a: Titrimetric Methods

• This practical uses a titrimetric method (a technique involving titration) to determine the order of reaction with respect to iodine in its acid-catalysed reaction with propanone

• Propanone reacts with iodine in the presence of an acid catalyst:

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

Aim

• To determine how the concentration of iodine, [I₂], affects the rate of the reaction

Experimental design

• The concentrations of the other reactants, propanone and H⁺, do not change

• A large excess of both propanone and the acid catalyst is used

  • This means that their concentrations will remain effectively constant throughout the experiment

Method (sampling and quenching)

• We need to find the concentration of iodine at various points in time, so we use a sampling and titration method

1. 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 

2. Add 50 cm³ of 0.02 mol dm^-3 iodine solution

   • Start the timer as soon this is added to the beaker

3. At regular intervals, use a pipette to withdraw 10 cm³ portion of the reaction mixture and transfer this to a conical flask 

4. Quench / stop the sample reacting further by adding a spatula of sodium hydrogen carbonate

5. Titrate the sample against 0.01 mol dm^-3 sodium thiosulfate(VI) solution using starch as an indicator

   • Starch is used as an indicator is used to determine the concentration of iodine

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

6. Record the result for each titration in a suitable table

Practical tips

• Have everything ready and set up before beginning the procedure

• Keep the timer running throughout the practical rather than stopping the clock 

• Record the time when the sodium hydrogencarbonate is added to the samples of reaction mixture

  • This is when the reaction actually stopped

  • This means that the concentration of iodine can be calculated accurately

Results

• Here is an example set of results

Analysis and conclusion

• Plot a graph of titre (y-axis) against time (x-axis)

• For this reaction, the graph is a straight line with a negative gradient

  • A straight, descending line on a concentration-time graph means the rate of reaction is constant.

  • This shows that the rate is independent of the concentration of iodine.

  • Therefore, the reaction is zero order with respect to iodine

• The reaction is first order with respect to propanone and H⁺

• This experiment determines that the reactions is zero order with respect to iodine

• So, the overall rate equation is:

Rate = k[CH₃COCH₃ (aq)] [H⁺ (aq)]

• [I₂] does not appear in the rate equation because its order is zero