How Equilibria Respond to Pressure Changes

Le Chatelier's Principle

Le Chatelier's Principle: A system at equilibrium, when subjected to a disturbance, responds in a way that tends to minimize the effect of the disturbance.

When pressure changes, the equilibrium shifts to counteract that change.

Read in details Le Chatelier's Principle

Key Rules for Pressure Changes

• Increasing pressure: Equilibrium shifts toward the side with fewer gas molecules.
• Decreasing pressure: Equilibrium shifts toward the side with more gas molecules.
• Equal moles of gas on both sides: Pressure has no effect on equilibrium position.

Summary Table

Stress Applied	System Response	Effect on Equilibrium
Pressure increase	Produces fewer gas molecules	Shifts to side with fewer moles of gas
Pressure decrease	Produces more gas molecules	Shifts to side with more moles of gas
Equal gas moles on both sides	No shift needed	No effect on equilibrium

Example 1: Haber Process (Ammonia Formation)

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
(4 gas molecules)          (2 gas molecules)

Effect of Increasing Pressure

Increasing pressure favors the forward reaction (→ NH₃ formation) because the product side has fewer gas molecules (2 vs 4).

This decreases the total number of gas molecules, reducing pressure and relieving the stress.

Effect of Decreasing Pressure

Decreasing pressure favors the reverse reaction (← NH₃ decomposition) because the reactant side has more gas molecules.

Example 2: Hydrogen Chloride Formation

H₂(g) + Cl₂(g) ⇌ 2HCl(g)
(2 gas molecules)          (2 gas molecules)

Since both sides have equal moles of gas, pressure changes have no effect on the equilibrium position.

Important Points to Remember

1. Only Gases Are Affected

Changing pressure has a negligible effect on liquids and solids. Only gases respond significantly to pressure changes.

2. Equilibrium Constant (K) Remains Unchanged

Like changes in concentration, K_eq (or K_c) is unchanged by pressure changes.

The thermodynamic equilibrium constant is independent of pressure.

3. Mechanism: Molecular Collisions

When pressure increases, gas molecules collide more frequently, and equilibrium favors the consumption of gas molecules to reduce pressure.

Summary

Equilibria respond to pressure changes according to Le Chatelier's Principle:

• ✓ Increase pressure → shift to fewer gas molecules.
• ✓ Decrease pressure → shift to more gas molecules.
• ✓ Equal gas moles → no shift.
• ✓ K_c remains constant.
• ✓ Only gases are affected.