Chemistry 9701/43 — October/November 2015

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

questions

100

marks

120

minutes

Topics Introduction to A Level Organic Chemistry · Transition Elements · Nitrogen Compounds · Chemical Energetics · Group 2 · Electrochemistry · +4 more

Q1Chemical EnergeticsGroup 2Free sample

(a)

The dissolving of an ionic compound in water is accompanied by an energy change, the enthalpy change of solution, $\Delta H_{\text{sol}}$ .

\text{MgCl}_2(\text{s}) + \text{aq} \rightarrow \text{Mg}^{2+}(\text{aq}) + 2\text{Cl}^-(\text{aq})

Describe, in terms of bond breaking and bond making, what happens to the solid ionic lattice when an ionic compound dissolves in water.

(b)

(i)

What is meant by the term enthalpy change of solution, $\Delta H_{\text{sol}}$ ?

(ii)

Use the following data to calculate the standard enthalpy change of hydration, $\Delta H^{\ominus}_{\text{hyd}}$ , of chloride ions, $\text{Cl}^-(\text{g})$ .
You may find it helpful to construct an energy cycle.

enthalpy change	value
$\Delta H^{\ominus}_{\text{hyd}}(\text{Mg}^{2+}(\text{g}))$	$-1925 \text{ kJ mol}^{-1}$
lattice energy of $\text{MgCl}_2(\text{s})$	$-2524 \text{ kJ mol}^{-1}$
enthalpy change of solution for $\text{MgCl}_2(\text{s})$	$-155 \text{ kJ mol}^{-1}$

\Delta H^{\ominus}_{\text{hyd}}(\text{Cl}^-(\text{g})) = \text{.........................} \text{ kJ mol}^{-1}

(iii)

The enthalpy change of hydration for $\text{Na}^+$ , $\Delta H^{\ominus}_{\text{hyd}}(\text{Na}^+(\text{g}))$ , is $-410 \text{ kJ mol}^{-1}$ .

Suggest an explanation for why the $\Delta H^{\ominus}_{\text{hyd}}$ of the $\text{Na}^+$ ion is less exothermic than the $\Delta H^{\ominus}_{\text{hyd}}$ of the $\text{Mg}^{2+}$ ion.

(c)

Describe and explain how the solubility of the Group II sulfates varies down the group.

The rest of this paper

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