Question

For the reaction below, describe the temperature and pressure conditions that would produce the highest yield for the forward reaction. Explain your answer in terms of Le Châtelier's principle.

N2 (g) + 3 H2 (g) 2 NH3 (g) H = -92 kJ/mol

Answer 1

The negative value for the enthalpy means the forward reaction is exothermic. LeChatlier's principle would favor the forward reaction if the conditions are cold. The sum of the coefficients of the reactants (1 + 3 = 4) is greater than the products (2). LeChatlier's principle would favor the forward reaction if pressure was increased. Thus, lower temperature and higher pressure would result in the highest yield for the forward reaction.


Aside: it is somewhat ironic that this particular chemical equilibrium (the Haber process) was chosen to demonstrate LeChatlier's principle, as in practice both high temperature and high pressure have been standard conditions for ammonia production since the first world war. Can anyone else support/critique this fact? Thanks in advance.