Standard enthalpy of formation of a compound is the change in enthalpy that takes place during the formation of 1 mole of a substance in its standard form from its constituent elements in their standard state. Re-writing the given equation for 1 mole of NH3(g), 3(g)
= ½ ∆rHθ
= ½ (–92.4 kJ mol–1 )
= –46.2 kJ mol–1
