Question 11 · Complements de química URV Exam Bridging Courses in Chemistry

Question 11 (1.25 points). Lattice energy and covalent contribution in LiF

Lithium fluoride (LiF) crystallizes with the NaCl structure type. Calculate both the experimental lattice energy ($U_0^{\text{exp}}$) using a Born-Haber cycle and the theoretical lattice energy ($U_0^{\text{theor}}$) using the Born-Landé equation. From their difference, evaluate the non-ionic (covalent) percentage contribution in LiF.

Thermodynamic and structural data:

$\Delta H_f^\circ(\text{LiF}) = -617\text{ kJ/mol}$

$\Delta H_{\text{sub}}(\text{Li}) = 161\text{ kJ/mol}$

$\Delta H_{\text{ion}}(\text{Li}) = 520\text{ kJ/mol}$

$\Delta H_{\text{diss}}(\text{F}_2) = 154\text{ kJ/mol}$

$\text{EA}(\text{F}) = -328\text{ kJ/mol}$

$r(\text{Li}^+) = 0.68\text{ \AA},\ r(\text{F}^-) = 1.33\text{ \AA}$

Atomic numbers: $Z(\text{Li}) = 3,\ Z(\text{F}) = 9$

$N_A = 6.023 \cdot 10^{23}\text{ mol}^{-1}$

$e = 4.8 \cdot 10^{-10}\text{ esu (statC)}$

$1\text{ \AA} = 10^{-8}\text{ cm};\ 1\text{ erg} = 10^{-7}\text{ J}$

Structure

Madelung constant ($A$)

Cesium chloride · 1.76267

Sodium chloride · 1.74756

Blende of zinc · 1.63806

Wurtzite · 1.64132

Fluorite · 2.51939

Antifluorite · 2.51900

Rutile · 2.40800

Configuration ion

Born exponent ($n$)

He · 5

Ne · 7

Ar, Fe$^{2+}$ · 9

Kr, Ag$^+$ · 10

Xe, Au$^+$ · 12

a)

b)

c)

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