Chemistry 9701/53 — October/November 2017

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Analysis, Conclusions and Evaluation · Planning

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Iodide ions, $\text{I}^-$ , and persulfate ions, $\text{S}_2\text{O}_8^{2-}$ , react according to the following equation.

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

The rate of reaction between these ions can be determined from the time it takes for a certain amount of iodine, $\text{I}_2(\text{aq})$ , to be produced.

A mixture of solutions is prepared, containing known volumes of
- aqueous ammonium persulfate, $(\text{NH}_4)_2\text{S}_2\text{O}_8(\text{aq})$ ,
- aqueous sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3(\text{aq})$ ,
- starch indicator.
A known volume of aqueous potassium iodide, $\text{KI}(\text{aq})$ , is added to this mixture and a timer is started.
After the reactants are mixed, they react slowly to produce iodine, $\text{I}_2(\text{aq})$ .
Any iodine initially produced is removed by a reaction with thiosulfate ions.

\text{I}_2(\text{aq}) + 2\text{S}_2\text{O}_3^{2-}(\text{aq}) \rightarrow 2\text{I}^-(\text{aq}) + \text{S}_4\text{O}_6^{2-}(\text{aq})

Iodine, $\text{I}_2(\text{aq})$ , is continuously removed until all of the thiosulfate ions have been used up.
After that time any $\text{I}_2(\text{aq})$ that is produced turns the starch indicator blue.
The time of the first appearance of the blue colour is recorded.
This procedure is repeated with different volumes of reactants, keeping the total volume of the reaction mixture constant by adding the required volume of distilled water.

You are to plan a series of experiments to determine the effect of changing the concentration of iodide ions on the rate of reaction.

You are provided with the following materials.

solid ammonium persulfate, $(\text{NH}_4)_2\text{S}_2\text{O}_8(\text{s})$
$0.20\text{ mol dm}^{-3}$ aqueous $\text{KI}$ , a source of $\text{I}^-(\text{aq})$
$0.0050\text{ mol dm}^{-3}$ aqueous $\text{Na}_2\text{S}_2\text{O}_3$ , a source of $\text{S}_2\text{O}_3^{2-}(\text{aq})$
starch indicator

(a)

(i)

Calculate the mass of $(\text{NH}_4)_2\text{S}_2\text{O}_8(\text{s})$ that would be required to prepare $250\text{ cm}^3$ of a standard solution of concentration $1.00\text{ mol dm}^{-3}$ .

$[A_r \text{ values: N, 14.0; H, 1.0; S, 32.1; O, 16.0}]$

(ii)

Describe how, after weighing the mass calculated in (i), you would prepare this standard solution for use in your experiment.

Give the name and capacity, in $\text{cm}^3$ , of any apparatus used.

(iii)

Explain how the use of starch solution improves the accuracy of the experiment.

(b)

A student planned five experiments to investigate the effect of iodide concentration, $[\text{I}^-]$ , on the rate of reaction. The table shows the volumes used in experiment 1.

Complete the table for experiments 2 to 5.

experiment	volume of $1.00\text{ mol dm}^{-3}$ $(\text{NH}_4)_2\text{S}_2\text{O}_8(\text{aq})$ / $\text{cm}^3$	volume of $0.20\text{ mol dm}^{-3}$ $\text{KI}(\text{aq})$ / $\text{cm}^3$	volume of water / $\text{cm}^3$	volume of $0.0050\text{ mol dm}^{-3}$ $\text{Na}_2\text{S}_2\text{O}_3(\text{aq})$ / $\text{cm}^3$	volume of starch solution / $\text{cm}^3$
1	25.0	10.0	0.0	5.0	1.0
2				5.0	1.0
3				5.0	1.0
4				5.0	1.0
5				5.0	1.0

(c)

In a different experiment, a student mixed the following solutions and measured the time taken for the reaction.

$10.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}$ $(\text{NH}_4)_2\text{S}_2\text{O}_8(\text{aq})$
$5.0\text{ cm}^3$ of $0.0050\text{ mol dm}^{-3}$ $\text{Na}_2\text{S}_2\text{O}_3(\text{aq})$
$5.0\text{ cm}^3$ of $0.20\text{ mol dm}^{-3}$ $\text{KI}(\text{aq})$
$1.0\text{ cm}^3$ of starch indicator

(i)

The time taken for the blue colour to appear was 134 seconds (to the nearest second).

Calculate the rate of production of moles of $\text{I}_2$ , in $\text{mol dm}^{-3}\text{ s}^{-1}$ .

(ii)

What should the student have done to make sure that the results were reliable?

(iii)

The $5.0\text{ cm}^3$ of $0.0050\text{ mol dm}^{-3}$ $\text{Na}_2\text{S}_2\text{O}_3(\text{aq})$ was measured using a $50\text{ cm}^3$ burette which had graduations every $0.1\text{ cm}^3$ .

Calculate the maximum percentage error in the measured volume of this solution.

(d)

A second student tried to perform the same experiment but found that the reaction mixture turned blue immediately after $\text{KI}(\text{aq})$ was added.

State what error the student had made.

(e)

The following information gives some of the hazards associated with the chemicals used in the procedure.

Chemical	Hazard Information
Ammonium persulfate	Solid is oxidising and hazardous to the environment. Contact with combustible material may cause fire. It is classified as health hazard, is harmful if swallowed and is irritating to eyes, respiratory system and skin. Solutions equal to or more concentrated than $0.2\text{ mol dm}^{-3}$ should be labelled health hazard and hazardous to the environment. Solutions equal to or more concentrated than $0.05\text{ mol dm}^{-3}$ but less concentrated than $0.2\text{ mol dm}^{-3}$ should be labelled health hazard.
Potassium iodide	All solutions are low hazard.
Sodium thiosulfate	All solutions are low hazard.

Describe one relevant precaution, other than eye protection and a lab coat, that should be taken to keep the risk associated with the chemicals used to a minimum. Explain your answer.

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