Chemistry 9701 · AS & A Level · The Periodic Table: chemical periodicity

The Periodic Table: chemical periodicity — practice question

Every third-period element from sodium through chlorine forms an oxide.
(a)[4]

Draw a diagram showing the shape of each oxide molecule, $\text{SO}_3$ and $\text{Cl}_2\text{O}$. Name each shape. In $\text{SO}_3$, each oxygen atom is double-bonded to the sulfur atom.

(b(i))[2]

Explain why $\text{MgO}$ has a higher melting point than $\text{Na}_2\text{O}$.

(b(ii))[2]

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

(c)

$\text{SO}_3$ is formed when $\text{SO}_2$ reacts with $\text{O}_2$ in the Contact process. A dynamic equilibrium is reached. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \quad \Delta H = -196\,\text{kJ mol}^{-1}$

(c(i))[4]

Explain why raising the total pressure, at constant temperature, increases the rate at which $\text{SO}_3$ is produced and also increases the yield of $\text{SO}_3$.

(c(ii))[2]

Explain why the slopes of the $\text{SO}_2$ and $\text{O}_2$ lines get smaller with time.

(c(iii))[1]

Explain why all three lines level off.

(c(iv))[1]

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

(d)

A mixture containing $2.00$ moles of $\text{SO}_2(g)$ and $2.00$ moles of $\text{O}_2(g)$ is sealed in a container with a suitable catalyst, at constant temperature and pressure. At equilibrium, the mixture contains $1.98$ moles of $\text{SO}_3(g)$. The total volume of the equilibrium mixture is $40.0\,\text{dm}^3$. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$

(d(i))[1]

Write the expression for the equilibrium constant, $K_c$, for the reaction in which $\text{SO}_2(g)$ and $\text{O}_2(g)$ form $\text{SO}_3(g)$.

(d(ii))[2]

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

(d(iii))[3]

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

Worked solution & mark scheme

This 22-mark question has a full step-by-step worked solution and mark scheme. One marking point: Correctly shown shape of $SO_3$ with trigonal planar geometry

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