The first ionization energy of magnesium is 738 kJ/mol, the second ionization energy of magnesium is 1451 kJ/mol, and the electron affinity for chlorine is -349 kJ/mol. Prove mathematically using Hess's Law that the formation of magnesium chloride from magnesium metal and chlorine gas is exothermic.
You will also need:
Cl2(g) --> Cl(g) Heat formation = 121.7 kJ/mol
Mg(s) --> Mg(g) Heat formation = 147.1 kJ/mol