Chemistry 5070 · O Level · Rate of reaction

Rate of reaction — practice question

Mixing aqueous sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$(aq), with dilute hydrochloric acid gives a pale yellow sulfur precipitate, so the reaction mixture turns cloudy. $\text{Na}_2\text{S}_2\text{O}_3$(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + S(s) + SO_2(g) + H_2O(l)$
(a)[1]

Why are the experiments carried out in a fume cupboard?

(b(i))[1]

State one factor, apart from concentration, that is likely to change the rate of reaction.

(b(ii))[1]

What method would you use to stop the variable in (b)(i) changing during each experiment?

(c(i))[2]

Fill in the table with the two missing volumes.

(c(ii))[1]

Which one of the five experiments shows the highest rate of reaction?

(c(iii))[2]

What do the table results show about how the rate of reaction changes when the concentration of sodium thiosulfate is increased?

(d(i))[1]

Calculate how many moles of sodium thiosulfate are present in the beaker. (For experiment 1, the beaker contains $10\text{ cm}^3$ of $0.50\text{ mol dm}^{-3}$ sodium thiosulfate.)

(d(ii))[1]

What total volume do the liquids have in experiment 1?

(d(iii))[1]

Calculate the concentration of sodium thiosulfate in the beaker at the beginning of experiment 1.

(e)[2]

Give a test and observation for identifying sulfur dioxide gas.

(f)[2]

Give a test and result that show sulfite ions, $\text{SO}_3^{2-}$, are present in the aqueous solution.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Sulfur dioxide / SO₂ is poisonous or toxic” …

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