Given the equation representing a system at equilibrium: #N_2(g) + 3H_2(g) rightleftharpoons 2NH_3(g) + "energy"#, what changes occur when the temperature of this system is decreased?

Question

Given the equation representing a system at equilibrium:  #N_2(g) + 3H_2(g) rightleftharpoons  2NH_3(g) + "energy"#, what changes occur when the temperature of this system is decreased?

Peyton Adams · Accepted Answer

#"Le Chatelier's principle"# states that when a system at......

#"Le Chatelier's principle states that when a system at....."# #"equilibrium is subject to an external stress, the equilibrium"# #"will move so as to oppose the external stress"#. As I have said before (I am a broken record): #"oppose "!=" counteract"#. Consequently, we make an effort to change the following equilibrium: #N_2(g) + 3H_2(g) rightleftharpoons 2NH_3(g)+Delta# If the temperature drops, the reaction will effectively lose heat, so if everything else stays the same, the equilibrium should move to the right to make up for the lost heat. Naturally, a new equilibrium constant may apply given the altered reaction conditions, but we won't really be able to determine this until we are given more details. The industrial synthesis of ammonia, which has been INTENSELY STUDIED, is performed at a reasonably elevated temperature (approx. #500-600# #""^@C#) so as to maintain an acceptable rate and turnover of reaction. The ammonia gas, as it is formed, can be removed from the equilibrium on a cold finger, the reactant gases recycled, and thus the equilibrium can be driven to the right. Of course, a catalyst is used to provide an acceptable rate.

Jeremiah Adams · Answer

undefined When the temperature of the system represented by the given equation is decreased, the equilibrium position shifts in the direction that produces heat. According to Le Chatelier's Principle, since the forward reaction is exothermic (it releases energy), the system will shift to the right to counteract the decrease in temperature. Consequently, the concentrations of N2 and H2 decrease, while the concentration of NH3 increases to re-establish equilibrium.