The ammonia synthesis reaction exhibits a well-known exception to Le Châtelier's principle. If the initial mole fraction of nitrogen exceeds 0.5, the infinitesimal addition of more nitrogen at constant temperature and pressure causes the reaction to shift to the left, producing more nitrogen, in violation of Le Châtelier's principle. The value of 0.5, however, is obtained from an analysis that assumes ideal gas behavior. We revisit this exception, but now consider the effects of nonideality on the shifts in the reaction. The mole fraction of nitrogen at which the reaction shifts direction from right to left (upon the addition of more nitrogen) is found to be strongly dependent upon the temperature and pressure, approaching 0.1 under some conditions. We also consider the shifts in the direction of the reaction upon addition of finite amounts of nitrogen (to which Le Châtelier's principle no longer applies). In some cases, both infinitesimal and finite additions of nitrogen always cause the reaction to shift to the left. At other conditions, large enough additions of nitrogen also cause the reaction to shift to the left, even though the reaction satisfied Le Châtelier's principle (i.e., shifted to the right) upon smaller additions of nitrogen.

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