Potassium chloride, $\text{KCl}$, and magnesium chloride, $\text{MgCl}_2$, are both ionic solids. Table 1.1 energy change $\text{value}/\text{kJ}\,\text{mol}^{-1}$ --- --- standard enthalpy change of solution, $\Delta H^\ominus_{\text{sol}}$, of $\text{KCl}$ $+15$ lattice energy, $\Delta H^\ominus_{\text{latt}}$, of $\text{KCl(s)}$ $-701$ standard enthalpy change of hydration, $\Delta H^\ominus_{\text{hyd}}$, of $\text{K}^+$ $-322$ standard enthalpy change of hydration,…

Chemistry 9701/43 — October/November 2022

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

questions

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120

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Topics Transition Elements · Introduction to A Level Organic Chemistry · Hydrocarbons · Chemical Energetics · Electrochemistry · Equilibria · +6 more

Q1Chemical EnergeticsFree sample

Potassium chloride, $\text{KCl}$ , and magnesium chloride, $\text{MgCl}_2$ , are both ionic solids.

Table 1.1

energy change	$\text{value}/\text{kJ}\,\text{mol}^{-1}$
standard enthalpy change of solution, $\Delta H^\ominus_{\text{sol}}$ , of $\text{KCl}$	$+15$
lattice energy, $\Delta H^\ominus_{\text{latt}}$ , of $\text{KCl(s)}$	$-701$
standard enthalpy change of hydration, $\Delta H^\ominus_{\text{hyd}}$ , of $\text{K}^+$	$-322$
standard enthalpy change of hydration, $\Delta H^\ominus_{\text{hyd}}$ , of $\text{Cl}^-$	$-364$
standard enthalpy change of solution, $\Delta H^\ominus_{\text{sol}}$ , of $\text{MgCl}_2$	$-155$
lattice energy, $\Delta H^\ominus_{\text{latt}}$ , of $\text{MgCl}_2\text{(s)}$	$-2493$

(a)

Complete the energy cycle involving the enthalpy change of solution and the lattice energy of potassium chloride, $\text{KCl}$ , and the relevant enthalpy changes of hydration. Label your diagram.

State symbols should be used.

Fig. 1.1

(b)

Use the data in Table 1.1 to calculate the enthalpy change of hydration of magnesium ions, $\text{Mg}^{2+}$ . Show your working.

\Delta H^\ominus_{\text{hyd}} \text{ of magnesium ions, Mg}^{2+} = \text{..............................}\text{ kJ}\,\text{mol}^{-1}

(c)

Explain the reasons why the lattice energy of $\text{MgCl}_2$ is more exothermic than the lattice energy of $\text{KCl}$ .

(d)

Define the following terms.

(i)

enthalpy change of atomisation

(ii)

first electron affinity

(e)

(i)

Explain what is meant by entropy, $S$ .

(ii)

Potassium chloride is very soluble in water at $20\,^\circ\text{C}$ .

Explain the solubility of potassium chloride by reference to change in entropy, $\Delta S$ .

(iii)

Use the Gibbs equation and your answer to (e)(ii) to predict whether potassium chloride is more soluble in water at $20\,^\circ\text{C}$ or at $80\,^\circ\text{C}$ . Explain your answer.

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