Abundances of NH₃ as a function of time computed with two kinetic schemes that are fully reversed according to equilibrium constants but that differs by their nitrogen chemistry (nominal and GRIMECH as defined in Sects. 2.1.2 and 3.3). While they both converge towards the equilibrium (dotted line), they exhibit very different evolution. Initial condition is a mixture of H₂, CH₄, O₂, N₂, and He with solar elemental abundances.