CAIE A-Level Chemistry 23.1 Lattice Energy and Born–Haber Cycles

Practise defining lattice energy and constructing cycles from atomisation, ionisation and electron-affinity steps.

Exam points

define lattice energy for formation of one mole of ionic solid from its gaseous ions
place every element, atom, gaseous ion and electron at the correct level with state symbols
apply Hess’s law with correct stoichiometric multipliers and signs to solve a missing enthalpy

Covered syllabus points

23.1.1—Terms 23.1.2—Terms: first electron affinity, EA 23.1.3—Born–Haber cycles for ionic solids 23.1.4—Calculations: Born–Haber cycles 23.1.5—Explain, in qualitative terms, the effect

[Maximum number: 4]

Potassium chloride, KCl , and magnesium chloride, $\mathrm{MgCl}_{2}$ , are both ionic solids.

Table 1.1

(a)

Explain the reasons why the lattice energy of $\mathrm{MgCl}_{2}$ is more exothermic than the lattice energy of KCl.

[ 2 ]

$\quad \mathrm{Mg}^{2+}$ is smaller (than $\mathrm{K}^{+}$ )

- $\mathrm{Mg}^{2+}$ is greater charge (than $\mathrm{K}^{+}$ )

- greater attraction

between $\mathrm{Mg}^{2+}$ and $\mathrm{Cl}^{-} /$ between the ions (in $\mathrm{MgCl}_{2}$ )

OR stronger ionic bonds (in $\mathrm{MgCl}_{2}$ )

(b)

Define the following terms.

[ 2 ]

(i)

enthalpy change of atomisation

[ 1 ]

(ii)

first electron affinity

[ 1 ]

[Maximum number: 6]

(a)

Calcium metal reacts with chlorine gas to form calcium chloride, $\mathrm{CaCl}_{2}$ .

[ 6 ]

(i)

Write an equation, including state symbols, to represent the lattice energy of calcium chloride, $\mathrm{CaCl}_{2}$ .

[ 1 ]

$\mathrm{Ca}^{2+}(\mathrm{g})+2 \mathrm{Cl}^{-}(\mathrm{g}) \rightarrow \mathrm{CaCl}_{2}(\mathrm{~s})$ (state symbols required)

(ii)

Complete a fully labelled Born-Haber cycle that could be used to calculate the lattice energy, $\Delta H_{\text {latt }}^{\ominus}$ , for calcium chloride.

Figure for Question (ii) — CAIE A-Level Chemistry A2

[ 2 ]

(iii)

Use your answer to (ii) and the following data, together with relevant data from the Data Booklet, to calculate a value for $\Delta H_{\text {latt }}^{\ominus}$ for calcium chloride.

Table for Question (iii) — CAIE A-Level Chemistry A2

\Delta H_{\text {latt }}^{\ominus}=\ldots \ldots \ldots \ldots \ldots . . . . . . . . . . . . . . . . . . . . k J ~ m o l-1 ~

[ 3 ]

$\Delta \mathrm{H}_{\text {latt }}{ }^{\theta}=-796-242-178-590-1150+(2 \times 349)=-2258 \mathrm{~kJ} \mathrm{~mol}^{-1}$

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Covered syllabus points

23.1 Lattice energy and Born-Haber cycles question 1

Question (a)

Question (b)

Question (i)

Question (ii)

23.1 Lattice energy and Born-Haber cycles question 2

Question (a)

Question (i)

Question (ii)

Question (iii)