Effect of Pressure on Reversible Reactions at Equilibrium

Introduction

• Focus on understanding the effect of pressure on reversible reactions at equilibrium.
• Relevant for higher-tier students.
• Builds on previous lessons about changing conditions affecting equilibrium, based on Le Chatelier's Principle.

Key Concepts

Pressure Effects on Gas Reactions

• Pressure and Molecules: Pressure in gases is dependent on the number of molecules.
  • Two containers with gases at same temperature and volume but different molecule counts will have different pressures.
  • Example: Container with twice the molecules will have twice the pressure.

Example Reaction

• Nitrogen and Hydrogen to Ammonia:

  • Reaction: [ N_2 + 3H_2 \leftrightarrow 2NH_3 ]
  • Left side: 1 molecule of ( N_2 ) + 3 molecules of ( H_2 ) = 4 molecules total.
  • Right side: 2 molecules of ( NH_3 ).

Rules for Pressure Changes

• Increasing Pressure:
  • Equilibrium shifts to the side with fewer molecules.
  • Example: Above reaction shifts to the right (fewer molecules on right).
• Decreasing Pressure:
  • Equilibrium shifts to the side with more molecules.
  • Example: Above reaction shifts to the left (more molecules on left).

Another Reaction Example

• Iodine and Hydrogen to Hydrogen Iodide:
  • Reaction: [ I_2 + H_2 \leftrightarrow 2HI ]
  • Both sides have equal number of molecules (2 each).
  • Result: Changing pressure has no effect on equilibrium.

Resources

• Additional practice and questions available in the revision workbook.

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