A-Level AQA Chemistry Kinetics: This question is about rates of

Question

This question is about rates of reaction.
Iodine and propanone react together in an acid-catalysed reaction

CH₃C(O)CH₃(aq) + I₂(aq) → CH₃C(O)I(aq) + HI(aq)

A student completed a series of experiments to determine the order of reaction with respect to iodine.

Method
1. Transfer 25 cm³ of 1.0 mol dm⁻³ propanone solution into a conical flask.
2. Add 10 cm³ of 1.0 mol dm⁻³ HCl(aq).
3. Add 25 cm³ of 5.0 × 10⁻³ mol dm⁻³ I₂(aq) and start a timer.
4. After intervals of 1 minute, remove 1.0 cm³ sample of the mixture and add each sample to a separate beaker containing an excess of Na₂S₂O₃(aq).
5. Titrate the contents of each beaker with a standard solution of sodium thiosulfate until the endpoint is reached.

The volume of sodium thiosulfate solution used in each titration is proportional to the concentration of iodine in each beaker.

Table 5 shows the results of the experiment.

**Table 5**

| Time / minutes | Volume of sodium thiosulfate solution / cm³ |
|----------------|---------------------------------------------|
| 1              | 41                                          |
| 2              | 35                                          |
| 3              | 24                                          |
| 4              | 22                                          |
| 5              | 16                                          |
| 6              | 10                                          |

Use the results in Table 5 to draw a graph of volume of sodium thiosulfate solution against time.

Draw a line of best fit.

Explain how the graph shows that the reaction is zero-order with respect to iodine in the reaction between propanone and iodine.

The Arrhenius equation can be written as

ln k = −Ea/RT + ln A

Figure 8 shows a graph of ln k against 1/T for the reaction

2 HI(g) → H₂(g) + I₂(g)

**Figure 8**

| 1/T / K⁻¹ | ln k  |
|-----------|-------|
| 0.0012    | −2    |
| 0.0014    | −6    |
| 0.0016    | −8    |
| 0.0018    | −12   |
| 0.0020    | −14   |

Use Figure 8 to calculate a value for the activation energy (Ea) in kJ mol⁻¹ for this reaction.

The gas constant R = 8.31 J K⁻¹ mol⁻¹.

This question is about rates of reaction - AQA - A-Level Chemistry - Question 10 - 2021 - Paper 2

Worked Solution & Example Answer:This question is about rates of reaction - AQA - A-Level Chemistry - Question 10 - 2021 - Paper 2

Suggest why the 1.0 cm³ portions of the reaction mixture are added to an excess of Na₂S₂O₃ solution.

Suggest why the order of this reaction with respect to propanone can be ignored in this experiment.

Use the results in Table 5 to draw a graph of volume of sodium thiosulfate solution against time.

Explain how the graph shows that the reaction is zero-order with respect to iodine.

Use Figure 8 to calculate a value for the activation energy (Ea) in kJ mol⁻¹.

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