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6.1.1Unit 6. Practical Skills in Chemistry II

Rates of Reaction – Titrimetric Method — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 5 of 14 Unit quiz

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Part 1 of 6The Reaction Studied

6.1.1 Rates of Reaction - Titrimetric Method

The titration lets us measure the iodine concentration at intervals.

Equation
  • CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + HI(aq), catalysed by hydrogen ions.
Observation
  • Brown iodine is used up, but the colour change is too gradual to time, so samples are analysed by titration.
Rate equation
  • Experiments show rate = k[CH₃COCH₃][H⁺], so the reaction is zero order with respect to iodine.
Autocatalysis
  • Because H⁺ is also a product, the reaction speeds up slightly as it proceeds, so only early results are used.

Part 2 of 6Experimental Method

1
Mix known volumes of propanone, sulfuric acid and water in a flask, then add iodine solution and start the stopwatch at once.
2
At regular intervals remove a fixed sample, such as 10.0 cm³, with a pipette.
3
Add the sample straight to sodium hydrogencarbonate solution, which neutralises the acid catalyst and stops the reaction.
4
Titrate the remaining iodine with standard sodium thiosulfate, adding starch indicator when the solution is pale yellow.
5
The end point is the change from blue-black to colourless, and the titre is recorded for each time.

Part 3 of 6Analysing the Results

Analysing the Results
Titration equation
  • I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻, so 1 mol of iodine reacts with 2 mol of thiosulfate.
Concentration–time graph
  • A straight line with constant negative gradient shows zero order with respect to iodine.
Rate
  • The rate equals the gradient magnitude, and the units are mol dm⁻³ s⁻¹.
Order for propanone or H⁺
  • Repeat with doubled concentration of one reactant and compare the gradients.

Part 4 of 6Sources of Error

Quenching delay
  • If the sample is not added to the hydrogencarbonate quickly, the reaction continues during the transfer.
Reading the burette
  • The titre is read to ±0.05 cm³, so very small titres give a high percentage uncertainty.
Temperature
  • Rate depends on temperature, so the mixture should be kept in a water bath.
Timing
  • Record the time the sample is quenched, not the time of pipetting.

Part 5 of 6Worked Example

Question

  • A 10.0 cm³ sample needs 18.40 cm³ of 0.00500 mol dm⁻³ thiosulfate.
  • Find the iodine concentration.

  1. Moles of thiosulfate = 0.00500 × 0.01840 = 9.20 × 10⁻⁵ mol.
  2. Moles of iodine = 9.20 × 10⁻⁵ ÷ 2 = 4.60 × 10⁻⁵ mol.
  3. Concentration = 4.60 × 10⁻⁵ ÷ 0.0100 dm³ = 4.60 × 10⁻³ mol dm⁻³.

Answer [I₂] = 4.60 × 10⁻³ mol dm⁻³

Part 6 of 6Exam Checklist & Tips

6.1.1 – Key Points for the Exam

  • Quenching Stopping a reaction suddenly so a sample can be analysed.
  • Order of reaction The power to which a concentration is raised in the rate equation.
  • Zero order A reaction rate that does not change when the reactant concentration changes.
  • Standard solution A solution whose concentration is accurately known.

Common Mistakes

  • Forgetting to stop the reaction in the sample, so the titre keeps changing.
  • Using the 1 : 1 mole ratio for iodine and thiosulfate; it is 1 : 2.
  • Adding starch too early, which makes the end point harder to see.
  • Stating the order from a single concentration–time point.

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