Understanding Lattice Energy

Definition of Lattice Energy-Lattice energy is the energy released when one mole of an ionic compound is formed from its constituent gaseous ion. It is denoted by the expression ∆H_latt.

Mathematically we can write it as,

∆H_{latt.⁼∆G + T∆S}

You should confuse lattice energy with enthalpy of formation.

Lattice energy can be calculated using two methods:-

     The electrostatic method-the electrostatic method is a theoretical method and for calculation it is assumed that the bond is wholly ionic.

∆H_latt (Lattice energy)=

The unit of energy here should be in kJ mol^-1.

Where A and n are known as Madelung constant and born constant respectively.

 Z⁺  and  Z^-  are charges on ions. And  a is internuclear distance in picometer(for calculation you have to convert it in meter by multiplying it by 10^-12).

2.     Now arriving at the second method. This method is based on born-haber cycle. This method involves thermochemical calculation based on Hess’ law.

∆H_latt (Lattice energy)=

Where ∆H_s= Heat of sublimation.

∆H_d = Heat of dissociation.

∆H_f= Heat of formation

I= Ionisation energy of solid

E= Electron affinity of gas.

Example and solution of some common compounds

Born haber cycle for NaCl

Where ∆H_s= 108.

∆H_d = 121.

∆H_f= -411

I= Ionisation energy of Na=495

E=electron affinity of Cl= -348

Inserting the value in the above equation we get

∆H_{latt= -411-(108+495+121-348)} kJ/mol

              = -785 kJ/mol

Another example of  BaCl₂

 ? represent del operator and in BaCl2 equation the product should be BaCl2 instead of NaCl2.

Comments and suggestions are welcomed.