Given
N2(g)+3H2(g)→2NH3(g);ΔrHθ=−92.4 kJ mol−1
What is the standard enthalpy of formation of NH3 gas?
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),
12N2(g)+32H2(g)→NH3(g)
∴ Standard entalpy of formation of NH3(g)
=12ΔrHθ=12(–92.4 kJ mol−1)=−46.2 kJ mol−1