A-Level AQA Chemistry Physical Chemistry Practicals: Iodide ions are oxidised to iodi

Question

Iodide ions are oxidised to iodine by hydrogen peroxide in acidic conditions.

$$H_2O_2(aq) + 2H^+(aq) + 2I^-(aq) ightarrow I_2(aq) + 2H_2O(l)$$

The rate equation for this reaction can be written as

$$rate = k[H_2O_2]^{m}[H^+]^{n}$$

In an experiment to determine the order with respect to $H^+(aq)$, a reaction mixture is made containing $H^+(aq)$ with a concentration of $5.00 	ext{ mol dm}^{-3}$.

A large excess of both $H_2O_2$ and $I^-$ is used in this reaction mixture so that the rate equation can be simplified to

$$rate = k_i[H^+]^{t}$$

1. Explain why the use of a large excess of $H_2O_2$ and $I^-$ means that the rate of reaction at a fixed temperature depends only on the concentration of $H^+(aq)$.

2. Samples of the reaction mixture are removed at timed intervals and titrated with alkali to determine the concentration of $H^+(aq)$. State and explain what must be done to each sample before it is titrated with alkali.

3. A graph of the results is shown in Figure 1.

Explain how the graph shows that the order with respect to $H^+(aq)$ is zero.

4. Use the graph in Figure 1 to calculate the value of $k_i$. Give the units of $k_i$.

5. A second reaction mixture is made at the same temperature. The initial concentrations of $H^+(aq)$ and $I^-(aq)$ in this mixture are both $5.00 	ext{ mol dm}^{-3}$. There is a large excess of $H_2O_2$.
   The results are shown in Table 1.

Table 1

| Time / s | 0 | 100 | 200 | 400 | 800 | 1200 |
|----------|---|-----|-----|-----|-----|------|
| $[H^+]$ (mol dm^-3) | 0.50 | 0.44 | 0.39 | 0.31 | 0.24 | 0.19 |

Plot these results on the grid in Figure 2. The first three points have been plotted.

6. Draw a line of best fit on the grid in Figure 2.

7. Calculate the rate of reaction when $[H^+] = 0.35 	ext{ mol dm}^{-3}$. Show your working using a suitable construction on the graph in Figure 2.

8. A general equation for a reaction is shown.

$$A(aq) + B(aq) + C(aq) ightarrow D(aq) + E(aq)$$

In aqueous solution, A, B, C and D are all colourless but E is dark blue.

A reagent $(X)$ is available that reacts rapidly with $E$. This means that, if a small amount of $X$ is included in the initial reaction mixture, it will react with any $E$ produced until all of the $X$ has been used up.

Explain, giving brief experimental details, how you could use a series of experiments to determine the order of this reaction with respect to A. In each experiment you should obtain a measure of the initial rate of reaction.

Iodide ions are oxidised to iodine by hydrogen peroxide in acidic conditions - AQA - A-Level Chemistry - Question 1 - 2018 - Paper 3

Worked Solution & Example Answer:Iodide ions are oxidised to iodine by hydrogen peroxide in acidic conditions - AQA - A-Level Chemistry - Question 1 - 2018 - Paper 3

Explain why the use of a large excess of H2O2 and I– means that the rate of reaction at a fixed temperature depends only on the concentration of H+(aq).

State and explain what must be done to each sample before it is titrated with alkali.

Explain how the graph shows that the order with respect to H+(aq) is zero.

Use the graph in Figure 1 to calculate the value of ki. Give the units of ki.

Plot these results on the grid in Figure 2. The first three points have been plotted.

Draw a line of best fit on the grid in Figure 2.

Calculate the rate of reaction when [H+] = 0.35 mol dm–3. Show your working using a suitable construction on the graph in Figure 2.

Explain how you could use a series of experiments to determine the order of this reaction with respect to A.

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