Chemistry 9701/34 — October/November 2021

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

The thiosulfate ion, $\text{S}_2\text{O}_3^{2-}$ , is unstable in the presence of acid. The following reaction occurs.

\text{S}_2\text{O}_3^{2-}(\text{aq}) + 2\text{H}^+(\text{aq}) \rightarrow \text{S}(\text{s}) + \text{SO}_2(\text{aq}) + \text{H}_2\text{O}(\text{l})

The rate of this reaction can be measured by timing how long it takes for the solid sulfur that is formed to make the mixture too cloudy to see through.

You will investigate how the concentration of the thiosulfate ions affects the rate of this reaction.

Throughout these experiments care must be taken to avoid inhaling any $\text{SO}_2$ that is produced. It is very important that as soon as each experiment is complete, the contents of the beaker are emptied into the quenching bath and the beaker is rinsed thoroughly.

FB 1 is $0.100\text{ mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
FB 2 is $2.00\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
distilled water

(a)

Method

Experiment 1

Label one burette FB 1 and fill it with FB 1.
Run $45.00\text{ cm}^3$ of FB 1 from the burette into the $100\text{ cm}^3$ beaker.
Use the $25\text{ cm}^3$ measuring cylinder to measure $10.0\text{ cm}^3$ of FB 2.
Add FB 2 to FB 1 and start timing immediately.
Stir the mixture once and place the beaker on the printed insert.
View the print on the insert from above the mixture.
Stop timing when the print on the insert is no longer visible.
Record this reaction time to the nearest second.
Empty the contents of the beaker into the quenching bath.
Rinse and dry the beaker so it is ready for use in Experiment 2.

Experiment 2

Fill the second burette with distilled water.
Refill the burette labelled FB 1 with FB 1.
Run $20.00\text{ cm}^3$ of FB 1 into the $100\text{ cm}^3$ beaker.
Run $25.00\text{ cm}^3$ of distilled water into the same beaker.
Use the $25\text{ cm}^3$ measuring cylinder to measure $10.0\text{ cm}^3$ of FB 2.
Add FB 2 to the beaker and start timing immediately.
Stir the mixture once and place the beaker on the printed insert.
View the print on the insert from above the mixture.
Stop timing when the print on the insert is no longer visible.
Record this reaction time to the nearest second.
Empty the contents of the beaker into the quenching bath.
Rinse and dry the beaker so it is ready for use in the next experiment.

Experiments 3–5

Carry out three further experiments to investigate how the reaction time changes with different volumes of FB 1.

The combined volume of FB 1 and distilled water must always be $45.00\text{ cm}^3$ .
Do not use a volume of FB 1 that is less than $20.00\text{ cm}^3$ .

Record all your results in a table.
You should include the volume of FB 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments.
Calculate the rate of reaction using the following formula.

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

(b)

On the grid opposite, plot the rate on the $y$ -axis against the volume of FB 1 on the $x$ -axis. Identify any anomalous points. Draw a line of best fit.

Graph Grid

(c)

In these experiments, the volume of FB 1 is related to the concentration of the thiosulfate ions.

Use your graph to suggest the relationship between the rate of reaction and the concentration of the thiosulfate ions.

(d)

The quenching bath contains an aqueous mixture of sodium carbonate and universal indicator.

(i)

How does the quenching bath prevent the further production of $\text{SO}_2$ from the reaction?

(ii)

Suggest why the mixture contains universal indicator.

(e)

(i)

In each experiment the acid is in large excess.

Show, by calculation, that the acid is in large excess in Experiment 1.

(ii)

Suggest a reason why the acid used should be in large excess.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation9M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation13M

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