Rates of Reaction - Titrimetric Method (Edexcel IAL Chemistry)

Core Practical 9a: Titrimetric Methods

Spec point — 11.12

• 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

1. At regular intervals, use a pipette to withdraw 10 cm³ portion of the reaction mixture and transfer this to a conical flask
2. Quench/ stop the sample reacting further by adding a spatula of sodium hydrogen carbonate
3. 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)

1. 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

Concentration-time graphs of a zero-order reaction

• 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