Ammonia Synthesis: Incomplete Reaction
The ammonia synthesis reaction is less extreme than combustion:

(g) + (g) (g)
= -7.95 kcal per 2 moles of

The equilibrium constant is not the enormous exponential value that we saw for the HCl and reactions; it is



Note how the exponents on each concentration term are related to the number of molecules in the chemical reaction, and how all of the exponents determine the overall units of for . If concentrations were given in moles per liter instead of atmospheres, the numerical value of the equilibrium constant K would be different.
The rates of forward and reverse reactions become equal when appreciable amounts of both reactants and products are still present. In our crabapple analogy, the two opponents are more evenly matched in the ammonia reaction, and there is no overwhelming pileup at equilibrium on either side of the fence. The ammonia reaction is not "complete" in the way that the other two reactions are.

Photo: The Incitec ammonia synthesis plant, Brisbane, Queensland.