Enthalpy of formation of NH3 = − 46 kJ / mole ∴ N2+3H2 → 2NH3 ΔHf = −2 × 46 kJ mol Bond breaking is endothermic and Bond formation is exothermic Assuming 'x' is the bond energy of N − H bond (kJ mol−1) ∴ 712 + (3 × 436) − 6x = − 46 × 2 ∴ x = 352 kJ / mol