Edexcel IGCSE Chemistry 3.9 Catalysts

Practise rate experiments with catalysts and changing conditions, using gas volumes, mass loss, gradients and fair-test controls.

[Maximum number: 3]

Dilute hydrochloric acid reacts with a solution of sodium thiosulfate $\left(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\right)$ to form a precipitate.

The equation for the reaction is

\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{NaCl}(\mathrm{aq})+\mathrm{S}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{SO}_{2}(\mathrm{~g})

(a)

The reaction is often used to investigate rates of reaction.

The diagram shows the apparatus a student uses to investigate the effect of temperature on the rate of the reaction.

Figure for Question (a) — Edexcel IGCSE Chemistry

This is the student's method.
- pour $50 \mathrm{~cm}^{3}$ of cold sodium thiosulfate solution into a conical flask and heat it to $20^{\circ} \mathrm{C}$
- draw a cross ( X ) on a piece of paper and place it under the flask
- add $5 \mathrm{~cm}^{3}$ of dilute hydrochloric acid to the flask
- look at the cross from above and record the time taken until the cross cannot be seen

The student repeats the experiment four times, using sodium thiosulfate solution at a different temperature each time.

He keeps the volumes of sodium thiosulfate solution and hydrochloric acid constant in each experiment.

Give two other factors that the student should keep constant.

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(b)

The table shows the student's results.

Table for Question (b) — Edexcel IGCSE Chemistry

The highest temperature the student uses is 60 °C because he thinks the results might not be as accurate at temperatures higher than 60 °C.

Suggest a reason why the results might not be as accurate at temperatures higher than 60 °C.

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3.9 Experiments on factors affecting reaction rate question 1

Question (a)

Question (b)