A-Level AQA Chemistry Rate Equations: Iodide ions are oxidised to iodi

Question

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

H2O2(aq) + 2H+(aq) + 2I-(aq) → I2(aq) + 2H2O(l)

The rate equation for this reaction can be written as

rate = k[H2O2]^{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 0.500 mol dm-3.

A large excess of both H2O2 and I- is used in this reaction mixture so that the rate equation can be simplified to

rate = k'[H+]^{d}.

1. 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).

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 k1. Give the units of k1.

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 0.500 mol dm-3. There is a large excess of H2O2.

In this reaction mixture, the rate depends only on the concentration of I-(aq).

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 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) → 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 k1. Give the units of k1.

Plot these results on the grid in Figure 2.

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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