Le Chatelier's Principle: Effect of Pressure
By Abhishek Sengar | CHEMCA India
Pressure changes only affect reactions that contain Gases. To predict the shift, you must calculate the change in gaseous moles (Δng) between the reactants and products.
1. The Rule of Gaseous Moles
Pressure is directly proportional to the number of gas molecules. If you squeeze a container (Increase Pressure), the system feels crowded. To relieve this pressure, it shifts toward the side with FEWER gas molecules.
The Summary:
- Increase Pressure: Shifts to the side with FEWER moles of gas.
- Decrease Pressure: Shifts to the side with MORE moles of gas.
- If Δng = 0: Pressure has NO EFFECT!
- Increase Pressure: Shifts to the side with FEWER moles of gas.
- Decrease Pressure: Shifts to the side with MORE moles of gas.
- If Δng = 0: Pressure has NO EFFECT!
Practice Question
Q: Consider the reaction: H2(g) + I2(g) ⇌ 2HI(g). If the pressure of the vessel is doubled, in which direction will the equilibrium shift?
Answer: NO SHIFT. There are 2 moles of gas on the reactant side and 2 moles of gas on the product side (Δng = 0). Since the moles are perfectly equal, pressure changes cannot force a shift.
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