AS & A Level Chemistry - 9701 Paper 3 2019 Summer Zone 3

The thiosulfate ion, $S_{2}O_{3}^{2-}$, reacts in acidic conditions as shown.

$S_{2}O_{3}^{2-}(aq) + 2H^{+}(aq) \rightarrow S(s) + SO_{2}(g) + H_{2}O(l)$

You will investigate how the concentration of the thiosulfate ions affects the rate of this reaction. The rate can be measured by timing how long it takes for the solid sulfur that is formed to make the solution too cloudy to see through.

Small amounts of $SO_{2}$ gas may be produced during this reaction. Care must be taken to avoid inhaling this $SO_{2}$ gas.

It is very important that as soon as each experiment is complete the beaker containing the reaction mixture is emptied into the quenching bath.

FA 1 is $0.100\ \text{mol\ dm}^{-3}$ sodium thiosulfate, $Na_{2}S_{2}O_{3}$.
FA 2 is $2.00\ \text{mol\ dm}^{-3}$ hydrochloric acid, $HCl$.
distilled water

(a) Method

Experiment 1

● Fill the burette labelled FA 1 with FA 1.
● Run 45.00 cm³ of FA 1 from the burette into the 100 cm³ beaker.
● Use the measuring cylinder to measure 10.0 cm³ of FA 2.
● Add the FA 2 to the FA 1 in the beaker and start timing immediately.
● Stir the mixture once and place the beaker on the printed insert.
● Look down through the solution in the beaker at the print on the insert.
● Stop timing as soon as the precipitate of sulfur makes the print on the insert just invisible.
● Record this reaction time to the nearest second in your results table.
● Empty the contents of the beaker into the quenching bath.
● Wash out the beaker thoroughly.
● Shake the beaker to remove any excess water.

Experiment 2

● Fill a second burette with distilled water.
● Refill the burette labelled FA 1 with FA 1.
● Run 20.00 cm³ of FA 1 into the 100 cm³ beaker.
● Run 25.00 cm³ of distilled water into the same beaker.
● Use the measuring cylinder to measure 10.0 cm³ of FA 2.
● Add the FA 2 to the FA 1 in the beaker and start timing immediately.
● Stir the mixture once and place the beaker on the printed insert.
● Look down through the solution in the beaker at the print on the insert.
● Stop timing as soon as the precipitate of sulfur makes the print on the insert just invisible.
● Record this reaction time to the nearest second in your results table.
● Empty the contents of the beaker into the quenching bath.
● Wash out the beaker thoroughly.
● Shake the beaker to remove any excess water.

Experiments 3–5

Carry out three further experiments to investigate how the reaction time changes with different volumes of FA 1.
Note that the combined volume of FA 1 and distilled water must always be 45.00 cm³.
Do not use a volume of FA 1 that is less than 20.00 cm³.

Record all your results in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments. The rate of reaction can be calculated using the following expression.

$$\text{rate} = \frac{500}{\text{reaction time}}$$
[9]

(b) On the grid, plot a graph of the rate (y-axis) against the volume of FA 1 (x-axis). Label any anomalous points. Draw a line of best fit.
[4]



(c) In these experiments, the volume of FA 1 is related to the concentration of the thiosulfate ions. From your graph state the relationship between the rate of reaction and the concentration of the thiosulfate ions.
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[1]

(d) Assume that the error in the time measured for each experiment was ± 2 s.
Calculate the minimum value for the reaction rate you observed in Experiment 2.
Show your working.

minimum rate for Experiment 2 = .................................
[2]

(e) (i) A student suggested that, using a 250 cm³ beaker, the time recorded for Experiment 1 would be the same.
Discuss whether the student is correct.
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[1]

(ii) A student carried out a further experiment using the same procedure as in (a). The student used 5.00 cm³ of FA 1, 40.00 cm³ of distilled water and 10.0 cm³ of FA 2. The print on the insert never became invisible.
Explain why.
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[1]

In this experiment you will determine the enthalpy change of solution for hydrated sodium thiosulfate.

FA 3 is hydrated sodium thiosulfate, $Na_2S_2O_3·5H_2O$.

(a) Method
- Support the plastic cup in the 250 cm³ beaker.
- Rinse the measuring cylinder.
- Using the measuring cylinder, pour 25.0 cm³ of distilled water into the plastic cup.
- Measure the temperature of the water in the cup.
- Weigh the container with FA 3.
- Add all the FA 3 to the distilled water in the cup.
- Use the thermometer to stir the mixture gently until all the solid has dissolved.
- Measure the lowest temperature that is reached.
- Reweigh the container with any remaining FA 3.
- Record all your measurements.
- Calculate and record the mass of FA 3 added and the change in temperature.

[4]

(b) The enthalpy change of solution for FA 3 is the enthalpy change when 1 mole of FA 3 is dissolved in 1 dm³ of solution.

(i) Calculate how many moles of FA 3 were added to the water.

moles of FA 3 = ...................................... mol [1]

(ii) Calculate the energy change when the sample of FA 3 was added to the distilled water.
[Assume that 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0^∘C.]

energy change = .................................. J [1]

(iii) Calculate the enthalpy change of solution of FA 3.

enthalpy change of solution = ............ ............ ........................ sign value units [1]

(c) One way to improve this experiment would be to use a balance that reads to more decimal places.

Suggest two other ways in which this experiment could be altered to give a more accurate value for the enthalpy change. Explain how each would improve the accuracy.

Suggestion 1
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Suggestion 2
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[2]

(d) A student carrying out the experiment in Question 1 used all the FA 1. The student made up a fresh sample of FA 1 of the correct concentration by dissolving some FA 3 in water. This solution was then used immediately to repeat one of the experiments in Question 1 but the time was then much greater than had been measured previously.

Explain why the time was greater.
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[1]

(a) FA 4 is an aqueous solution containing a single cation and a single anion. The anion is either the sulfate ion, $\text{SO}_4^{2-}$, or the sulfite ion, $\text{SO}_3^{2-}$.

(i) To an approximately 1 cm depth of FA 4 in a test-tube, add aqueous sodium carbonate. Record your observations.
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(ii) Select reagents to identify the anion present in FA 4. Carry out a test with these reagents and record your observations.
reagents ..............................................................................................................................
observations ..........................................................................................................................
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(iii) Identify FA 4.
The formula of FA 4 is ....................................................... . [1]

(b) (i) FA 5 contains one cation and two anions. Two of these ions are listed in the Qualitative Analysis Notes. Carry out the following tests and record your observations.

[Table_1]

test | observations
Add a small spatula measure of FA 5 to a hard-glass test-tube. Heat the sample gently at first and then more strongly.
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Pour a 4 cm depth of dilute sulfuric acid into a boiling tube. Carefully add the remaining FA 5. Leave to stand until the reaction is complete. The solution produced is FA 6. Keep FA 6 for use in the following tests.

To a 1 cm depth of FA 6 in a test-tube add aqueous sodium hydroxide.
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To a 1 cm depth of FA 6 in a test-tube add aqueous ammonia.
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(ii) State the type of reaction observed when FA 5 was heated.
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(iii) Give the formula of the cation and one of the anions present in FA 5.
cation: ............................................. anion: ................................................... [1]