Chemistry 9701/23 — May/June 2018

Cambridge AS Level · AS Level Structured Questions · worked solutions for every part, with the mark scheme

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Topics Chemical Bonding · Atoms, Molecules and Stoichiometry · Introduction to Organic Chemistry · States of Matter · Equilibria · Reaction Kinetics · +6 more

Q1Chemical BondingStates of MatterEquilibriaReaction KineticsAtoms, Molecules and StoichiometryFree sample

The elements sodium to chlorine, in the third period, all form oxides.

(a)

Draw a diagram to show the shape of the molecule of each of the oxides, $\text{SO}_3$ and $\text{Cl}_2\text{O}$ .

Name each shape.

In $\text{SO}_3$ each oxygen atom forms a double bond with the sulfur atom.

(b)

(i)

Explain why the melting point of $\text{MgO}$ is higher than that of $\text{Na}_2\text{O}$ .

(ii)

Explain why the melting point of $\text{SiO}_2$ is much higher than that of $\text{SO}_3$ .

(c)

$\text{SO}_3$ is produced by the reaction between $\text{SO}_2$ and $\text{O}_2$ in the Contact process. A dynamic equilibrium is established.

2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{SO}_3(\text{g}) \quad \Delta H = -196 \text{ kJ mol}^{-1}

(i)

Explain why increasing the total pressure, at constant temperature, increases the rate of production of $\text{SO}_3$ and increases the yield of $\text{SO}_3$ .

(ii)

The graph shows how the concentrations of all three species in the system change with time for a typical reaction mixture. The gradients of all three lines decrease with time and then level off in this dynamic equilibrium.

Fig. 1.2

Explain why the gradients of the $\text{SO}_2$ and $\text{O}_2$ lines decrease with time.

(iii)

Explain why all three lines become horizontal.

(iv)

Suggest a reason why the initial gradient of the $\text{SO}_2$ line is steeper than that of the $\text{O}_2$ line.

(d)

$2.00$ moles of $\text{SO}_2(\text{g})$ and $2.00$ moles of $\text{O}_2(\text{g})$ are sealed in a container with a suitable catalyst, at constant temperature and pressure. The resulting equilibrium mixture contains $1.98$ moles of $\text{SO}_3(\text{g})$ .

The total volume of the equilibrium mixture is $40.0 \text{ dm}^3$ .

2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{SO}_3(\text{g})

(i)

Write the expression for the equilibrium constant, $K_c$ , for the reaction between $\text{SO}_2(\text{g})$ and $\text{O}_2(\text{g})$ to produce $\text{SO}_3(\text{g})$ .

(ii)

Calculate the amount, in moles, of $\text{SO}_2(\text{g})$ and $\text{O}_2(\text{g})$ in the equilibrium mixture.

(iii)

Use your answers to (d)(i) and (d)(ii) to calculate the value of $K_c$ for this equilibrium mixture. Give the units of $K_c$ .

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