6.1.2Unit 6. Practical Skills in Chemistry II
Rates of Reaction – Clock Reaction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 6 of 14 Unit quiz
Section 1 of 6Principle of a Clock Reaction
A fixed small amount of thiosulfate acts as a timer.
Main reaction
- Peroxodisulfate ions oxidise iodide ionsS₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂.
Clock reaction
- Iodine is removed at once by thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻, so no colour appears at first.
Colour change
- When all the thiosulfate is used, extra iodine reacts with starch and the mixture suddenly turns blue-black.
Measured time
- The time to the sudden colour change is inversely proportional to the initial rate.
Section 2 of 6Procedure
1
Measure fixed volumes of peroxodisulfate, thiosulfate and starch into one beaker and keep the total volume constant by adding water.
2
Measure a volume of potassium iodide solution in a separate container.
3
Pour the iodide into the mixture and start the stopwatch immediately.
4
Stop timing when the blue-black colour first appears.
5
Repeat with different iodide volumes, adding water each time so that all other concentrations stay unchanged.
Section 3 of 6Interpreting Results
Interpreting Results
Initial rate
- Rate is approximately the amount of iodine formed divided by the time taken to the colour change.
Order test
- If doubling [I⁻] halves the time, the rate has doubled, so the reaction is first order in iodide.
Graph
- A plot of 1 ÷ time against [I⁻] is a straight line through the origin for a first-order reaction.
Limitation
- This gives an average rate over the first few percent of reaction, which is close to the initial rate.
| Volume of KI (cm³) | Time (s) | 1 ÷ time (s⁻¹) |
|---|---|---|
| 10.0 | 80 | 0.0125 |
| 20.0 | 40 | 0.0250 |
| 40.0 | 20 | 0.0500 |
Section 4 of 6Controlling Variables
Constant volume
- Total volume must be the same in each run so that dilution does not change concentrations.
Constant temperature
- A water bath prevents rate changes caused by temperature.
Fresh solutions
- Iodide can slowly oxidise in air, so use fresh solutions.
Colour judgement
- One person should judge the end point each time to reduce human error.
Section 5 of 6Worked Example
Question
- 5.0 cm³ of 0.0100 mol dm⁻³ thiosulfate is in 50.0 cm³ of mixture and the blue colour appears after 40 s.
- Find the average rate of formation of iodine.
- Moles of thiosulfate = 0.0100 × 0.0050 = 5.0 × 10⁻⁵ mol;
- concentration in mixture = 5.0 × 10⁻⁵ ÷ 0.0500 = 1.0 × 10⁻³ mol dm⁻³.
- Iodine formed = ½ × 1.0 × 10⁻³ = 5.0 × 10⁻⁴ mol dm⁻³.
- Rate = 5.0 × 10⁻⁴ ÷ 40 = 1.25 × 10⁻⁵ mol dm⁻³ s⁻¹.
Answer Rate = 1.3 × 10⁻⁵ mol dm⁻³ s⁻¹ (2 s.f.)
Section 6 of 6Exam Checklist & Tips
6.1.2 – Key Points for the Exam
- Clock reaction A reaction in which a sudden colour change marks the end of a fixed time interval.
- Initial rate The rate at the start when concentrations are known.
- Starch An indicator that turns blue-black with iodine.
- Control variable A factor kept constant so the effect of one change can be seen.
Common Mistakes
- Changing the volume of iodide without adding water to keep the total volume constant.
- Believing the time is the rate; rate is proportional to 1 ÷ time.
- Using too much thiosulfate, so the reaction takes so long that concentrations change substantially.
- Starting the stopwatch before all the solutions are mixed.
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