TABLE OF CONTENTS Nitrogen and hydrogen will sit happily together in a sealed vessel without reacting to form ammonia, with the equilibrium for the reaction being completely over to the left hand side of the equation under ambient conditions.
| (4.1) |  |
According to Le Chatelier's principle the equilibrium will be shifted to the right-hand side by high pressures and, since the reaction is exothermic, by low temperatures. Indeed early work by Haber showed that at 200 C and 300 atmospheres pressure the equilibrium mix would contain 90% ammonia, whilst at the same pressure but at 700 C the percentage of ammonia at equilibrium would be less than 5%. Unfortunately the activation energy is such that temperatures well in excess of 1000 C are needed to overcome this energy barrier (Figure 4.1). The conclusion from this is that direct reaction is not a commercially viable option.
However, in the early 1900s Haber and, later, Bosch discovered that the reaction did proceed at reasonable temperatures (around 500 C) in the presence of osmium and subsequently iron based materials. These catalysts acted by lowering the activation energy of the reaction; in other words, by interacting with the starting materials they altered the reaction pathway to one of lower energy. Catalysts do not, however, alter the equilibrium position of a reaction, which is under thermodynamic control; therefore high pressures are still needed to force the reaction. Hence a catalyst is commonly defined as: A...
