Question

Le-Chatelier principle is not applicable to
    (A) ${{H}_{2(g)}}+{{I}_{2(g)}}\rightleftharpoons 2H{{I}_{(g)}}$
    (B) $F{{e}_{(s)}}+{{S}_{(s)}}\rightleftharpoons Fe{{S}_{(s)}}$
    (C) ${{N}_{2(g)}}+3{{H}_{2(g)}}\rightleftharpoons 2N{{H}_{3(g)}}$
    (D) ${{N}_{2(g)}}+{{O}_{2(g)}}\rightleftharpoons 2N{{O}_{(g)}}$

Answer 1

Hint: Three of the given reactions are in gaseous phase and one is in solid phase. Le Chatelier's principle is not applicable to homogeneous solid systems. From this information, we can check which of the given reactions is an example of a homogeneous solid system.


Complete step by step solution:
      - Le Chatelier principle is a principle in chemistry that is used to predict the effect of change in conditions on chemical equilibria. According to this principle when any system at equilibrium for a long period of time is subjected to a change in concentration, temperature, volume, or pressure, then the system changes to a new equilibrium, and this change partly counteracts the applied change.
      - We can simplify the principle as, when a settled system is disturbed, it will adjust to diminish the change that has been made to it. That is, when a change is made into a system that is in equilibrium, then the system will try to counteract the applied change.
     - As we mentioned, change in concentration, temperature, volume, or pressure plays a major role. Firstly, changing the concentration of a chemical will shift the equilibrium to the side that would counter that change in concentration. The chemical system will attempt to partly oppose the change affected to the original state of equilibrium.
    - Similarly, in the case of temperature whether increasing or decreasing the temperature would favor the forward or the reverse reaction can be determined by applying the same principle as with concentration changes. Volume and pressure also work in the same way.
  - In the given question every reaction is a homogeneous reaction. That is the reactants and products are in the same phase. Le-Chatelier principle is not applicable to pure solids and liquids because of the reason that they experience negligible change in concentration during chemical equilibrium which means that adding or removing a solid from a system at equilibrium has no effect on equilibrium.
   - Among our given options the reaction $F{{e}_{(s)}}+{{S}_{(s)}}\rightleftharpoons Fe{{S}_{(s)}}$ is an example for homogeneous solid system. Hence the Le Chatelier principle is not applicable to this reaction

Therefore, the answer is option (B).


Note: It should be noted that, if a reaction obeys Le Chatelier principle, then the position of equilibrium is changed if we change the concentration of either reactant or product. Even though a change in concentration of one of the substances causes a shift in equilibrium, the value of equilibrium constant does not change.