Question 8 · Complements de química URV Juny 2023

Question 8 (1.25 points). Non-ionic contribution in potassium bromide

Potassium bromide (KBr) adopts the crystal structure illustrated in the unit cell diagram below. By comparing the theoretical and experimental lattice energies, determine the non-ionic (covalent) character percentage of KBr.

Thermodynamic Data:

• Standard enthalpy of formation: $\Delta H_f^\circ(\text{KBr}) = -391\text{ kJ/mol}$

• Enthalpy of sublimation of potassium: $\Delta H_{\text{sub}}(\text{K}) = 88\text{ kJ/mol}$

• First ionization energy of potassium: $\text{IE}(\text{K}) = 418\text{ kJ/mol}$

• Enthalpy of vaporization of bromine: $\Delta H_{\text{vap}}(\text{Br}_2) = 30\text{ kJ/mol}$

• Dissociation enthalpy of bromine: $\Delta H_{\text{diss}}(\text{Br}_2) = 193\text{ kJ/mol}$

• Electron affinity of bromine: $\text{EA}(\text{Br}) = -324\text{ kJ/mol}$

Structural & Physical Constants:

• Ionic radii: $r(\text{K}^+) = 1.33\text{ Å}$, $r(\text{Br}^-) = 1.95\text{ Å}$

• Atomic numbers: $Z_{\text{K}} = 19$, $Z_{\text{Br}} = 35$

• $N_A = 6.023 \times 10^{23}\text{ mol}^{-1}$; $e = 4.8 \times 10^{-10}\text{ esu}$; $1\text{ Å} = 10^{-8}\text{ cm}$; $1\text{ erg} = 10^{-7}\text{ J} = 10^{-10}\text{ kJ}$

Structure · Madelung Constant ($A$)

Cesium chloride · 1.76267

Sodium chloride · 1.74756

Zinc blende · 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

1.

2.

3.

%