Chemistry 9701 · AS & A Level · Atoms, molecules and stoichiometry

Atoms, molecules and stoichiometry — practice question

At constant temperature, the reaction between $\text{Na}_2\text{S}_2\text{O}_3\text{(aq)}$ and $\text{HCl(aq)}$ is followed. $\text{Na}_2\text{S}_2\text{O}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{SO}_2 + \text{S} + \text{H}_2\text{O}$. Fig. 4.1 illustrates how the concentration of $\text{HCl(aq)}$ changes with time.
(a(i))[2]

Use Fig. 4.1 to determine the average rate of change of concentration of $\text{HCl(aq)}$ in this reaction between $0$-$100$ seconds and between $400$-$500$ seconds. Include units in your answers.

(a(ii))[1]

Use Fig. 4.1 to identify the limiting reagent. Justify your answer.

(a(iii))[1]

Explain why the rate of reaction changes with time.

(a(iv))[2]

The reaction between $\text{Na}_2\text{S}_2\text{O}_3\text{(aq)}$ and $\text{HCl(aq)}$ is repeated in a second experiment. In this second experiment, $25.0\,\text{cm}^3$ of $0.050\,\text{mol dm}^{-3}\,\text{Na}_2\text{S}_2\text{O}_3\text{(aq)}$ reacts with $0.0020\,\text{mol}$ of $\text{HCl(aq)}$. Calculate the number of sulfur atoms produced.

(a(v))[1]

Explain why the rate of reaction cannot be monitored accurately by measuring the volume of $\text{SO}_2\text{(g)}$ produced in this reaction.

(b(i))[2]

Fig. 4.2 shows one possible arrangement of outer-shell electrons in a single $\text{SO}_2$ molecule. Use Fig. 4.2 to predict the shape and bond angle of a molecule of $\text{SO}_2$.

(b(ii))[2]

Use Table 4.1 to predict the strength of the dipole moment of $\text{SO}_2$, if any, compared with that of $\text{H}_2\text{O}$. Explain your answer.

(iv)[2]

The reaction between $\text{Na}_2\text{S}_2\text{O}_3(aq)$ and $\text{HCl}(aq)$ is repeated in a second experiment. In this second experiment, $25.0\text{ cm}^3$ of $0.050\text{ mol dm}^{-3}$ $\text{Na}_2\text{S}_2\text{O}_3(aq)$ reacts with $0.0020\text{ mol}$ of $\text{HCl}(aq)$. Calculate the number of sulfur atoms produced.

(v)[1]

Explain why the rate of reaction cannot be monitored accurately by measuring the volume of $\text{SO}_2(g)$ produced in this reaction.

Worked solution & mark scheme

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