HSC · Topic explainer
What does an inert gas do to equilibrium at fixed volume?
At fixed volume and temperature, adding an inert gas increases total pressure but leaves each reacting gas’s partial pressure unchanged. The reaction quotient stays the same, so the equilibrium composition does not shift. State the conditions before drawing a conclusion about a change in total pressure.
Worked example
A sealed rigid vessel contains N₂O₄ and NO₂ at equilibrium. Add argon while keeping the temperature constant. The amount of each reacting gas is unchanged at the moment argon enters, and the container volume is the same.
For each gas, its partial pressure is nRT/V. The NO₂ and N₂O₄ partial pressures therefore stay the same, as does their reaction quotient. At constant total pressure, the container could expand instead, so the same conclusion would not follow.
Official exam practice
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