Question

Arrange the following ionic compounds in
order of increasing lattice energy KC1, KBr,
CaCl2 and MgCl2
Give reason for your order

Answer 1

Question #48812, Chemistry, Inorganic Chemistry

Arrange the following ionic compounds in order of increasing lattice energy KC1, KBr, CaCl2 and MgCl2

Give reason for your order

Answer:

To determine the energy of the crystal lattice can be used thermodynamic cycle Bourne.

For example, analyze KCl:

$\Delta \mathrm{H}_{\mathrm{KCl}} = -(\Delta \mathrm{H}_{1} + \Delta \mathrm{H}_{2} + \Delta \mathrm{H}_{3} + \Delta \mathrm{H}_{4} + \Delta \mathrm{H}_{5}) = 730 \mathrm{~kJ} / \mathrm{mol}$

$\Delta \mathrm{H}_{1}$ -ionization potential of potassium (with opposite sign)

$\Delta \mathrm{H}_{2}$ -electron affinity of chlorine atom (with opposite sign)

$\Delta \mathrm{H}_{3}$ -thermal effect in the condensation of gaseous potassium

$\Delta \mathrm{H}_{4}$ -thermal effect on recombination of chlorine atoms

$\Delta \mathrm{H}_{5}$ -Standard enthalpy of formation combustion $K_{\text{(metal)}}$ in $\mathrm{Cl}_{2}$ (gas)

$\Delta \mathrm{H}_{\mathrm{KBr}} = 677.5 \mathrm{~kJ} / \mathrm{mol}$

$\Delta \mathrm{H}_{\mathrm{MgCl2}} = 2500 \mathrm{~kJ} / \mathrm{mol}$

$\Delta \mathrm{H}_{\mathrm{CaCl2}} = 2240 \mathrm{~kJ} / \mathrm{mol}$

You can calculate the energy of the formula Bourne:

$$\Delta U = N _ {a} * A \frac {Z _ {1} Z _ {2} e ^ {2}}{4 \pi \varepsilon r} (1 - 1 / n)$$ $$n = 1 + \frac{18r^4}{Ae^2\beta}$$

$\beta$-compressibility factor of the crystal

A - Madelung constant

$$KBr < KCl < CaCl_2 < MgCl_2$$

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