Chemistry 9701/33 — February/March 2018

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

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.

You will investigate how increasing temperature affects the rate of a reaction.

Sodium thiosulfate reacts with acid to form a pale yellow precipitate of sulfur. The ionic equation for the reaction is given.

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

You will measure the time it takes for the sulfur formed in the reaction to obscure the print on the Insert supplied.

Record your results in a table on page 4. Your table should include the rate of reaction for each experiment.

FA 1 is an $18.1\text{ g dm}^{-3}$ solution of hydrated sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3\cdot5\text{H}_2\text{O}$ .
FA 2 is a $0.050\text{ mol dm}^{-3}$ solution of a strong monoprotic acid, $\text{HZ}$ .

(a)

Method

Approximately half fill the $250\text{ cm}^3$ beaker with tap water and place it on the tripod and gauze over the Bunsen burner.
Heat the water in the beaker to about $55\text{ }^\circ\text{C}$ and then switch off the Bunsen burner. This will be your hot water bath.
Use the $25\text{ cm}^3$ measuring cylinder to transfer $10\text{ cm}^3$ of FA 1 into boiling tube 1. Place boiling tube 1 into your hot water bath.
Use the $50\text{ cm}^3$ measuring cylinder to transfer $20\text{ cm}^3$ of FA 2 into boiling tube 2. Place boiling tube 2 into your hot water bath.
Leave boiling tubes 1 and 2 in the hot water bath to heat up for use in Experiment 2.
Start Experiment 1.

Experiment 1

Use the $50\text{ cm}^3$ measuring cylinder to transfer $20\text{ cm}^3$ of FA 2 into the $100\text{ cm}^3$ beaker.
Measure and record the temperature of FA 2.
Use the $25\text{ cm}^3$ measuring cylinder to transfer $10\text{ cm}^3$ of FA 1 into the same beaker and start timing immediately.
Swirl the beaker once to mix the solutions and place the beaker on the Insert.
Look down through the beaker and contents onto the Insert.
Stop timing as soon as the precipitate of sulfur obscures the print on the Insert.
Record the 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

Measure and record the temperature of FA 2 in boiling tube 2.
Carefully transfer the hot contents of boiling tube 2 into the $100\text{ cm}^3$ beaker.
Carefully transfer the hot contents of boiling tube 1 into the same beaker and start timing immediately.
Swirl the beaker once to mix the solutions and place the beaker on the Insert.
Look down through the beaker and contents onto the Insert.
Stop timing as soon as the precipitate of sulfur obscures the print on the Insert.
Record the 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 3.

Experiment 3

Use the $25\text{ cm}^3$ measuring cylinder to transfer $10\text{ cm}^3$ of FA 1 into boiling tube 1. Place boiling tube 1 into your hot water bath.
Use the $50\text{ cm}^3$ measuring cylinder to transfer $20\text{ cm}^3$ of FA 2 into boiling tube 2. Place boiling tube 2 into your hot water bath.
Place the thermometer in boiling tube 2. When the temperature of FA 2 is about $8\text{ }^\circ\text{C}$ lower than that for Experiment 2 record the temperature. Remove the thermometer and transfer the contents of boiling tube 2 into the $100\text{ cm}^3$ beaker.
Transfer the contents of boiling tube 1 into the same beaker and start timing immediately.
Swirl the beaker once to mix the solutions and place the beaker on the Insert.
Look down through the beaker and contents onto the Insert.
Stop timing as soon as the precipitate of sulfur obscures the print on the Insert.
Record the 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 Experiments 4 and 5.

Experiments 4 and 5

Repeat the method for Experiment 3 but at two different temperatures.
Keep the temperature of FA 2 between room temperature and $55\text{ }^\circ\text{C}$ . Do not exceed $55\text{ }^\circ\text{C}$ .

Record all your results in your table on page 4.

Results

The rate of reaction can be calculated as shown.

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

Calculate the rate of reaction for each of your five experiments. Record these rates in your table.

(b)

On the grid plot a graph of rate of reaction on the $y$ -axis, starting at zero, against temperature on the $x$ -axis. Select a scale for the $x$ -axis which includes a temperature of $15.0\text{ }^\circ\text{C}$ . Label your axes and any points you consider anomalous.

Draw a line of best fit and extrapolate it to $15.0\text{ }^\circ\text{C}$ .

grid_plot

(c)

Use your graph to calculate the time to the nearest second that the reaction would have taken if you had carried it out at $17.5\text{ }^\circ\text{C}$ . Show on the grid how you obtained your answer.

(d)

Explain, by referring to your graph or your table of results, how the rate of reaction is affected by increasing temperature.

(e)

Calculations

(i)

Calculate the concentration of hydrated sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3\cdot5\text{H}_2\text{O}$ , in FA 1 in $\text{mol dm}^{-3}$ .

(ii)

Calculate the concentration of the strong monoprotic acid, HZ, in the solution immediately after FA 1 was added to FA 2 in the beaker.

(iii)

Use the equation on page 2 to determine which reagent, FA 1 or FA 2, was in excess.

(f)

(i)

Calculate the maximum percentage error in measuring the reaction time you recorded for Experiment 2. Assume that the maximum error of the timer is $\pm 0.5\text{ s}$ .

(ii)

A student suggested that the error in measuring the reaction time in Experiment 1 was greater than for Experiment 2.

Give one reason why the student could be correct.

(g)

Suggest two ways to improve the accuracy of the results of these experiments.

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Q2Qualitative Analysis · Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation16M

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