It is an application of Hess’s law used to look at the enthalpy changes that occur when an ionic compound is formed from its element. It is used to outline the thermodynamic changes during the formation of ionic salts. It is used to calculate Lattice enthalpy.
Standard lattice enthalpy of formation is the enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard condition. Standard lattice enthalpy of dissociation of enthalpy change when one mole of an ionic lattice completely dissociates into gaseous ions under standard condition. Lattice enthalpy gives an indication of strength of ionic bond. The experimental lattice enthalpy value may differ from calculated value, giving evidence that some ionic compounds have partially covalent character.
It is a thermodynamic quantity which is measure of degree of disorder within any system. The greater the degree of disorder then the higher the entropy is. Its symbol is S.
Entropy change is given by:
ΔS = ΣSθproduct -ΣSθreactants
Where, ΔS = ΔH/T
ΣSθproduct = total standard entropy of the products
ΣSθreactants = total standard entropy of the reactants
Gibbs free energy
The thermochemical quantity which allows prediction to be made as to whether a chemical reaction is feasible. Whether a reaction will be spontaneous depends on temperature enthalpy change and entropy change.
The Gibbs free energy is given by:
ΔG = ΔH – TΔS
ΔG = Change in free energy of the system
ΔH = Change in enthalpy of the system
T = temperature of the system
ΔS = change in entropy of the system
The reaction is only feasible if ΔG is zero or negative. ΔG depends on the temperature which means that some reaction may be feasible at one temperature and not at another.