Pressure can be used to shift the actual equilibrium conditions of any reaction during which the total number of moles of gas changes. In the ammonia reaction three moles of and one of react to form two moles of ammonia. Increasing the pressure shifts the equilibrium in the direction of more ammonia formation, and decreasing the pressure leads to more dissociation of ammonia (see above). The value of the equilibrium constant does not change,

but the relative amounts of , , and do change. You can understand this by imagining that the total pressure on the reaction vessel is doubled, so that the partial pressures of each of the gases also are doubled.