For the gas phase reaction N2 + 3 H2 2 NH3 ΔHº = - 92 kJ for the forward reaction. In order to increase the amount of H2 present, the reaction should be run

In order to increase the amount of H₂ present, the reaction should be run at high temperatures and low pressures.

Explanation:

Let's consider the following gas phase reaction.

N₂ (g) + 3 H₂ (g) ⇄ 2 NH₃ (g) ΔHº = - 92 kJ

To predict how the reaction will proceed when we introduce modifications we need to consider Le Chatelier's Principle: If a system at equilibrium suffers a perturbation, it will react to counteract such perturbation.

ΔHº < 0 means that the reaction is exothermic. If we increase the temperature, the system will decrease it favoring the reverse reaction to absorb heat. As a side-effect the amount of H₂ increases.

If we decrease the pressure, the system will increase it by favoring the reverse reaction so that there are more gaseous moles and more pressure. As a side-effect the amount of H₂ increases.