In some gaseous industrial reactions, an inert gas is added to the reaction mixture. This is a gas that doesn't react with reactants or products. The effect, if there is one, on the equilibrium position depends on whether the volume is kept constant and the total pressure is allowed to increase or whether the pressure is kept constant and the volume is allowed to increase.

The effect of adding an inert gas cannot be predicted using Le Chatelier's Principle.

Let's look at what happens when an inert gas is added to an equilibrium mixture when the volume is kept constant. 

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What kind of apparatus might be used to add an inert gas at constant volume?

This could be achieved using a metal cylinder with a valve through which gases can be added.

Why do the partial pressures of the gases remain unchanged?

The mole fraction is halved but the total pressure is doubled. Partial pressures depend on both of these values. The two changes cancel out, meaning that there is no overall change.

Now let's look at what happens when an inert gas is added to an equilibrium mixture and the pressure is kept constant. 

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What kind of apparatus might be used to add an inert gas at constant pressure?

This could be achieved using a gas syringe connected to a tap through which gases can be added.

Why do the partial pressures of gases A, B and C change?

Because the mole fractions decrease but the total pressure remains the same.

In terms of number of molecules, to what side of the reaction does the equilibrium position shift when an inert gas is added at constant pressure?

The side of the reaction that has the larger number of molecules.