N2 + 3H2 → 2NH3 ;ΔH = -46K.Cals. From the above reaction, heat of formation of ammonia is

Enthalpy of formation is the enthalpy change observed when one mole of a substance is formed from its constituent elements;In the given reaction two moles of Ammonia is formed;Thus ∆f0 of NH3=-462kCals/mole=-23 kCal/mole;