Hey there!
Given the reaction :
CH₃I + Cl⁻ → CH₃Cl + I⁻
Change in enthalpy for the reaction :
ΔrH = (Bond energies of breaking) - (Bond energies of formation)
Bond energy of breaking = CH₃ - I = 215 kJ/mol
Bond energy of formation = CH₃ - Cl = 330 kJ/mol
ΔrH = 215 kJ/mol - 330 kJ/mol = -115 kJ/mol
4.184 Kj/mol = 1 Kcal/mol
-115 kJ/mol = -115/4.184 = -27.49 Kcal/mol
