The reaction $\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3 + 92\,kJ}$ is shown with two arrows because—

• ait is irreversible
• bit is reversible — $\mathrm{NH_3}$ also breaks back to $\mathrm{N_2}$ and $\mathrm{H_2}$
• cit is a redox reaction
• dit produces a precipitate