So to write the enthalpy reactions required in the formation of MgF 2(s), we need to write the reactions in the direction required and we need to adjust for the number of moles of each reactant used: Remember if we reverse the direction of the reaction, the sign of the change in enthalpy will change. To calculate the total enthalpy change using Hess’s law we will need to find the enthalpy change for each component of the Born-Haber cycle and add them together.

Mg 2 +(g) + 2F -(g) –> MgF 2(s) ΔH = Lattice Energy Remember the lattice energy is energy released when ions in the gaseous form combine to form a solid. Hess’s law states that the total enthalpy change during the complete course of a reaction is same whether the reaction is made in one step or in several steps. Now, writing the component reactions of the Born-Haber Cycle:

Enthalpy of formation of MgF 2 (s) = ΔH o f = –1124 kJ/mol.

Electron Affinity of F (g) = EA = –328 k/J/mol.

Bond dissociation Energy for F 2 (g) = BE = +159 kJ/mol.

2nd ionization energy for Mg = IE 2 = +1451 kJ/mol.

1st ionization energy for Mg = IE 1 = +738 kJ/mol.

Enthalpy of Sublimation for Mg = ΔH sub = + 146 kJ/mol.

The known enthalpies of each of the steps of the Born-Haber cycle are obtained from a table: Problem: Use the Born-Haber Cycle to calculate the lattice energy of MgF 2

Connect to WiFi to avoid cellular charges for video streamingĬontinuing to solve the problem discussed at the end of Part II….

Lattice energy equation components