Question 8

Some students investigated how the concentration of peroxodisulfate ions affects the rate of reaction with iodide ions. The equation for the reaction is:

\text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{SO}_4^{2-}(\text{aq}) + \text{I}_2(\text{aq})

They used an iodine clock reaction method, where sodium thiosulfate and starch indicator are added to the reaction mixture. The iodine produced reacts immediately with thiosulfate until the thiosulfate is completely consumed, at which point any further iodine produced turns the starch indicator dark blue.

The students used the following method:

Place a measured volume of potassium iodide solution, sodium thiosulfate solution, and starch indicator in a beaker.
In a separate boiling tube, mix a variable volume of sodium peroxodisulfate solution with water.
Add the contents of the boiling tube to the beaker, stir, and start a timer.
Record the time taken for the mixture to suddenly turn dark blue.

The students kept the volume of potassium iodide, sodium thiosulfate, and starch constant in all experiments. They aimed to keep the total volume of sodium peroxodisulfate solution and water constant at $cm330\text{ cm}^3$ in each experiment, and performed all experiments at the same temperature.

The table shows their results:

Experiment	Volume of sodium peroxodisulfate solution in $cm3\text{cm}^3$	Volume of water in $cm3\text{cm}^3$	Time in $s\text{s}$
1	30	0	14.5
2	25	5	17.6
3	20	10	21.8
4	15	15	29.1
5	10	20	43.5
6	5	25	87.2
7	2	38	215.0

Explain why the results of experiment 7 should not be used.

The rate of reaction can be calculated using the following equation:

\text{rate of reaction} = \frac{1000}{\text{time taken}}

Calculate the rate of reaction for experiment 4. Give your answer to one decimal place.

Another group plotted a graph of the rate of reaction against the concentration of peroxodisulfate ions. The graph shows a straight line passing through the origin $(0, 0)$ .

Describe the relationship between the concentration of peroxodisulfate ions and the rate of reaction shown by this graph.

Explain, in terms of collision theory, why increasing the concentration has this effect on the rate.