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The acid-catalysed iodination of propanone consumes iodine. Take equal-volume samples at recorded times and determine the iodine remaining by titration with standard thiosulfate. The observation is iodine left in each aliquot, not the cumulative product gas used in a continuous gas experiment. Keep reaction temperature and initial mixture conditions controlled.
Video by Royal Society of Chemistry. The demonstration introduces rate-measurement approaches. This lesson uses the distinct timed-aliquot and titration method, including quenching and its own concentration conversion.
Viewing focus: Explain timed aliquots and quenching in a rate investigation
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: How does a timed-aliquot titration differ from measuring gas volume continuously?
It removes samples at assigned times and determines the reactant remaining in each after quenching. A gas curve measures product accumulation in one continuously monitored mixture.
After watching 2: Why must the method state how a sample is stopped from continuing to react?
The concentration must represent its assigned reaction time. Delay or ineffective quenching lets extra iodine react before analysis and biases the recorded time series.
What you'll be able to do
The acid-catalysed iodination of propanone consumes iodine. Take equal-volume samples at recorded times and determine the iodine remaining by titration with standard thiosulfate. The observation is iodine left in each aliquot, not the cumulative product gas used in a continuous gas experiment. Keep reaction temperature and initial mixture conditions controlled.
A sample would continue reacting after removal unless its conditions are changed. In the approved method, hydrogencarbonate neutralises the acid catalyst to quench or strongly slow further iodination before analysis. Record the time associated with quenching, not the later time the titration finishes. Incomplete or delayed quenching changes the apparent iodine concentration at the assigned time.
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻ gives n(I₂) = c(thiosulfate)V(titre)/2. Divide by the original reaction-mixture aliquot volume to find its iodine concentration. Water or quench solution added after sampling dilutes the analysis mixture but does not change the original iodine amount; do not replace the original aliquot denominator with the final titration-flask volume.
Use the specified near-endpoint starch procedure and consistent sampling, quenching and titration technique. Propanone-containing mixtures are flammable and volatile; iodine, acid and quench handling require the laboratory’s risk assessment. Carbon dioxide released during neutralisation must not be trapped in a sealed vessel.
For equal aliquots and the same thiosulfate concentration, titre is proportional to remaining iodine concentration. A linear decline with time supports zero order in iodine over the measured interval if propanone and acid concentrations are effectively constant. It does not imply zero order in every reactant or that the rate constant is zero.
A positive reaction-rate magnitude is the negative concentration–time gradient for iodine disappearance. Use repeated or independently prepared runs to assess scatter and check how small titres increase relative burette uncertainty. Delayed sampling or quenching can produce systematic shifts that no fitted straight line removes. Avoid extrapolating an order beyond the interval in which the assumptions remain credible.
| Quench time / s | Thiosulfate titre / cm³ | [I₂] in original aliquot / mol dm⁻³ |
|---|---|---|
| 0 | 20.0 | 0.0100 |
| 60 | 18.0 | 0.00900 |
| 120 | 16.0 | 0.00800 |
| 180 | 14.0 | 0.00700 |
| 240 | 12.0 | 0.00600 |
Choose two measurements and calculate the signed gradient. Then explain how it relates to the positive magnitude of iodine disappearance.
Blue: data with straight joining guides. Gold: selected interval. Scroll the plot horizontally on a small screen.
Try Δy ÷ Δx on paper first. Keep the sign and use the units on the axes.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Using the table, find the iodine-disappearance rate magnitude and the supported iodine order.
1.67 × 10⁻⁵ mol dm⁻³ s⁻¹; zero order in iodine for this interval and model.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.