Chemical Equilibrium in the Haber Process

The number of moles at equilibrium is calculated for the Haber process, the reversible, exothermic reaction that synthesizes ammonia

(

NH

3

) from hydrogen (

H

2

) and nitrogen (

N

2

). The reaction is typically carried out at around 200 bar and 675–725 K. You can vary the pressure and temperature in this Demonstration. Gases are assumed to behave ideally, although realistically at the high pressures used in this reaction, there is significant deviation from ideal behavior. Four moles of reactants form two moles of product, so raising the pressure shifts equilibrium toward products. Initially, the system is filled with one mole of ammonia and allowed to reach equilibrium. At equilibrium, you can add additional nitrogen, hydrogen, and/or ammonia at constant pressure, and the effect on the equilibrium is observed. Le Chatelier's principle predicts that when nitrogen or hydrogen are added, the reaction goes to the right, whereas when ammonia is added, the reaction shifts to the left. However, when the nitrogen/hydrogen ratio is sufficiently high, adding nitrogen shifts reaction to the left (i.e. adding nitrogen decreases the amount of ammonia and increases the amount of hydrogen), contrary to what Le Chatelier's principle predicts. This happens because adding nitrogen decreases the mole fraction of hydrogen, and because the hydrogen mole fraction is cubed in the equilibrium expression, ammonia reacts to increase the number of moles of hydrogen and nitrogen.