The standard enthalpy of formation of BrCl (g) is 14.7 kJmol-1. The standard enthalpies for the atomization of Br2 (l) and Cl2 (g) are + 112 and + 121.0 kJ per mol of atoms respectively, calculate the average enthalpy of the Br-Cl bond in BrCl (g).

Equation of the reaction:

Br2 (l) + Cl2 (g) - -> 2BrCl (g)

The enthalpy change for this reaction will be equal to twice the standard enthalpy change of formation for bromine monochloride, BrCl.

The standard enthalpy change of formation for a compound,

ΔH°f, is the change in enthalpy when one mole of that compound is formed from its constituent elements in their standard state at a pressure of 1 atm.

This means that the standard enthalpy change of formation will correspond to the change in enthalpy associated with this reaction

1/2Br2 (g) + 1/2Cl2 (g) → BrCl (g)

Here, ΔH°rxn = ΔH°f

This means that the enthalpy change for this reaction will be twice the value of ΔH°f = 2 moles BrCl

Using Hess' law,

ΔH°f = total energy of reactant - total energy of product

= (1/2 * (+112) + 1/2 * (+121)) - 14.7

= 101.8 kJ/mol

ΔH°rxn = 101.8 kJ/mol.