Question

In reaction ${{N}_{2}}(g)+3{{H}_{2}}(g)\rightleftharpoons 2N{{H}_{3}}(g)$; $\Delta H=-93.6kJ$, the yield of ammonia does not increase when [CPMT$1988$]
A.Pressure is increased
B.Temperature is lowered
C.Pressure is lowered
D.Volume of the reaction vessel is decreased

Answer 1

Hint: In the manufacture of ammonia by Haber’s process nitrogen reacts with hydrogen. Increasing the yield of ammonia follows the Le-Chatelier principle. In this reaction the number of moles of ammonia is lesser than reactants, then the system will try to shift the equilibrium towards the side of the lower volume.

Complete answer:Le-Chatelier’s principle tells how an equilibrium reaction will shift in response to changes in pressure, concentration, volume, and temperature. Ammonia synthesis is a reversible reaction and involves a reaction equilibrium. Here Le-Chatelier’s principle helps us to predict how reaction conditions will impact the production of ammonia, $N{{H}_{3}}$in this process.
${{N}_{2}}(g)+3{{H}_{2}}(g)\rightleftharpoons 2N{{H}_{3}}(g)$ ,$\Lambda H=-93.6kJ$

Since the forward reaction of ammonia synthesis is exothermic, therefore a lower temperature is needed to increase the yield of ammonia.

Increasing the pressure of the system causes the reaction equilibrium to shift towards the side with fewer moles. Here also the number of gaseous ammonia molecules is lesser than reactant molecules. Hence higher pressure favors the yields of ammonia.

If the pressure of the system decreases the reaction equilibrium shifts the equilibrium to the side of higher moles of gaseous molecules that is towards the reactant side of the given reaction. Hence the yield of ammonia decreases as the number of reactants increases at low pressure. This is not a favorable condition.

Again as the increasing pressure enhances the yield of ammonia, thereby volume decreases. As we know pressure is inversely proportional to volume. Thus, the yield of ammonia increases as the volume of the vessel decreases.
Therefore the yield of ammonia does not increase when pressure is lowered.

Thus, option (C) is correct.

Note: Since the forward reaction of ammonia synthesis is an exothermic reaction and backward reaction favors. But the rate of this backward process is very slow at low temperatures. Thus this reaction is carried out at a higher temperature to overcome this kinetic barrier.