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I am an issue understanding the changes that take place when we decrease the pressure of a reaction. I have understood that when we increase the pressure the side having more number of moles wants its moles to go to the other side where it is more empty(this is how I learnt it). But I fail to understand why if we decrease the pressure by increasing volume the side having less moles goes to the side having more moles? Also in the case of adding inert gases why does the equilibrium shift in the direction in which larger no of moles of gas are formed? Please help.

I am an issue understanding the changes that take place when we decrease the pressure of a reaction. I have understood that when we increase the pressure the side having more number of moles wants its moles to go to the other side where it is more empty(this is how I learnt it). But I fail to understand why if we decrease the pressure by increasing volume the side having less moles goes to the side having more moles? Also in the case of adding inert gases why does the equilibrium shift in the direction in which larger no of moles of gas are formed? Please help.

Grade:12

1 Answers

Aarti Gupta
askIITians Faculty 300 Points
9 years ago
According to Le-Chatelier’s Principle – If a system is subjected to a change of concentration,pressure or temperature,the equilibrium will shifts in the direction that tends to undo the effect of the change.
Effect of change of pressure- The change of pressure effected only those equilibria which involve gaseous substances and proceed with the change in the number of moles of the gases.According to Le-Chatelier’s Principle-
1.Increase of external pressure effect the equilibrium in such a way as to reduce the pressure thus it will shifts in the direction which has smaller number of moles of the gaseous substances.For ex-
N2 + 3H2 ---------------> 2NH3
If we increase the pressure in the above caes,volume will decreases thereby increasing the number of moles per unit volume .So to nullify this effect ,equilibrium gets shifted in the direction involving the decrease in the no. of moles,which could happen only if nitrogen and hydrogen combine to form ammonia.Thus it will shifts in the forward direction.So we can summarize the effect of change in pressure as-
1. Increase in pressure- ---------- equilibrium shifts towards lesser no. of gaseous moles.
2.Decrease in pressure ----------- equilibrium shifts towards more no. of gaseous moles.
Thus remember that pressure changes effect only those equilibria which involves gaseous substances,so please first check the state of the species involved in the reaction.Another thing is that decrease in V on increasing P,increases the no. of moles per unit volume and vice-versa.
Effect of addition of inert gas-
Addition of inert gas at constant pressure- When an inert gas is added to the system at constant pressure,it will increase the volume which in turn decreases the no.of moles per unit volume of various reactants and products.Thus to nullify this stress.equilibrium shifts to the direction where no. of moles are increased.For ex-
SO2 + ½ O2 -------->. SO3
If inert gas is added at constant P equilibrium shifts to the backward direction.
PCl5 ------------> PCl3 + Cl2
in this case addition of inert gas at constant P ,shifts the equilibrium to the forward direction.

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