Chemistry 9701/36 — October/November 2018

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Quantitative Analysis

Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided.

Show your working and appropriate significant figures in the final answer to each step of your calculations.

1 Iron(III) ions oxidise iodide ions, $\text{I}^-$ , to iodine, $\text{I}_2$ .

2\text{Fe}^{3+}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{Fe}^{2+}(\text{aq}) + \text{I}_2(\text{aq})

In this experiment you will investigate how the rate of this reaction is affected by the concentration of $\text{Fe}^{3+}$ ions. To do this you will add thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ , and starch indicator to a mixture of $\text{Fe}^{3+}(\text{aq})$ and $\text{I}^-(\text{aq})$ . The iodine produced by the reaction reacts immediately with the thiosulfate ions and is reduced back to iodide.

\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})

When all the thiosulfate has reacted, the iodine remaining in solution turns the starch indicator blue-black. The rate of reaction can be determined by timing how long it takes for the reaction mixture to turn blue-black.

FB 1 is $0.0500\text{ mol dm}^{-3}$ acidified iron(III) chloride, $\text{FeCl}_3$ .
FB 2 is $0.0500\text{ mol dm}^{-3}$ potassium iodide, $\text{KI}$ .
FB 3 is $0.00500\text{ mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
FB 4 is starch indicator.

(a)

Method

Experiment 1

Fill the burette labelled FB 1 with FB 1.
Run $20.00\text{ cm}^3$ of FB 1 into a $100\text{ cm}^3$ beaker.
Using the measuring cylinder add the following to the second $100\text{ cm}^3$ $100 cm^{3}$ beaker:
- $10\text{ cm}^3$ of FB 2
- $20\text{ cm}^3$ of FB 3
- $10\text{ cm}^3$ of FB 4
Add the contents of the first beaker to the second beaker and start timing immediately.
Stir the mixture once and place the beaker on a white tile.
Stop timing as soon as the solution turns blue-black. Ignore any colour changes that occur before the intense blue-black colouration.
Record this reaction time to the nearest second in the space provided on page 4.
Rinse both beakers and shake dry. Rinse and dry the glass rod.

Experiment 2

Fill a second burette with distilled water.
Run $10.00\text{ cm}^3$ of FB 1 into a $100\text{ cm}^3$ beaker.
Run $10.00\text{ cm}^3$ of distilled water into the same beaker containing FB 1.
Using the measuring cylinder add the following to the second $100\text{ cm}^3$ $100 cm^{3}$ beaker:
- $10\text{ cm}^3$ of FB 2
- $20\text{ cm}^3$ of FB 3
- $10\text{ cm}^3$ of FB 4
Add the contents of the first beaker to the second beaker and start timing immediately.
Stir the mixture once and place the beaker on a white tile.
Stop timing as soon as the solution turns blue-black. Ignore any colour changes that occur before the intense blue-black colouration.
Record this reaction time to the nearest second in the space provided on page 4.
Rinse both beakers and shake dry. Rinse and dry the glass rod.

Experiments 3–5

Carry out three further experiments to investigate how the reaction time changes with different volumes of FB 1.
Remember that the combined volume of FB 1 and distilled water must always be $20.00\text{ cm}^3$ .
Do not carry out an experiment using $15.00\text{ cm}^3$ of FB 1.
Do not use a volume of FB 1 that is less than $5.00\text{ cm}^3$ .

Keep all FB labelled solutions for use in (e) and in Question 2.

Record all your results in a single table. You should include the volume of FB 1, the volume of distilled water and the reaction time.

The relative rate for the reaction is given by the following expression.

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

Use this expression to calculate the relative rate for each of your experiments and record the values in your results table.

10M

(b)

On the grid opposite, plot the relative rate against the volume of FB 1. Include the origin in your plot. Label any points you consider anomalous. Draw a line of best fit.

Fig. 1.1

(c)

From your graph, what conclusion can you make about the relationship between the relative rate for the reaction and the volume of FB 1 used? Explain your answer.

(d)

A student carried out the same experiment but used $15.00\text{ cm}^3$ of FB 1. The student recorded a value for the reaction time of $28\text{ s}$ .

(i)

Use your graph to calculate the time you would have expected to record if you had carried out an experiment using $15.00\text{ cm}^3$ of FB 1.

Show the construction lines on your graph and show your working in the calculation.

(ii)

Calculate the percentage difference between your value and that of the student.

Show your working.

(e)

You are to carry out a sixth experiment. The concentrations of iron(III) chloride, sodium thiosulfate and starch indicator should all be the same as in Experiment 2 but the concentration of iodide ions should be twice the value that it is in Experiment 2.

State the volume of each solution used and record the reaction time to the nearest second.

(f)

(i)

$20.00\text{ cm}^3$ of $0.0500\text{ mol dm}^{-3}$ $\text{FeCl}_3$ , FB 1, were reacted with excess $\text{KI}$ , FB 2.

Using the information on page 2, calculate the number of moles of $\text{I}_2$ produced.

(ii)

The iodine produced in (i) required $35.00\text{ cm}^3$ of a different solution of sodium thiosulfate for complete reaction.

Calculate the concentration of the solution of sodium thiosulfate used.

The rest of this paper

1 more questions

Q2Qualitative Analysis · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation16M

Loading the full paper…