Le Chatelier's Principle: Addition of Inert Gas | CHEMCA

Le Chatelier's Principle: Addition of Inert Gas

By Abhishek Sengar | CHEMCA India

Adding an inert gas (like Helium or Argon) to a reaction mixture does NOT participate chemically. However, it can physically alter the equilibrium depending entirely on how you add it: at Constant Volume or Constant Pressure.

1. Addition at Constant Volume

If you pump inert gas into a rigid, closed steel tank, the total pressure increases. BUT, because the volume of the tank is locked, the partial pressures (and concentrations, n/V) of the actual reacting gases remain exactly the same. Result: NO EFFECT on equilibrium.

2. Addition at Constant Pressure

If you pump inert gas into a flexible container (like a piston), the container must expand (Volume increases) to keep the total pressure constant. As the Volume increases, the concentration (n/V) of every reacting gas drops! To fix this drop, the system shifts to produce more gas molecules.

The Constant Pressure Rule:
Adding an inert gas at Constant Pressure always shifts the equilibrium toward the side with MORE MOLES OF GAS (higher stoichiometric sum).

Practice Question

Q: PCl_5(g) ⇌ PCl_3(g) + Cl_2(g). If Helium gas is added to this mixture in a cylinder fitted with a movable piston, what will happen to the dissociation of PCl₅?

Answer: The dissociation will INCREASE. A movable piston implies Constant Pressure. Adding inert gas increases Volume, causing a shift toward the side with more moles of gas. The product side has 2 moles, while the reactant side has 1 mole. Thus, it shifts FORWARD.

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