Le Châtelier's Principle in haber process
Applying Le Châtelier's principle to determine optimum conditions
- The pressure
In the reaction, N2(g) + 3H2(g) <--> 2NH3(g) notice that there are 4 molecules on the left-hand side of the equation, but only 2 on the right. According to Le Chatelier's Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again. Hence, the reaction will be more product favoured. However, building a very high pressure condition is very expensive and might cause the business to be less profitable. So, the optimum pressure is 200atm.
- The temperature
You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of ammonia in the equilibrium mixture. The forward reaction N2(g) + 3H2(g) <--> 2NH3(g) (the production of ammonia) is exothermic. Therefore, according to Le Chatelier's Principle, this will be favoured if you lower the temperature. The system will respond by moving the position of equilibrium to counteract this - in other words by producing more heat by producing more products. However, a very low temperature will cause reaction to occur very slowly and hence, not efficient. Therefore, 400 - 450°C is a compromise temperature producing a reasonably high proportion of ammonia in the equilibrium mixture (even if it is only 15%), but in a very short time.
- The concentration
The mixture of nitrogen and hydrogen going into the reactor is in the ratio of 1 volume of nitrogen to 3 volumes of hydrogen. In some cases, excess of one reactants is added to ensure that the other more expensive reactants fully react. However, this does not apply in this case.
- The catalyst
In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor. However, catalyst does not affect the position of equilibrium. Hence, in the case of Haber Process an iron catalyst is used.
- The pressure
In the reaction, N2(g) + 3H2(g) <--> 2NH3(g) notice that there are 4 molecules on the left-hand side of the equation, but only 2 on the right. According to Le Chatelier's Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again. Hence, the reaction will be more product favoured. However, building a very high pressure condition is very expensive and might cause the business to be less profitable. So, the optimum pressure is 200atm.
- The temperature
You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of ammonia in the equilibrium mixture. The forward reaction N2(g) + 3H2(g) <--> 2NH3(g) (the production of ammonia) is exothermic. Therefore, according to Le Chatelier's Principle, this will be favoured if you lower the temperature. The system will respond by moving the position of equilibrium to counteract this - in other words by producing more heat by producing more products. However, a very low temperature will cause reaction to occur very slowly and hence, not efficient. Therefore, 400 - 450°C is a compromise temperature producing a reasonably high proportion of ammonia in the equilibrium mixture (even if it is only 15%), but in a very short time.
- The concentration
The mixture of nitrogen and hydrogen going into the reactor is in the ratio of 1 volume of nitrogen to 3 volumes of hydrogen. In some cases, excess of one reactants is added to ensure that the other more expensive reactants fully react. However, this does not apply in this case.
- The catalyst
In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor. However, catalyst does not affect the position of equilibrium. Hence, in the case of Haber Process an iron catalyst is used.