Combining these values we get for the enthalpy of reaction. Energy Out 2 x 432 864 KJ Mole. H2g F2g 2 HFg DH 2.
To calculate bond energy for molecules in a liquid state you need to also look up the enthalpy change of vaporization.
Br-Br 193 kJmol. Use the following mean bond enthalpy data to calculate the enthalpy of formation of NH3. In the second step of the reaction two moles of H-Cl bonds are formed. Notice the formula will automatically convert the bond energies of bonds broken in reactants to an endothermic term ve and the bond energies of bonds formed in products to an exothermic term -ve.