Chemistry 9701/35 — October/November 2011

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q1Presentation of Data and ObservationsManipulation, Measurement and ObservationAnalysis, Conclusions and EvaluationFree sample

You are required to investigate the effect of temperature on the rate of reaction of peroxydisulfate ions with iodide ions. Iodide ions are oxidised to iodine by peroxydisulfate ions.

\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})

FA 1 is aqueous potassium peroxydisulfate, $\text{K}_2\text{S}_2\text{O}_8$ .
FA 2 is an aqueous solution containing a mixture of potassium iodide, $\text{KI}$ , sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ , and starch.

When FA 1 and FA 2 are mixed together the potassium peroxydisulfate reacts with the potassium iodide to make iodine. As soon as this iodine is formed, it reacts with the sodium thiosulfate and is turned back into iodide ions. Only when all the sodium thiosulfate has reacted does iodine remain in the solution. The solution then turns blue-black because of the presence of the starch indicator. The rate of reaction can be determined by the time it takes for a blue-black colour to first appear in the colourless mixture.

(a)

Method

Read through the method and prepare a table on page 3 to record the initial and final temperatures and the reaction time for each experiment, before starting any practical work.

Half-fill a $250\text{ cm}^3$ beaker with water to act as a water bath.
Place it on a tripod and gauze and heat it with a Bunsen burner to about $65\text{ }^{\circ}\text{C}$ then remove the Bunsen. While your water is being heated continue with the following steps of the method.
Fill the burette, labelled FA 1, with the aqueous potassium peroxydisulfate, FA 1.
Fill the burette, labelled FA 2, with the mixture of solutions, FA 2.
Measure $10.0\text{ cm}^3$ of FA 1 into a boiling tube.
Measure $10.0\text{ cm}^3$ of FA 2 into a second boiling tube.
Place both boiling tubes in the water bath.
Clamp one of the tubes and place a thermometer in this tube.
When the temperature of this solution has reached about $60\text{ }^{\circ}\text{C}$ , pour the contents of the second tube into the clamped tube. Start timing immediately, note the temperature and stir the mixture.
Record this initial temperature.
Stop timing as soon as the blue-black colour appears. Record this reaction time to the nearest second and record the final temperature.
Repeat the experiment at decreasing temperatures as many times as necessary to generate data for plotting a graph. The experiment should not be performed at a temperature below about $30\text{ }^{\circ}\text{C}$ . The temperature of the water bath may be adjusted by adding cold water or by reheating. (Boiling tubes may be rinsed and reused.)

(b)

The rate of reaction for each experiment can be represented by the following.

\text{'rate'} = \frac{1000}{\text{reaction time in seconds}}

Complete the following table for each of your experiments. The mean temperature is the average of the initial and final temperature for the experiment.

mean temperature / °C	'rate'

(c)

(i)

Using your values in (b), plot a graph of 'rate' ( $y$ -axis) against mean temperature ( $x$ -axis). Choose suitable scales to allow you to extrapolate the graph to include the 'rate' at $20\text{ }^{\circ}\text{C}$ .

Graph Grid

(ii)

Use your graph to obtain a value for the 'rate' at $20\text{ }^{\circ}\text{C}$ .

'rate' at $20\text{ }^{\circ}\text{C}$ is .................................................

(d)

It has been suggested that an increase in temperature of $10\text{ }^{\circ}\text{C}$ will double the rate of reaction. Use two pairs of temperatures from your graph to confirm or deny this statement.

(e)

(i)

Which of your experiments has the greatest percentage error in timing?

(ii)

Calculate the percentage error in (i).

You may assume that the error in measuring the time for a reaction is $\pm 0.5$ seconds.

error in (i) = ............................................ %

(f)

A student had difficulty in drawing a line of best fit. Identify a source of error in the experimental procedure. Do not include any errors involving the precision of apparatus.

(g)

Suggest a modification that could be used to reduce this error.

(h)

Using FA 1, FA 2 and distilled water, describe how you could investigate the effect of concentration of potassium peroxydisulfate on the rate of reaction.

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