Free Science Lessons: Le Chatelier's Principle and Pressure in Reversible Reactions

Overview

• Main Focus: Predicting the effect of changing pressure on reversible reactions at equilibrium using Le Chatelier's Principle.
• Previous Topic: Introduction to Le Chatelier's Principle.

Le Chatelier's Principle

• Definition: When an external change is applied to a system at equilibrium, the equilibrium shifts in the direction that reduces the effect of that change.

Application to Gaseous Reactions

• Example Reaction: Haber process involving nitrogen, hydrogen, and ammonia.
• Reaction Detail:
  • Nitrogen (N₂) + Hydrogen (H₂) ↔ Ammonia (NH₃)
  • Reversible reaction: Ammonia can revert to nitrogen and hydrogen.

Gaseous Pressure and Moles

• Key Concept: Gas pressure is proportional to the number of moles of gas present.
• Haber Process Stoichiometry:
  • Reactants: 1 mole of N₂ + 3 moles of H₂ = 4 moles
  • Products: 2 moles of NH₃
  • Pressure: More moles mean higher pressure.

Effects of Pressure Changes at Equilibrium

• Increasing Pressure:
  • Equilibrium shifts towards fewer moles to reduce pressure.
  • In the Haber process, equilibrium shifts right (towards products).
• Decreasing Pressure:
  • Equilibrium shifts towards more moles to increase pressure.
  • In the Haber process, equilibrium shifts left (towards reactants).

Example Reaction: Dinitrogen Tetroxide and Nitrogen Dioxide

• Reaction:
  • Dinitrogen Tetroxide (N₂O₄, colorless) ↔ Nitrogen Dioxide (NO₂, brown)
• Pressure Increase:
  • Equilibrium shifts from 2 moles (NO₂) to 1 mole (N₂O₄) to reduce pressure.
  • Reaction mix less brown as NO₂ converts to N₂O₄.

Equilibrium Influence by Pressure

• Condition for Pressure Effect: Equilibrium affected by pressure only if total moles are different on either side of the equation.
• Example: Reaction of hydrogen and bromine to form hydrogen bromide.
  • Equal moles on both sides: Pressure change has no effect.

Next Steps

• Upcoming Topic: Effect of temperature on reversible reactions.