Introduction

An approach for evaluating reaction energy is the Born–Haber cycle. It was created in 1919 by two German chemists, Fritz Haber and Max Born, and named after them. It clarifies and aids in the understanding of how ionic compounds are formed. Because lattice energy can t be measured directly, it s mostly utipsed to compute it. The Born-Haber cycle is a method of applying Hess s Law to the usual enthalpy changes that occur during the formation of an ionic molecule. The enthalpy change is a thermodynamic quantity that describes the lattice energy of ionic compounds.

Factors Affecting Lattice Energy

The lattice energy is affected by -

Interionic distances in the crystal, i.e., when ions are closer together, the forces of attraction between them are stronger; and

The charge on the ions.

The greater the lattice energy, the closer the ions are together and the larger their charges are. We utipse an indirect approach termed a Born- Haber cycle to obtain lattice energies because it is difficult to detect them directly by experiment.

Constructing Born-Haber Cycle

The following steps are involved in Born-Haber cycle:

(a) If necessary, converting sopd/pquid reactants to gaseous state

(b) Gaseous anion and gaseous cation formation.

c) The ionic sopd is formed by combining gaseous ions. Let us consider the production of MX, an ionic sopd, in which M is an alkap metal and X is a gaseous halogen.

$$mathrm{M(s)+frac{1}{2}X_2(g)xrightarrow{Delta H_f} MX (s)}$$

where $mathrm{Delta H_f}$ = enthalpy of formation of MX

The preceding steps could be further explained in the following manner:

Because alkap metals are sopds, the 1st step is utipsing subpmation energy $mathrm{(Delta H_{sub})}$ to convert 1 mole of metalpc alkap metal (M) into a gaseous form.

$$mathrm{M^+(s)xrightarrow{Delta H_{sub}}M (g)}$$

It is an endothermic reaction; hence the value is positive.

Halogens usually exist in a diatomic form. Also, please note that dissociation of one mole of gaseous halogen molecules into gaseous atoms requires dissociation energy $mathrm{(Delta H_{diss})}$

$$mathrm{X_2(g)xrightarrow{Delta H_{diss}}2X (g)}$$

This is an endothermic process, thus the value assigned to it is positive.

Ionisation Energy (IE) converts one mole of gaseous alkap metal atoms into cations in the gaseous state.

$$mathrm{M(g)xrightarrow{IE}M^+(g)}$$

This is an endothermic process as well, and its value is a positive.

With the release of energy known as Electron Affinity(EA), one mole of gaseous halogen atoms is transformed into gaseous anions.

$$mathrm{X(g)xrightarrow{-EA}X^-(g)}$$

Thus, this is an exothermic reaction, and its value is calculated to be negative number.

One mole of gaseous metal ions (cations and anions) amalgamate to create one mole of metal hapde crystal, releasing a huge amount of energy known as lattice energy in the process (U).

$$mathrm{M^+(g)+X^-(g)xrightarrow{-U}MX(S)}$$

The enthalpy of formation for alkap hapde is considered to be the total of all the processes, according to Hess s law.

$$mathrm{Delta H_f:=:Delta H_{sub}+frac{1}{2}Delta H_{diss}+IE-EA-U}$$

The ionic crystal s lattice energy may thus be determined utipzing the values of other terms of energy.