Pressure and Reversible Reactions

Introduction

• By the end of this video, you should be able to describe the effect of pressure on a reversible reaction at equilibrium.

• Higher-tier students only.

• Le Chatelier’s Principle:
  If a system is at equilibrium and a change is made to the conditions, the system responds to counteract the change.

Key Factors

1. Pressure affects reactions involving gases.

   • Example: two containers with gas at the same temperature and volume.

   • Container on the right has twice the number of molecules as the one on the left.

2. Pressure of a gas depends on the number of molecules.

   • More molecules → higher pressure.

Reversible Reactions Example

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

• Left-hand side: 1 N₂ + 3 H₂ = 4 molecules.

• Right-hand side: 2 NH₃ = 2 molecules.

Rules for Pressure Changes:

• Increase pressure → equilibrium shifts to the side with fewer molecules.

  • Here, equilibrium shifts right, producing more NH₃.

• Decrease pressure → equilibrium shifts to the side with more molecules.

  • Here, equilibrium shifts left, producing more N₂ and H₂.

Another Example

H₂(g) + I₂(g) ⇌ 2HI(g)

• Left-hand side: 1 H₂ + 1 I₂ = 2 molecules.

• Right-hand side: 2 HI = 2 molecules.

• Observation: Number of molecules is the same on both sides.

• Conclusion: Changing the pressure has no effect on the position of equilibrium.

✅ Remember:

• Count the number of molecules on each side of the equation.

• Equilibrium shifts towards fewer molecules if pressure is increased, and towards more molecules if pressure is decreased.