Lecture Notes: Reversible Reactions and Equilibrium

Introduction to Reversible Reactions

• Reversible Reaction: Involves a double arrow (⇌) indicating two-way reactions.
  • Example: Ammonium chloride ⇌ Ammonia + Hydrogen Chloride
  • Forward Reaction: Ammonium chloride breaks down into ammonia and hydrogen chloride.
  • Backward Reaction: Ammonia and hydrogen chloride recombine to form ammonium chloride.

Rates of Reaction

• Initial stage:
  • High concentration of ammonium chloride, no ammonia or hydrogen chloride.
  • Fast forward reaction, backward reaction has not started.
• As reaction progresses:
  • Forward reaction slows down, backward reaction speeds up.
  • Eventually, both reactions occur at the same rate.

Concept of Equilibrium

• Equilibrium: The state where both forward and backward reactions occur at equal rates.
  • Concentrations of reactants and products remain constant.
  • Reactions still occur, but there's no overall change in concentration.
• Concentration at Equilibrium:
  • Does not imply equal concentrations of reactants and products.
  • Example scenarios:
    • High ammonium chloride, low ammonia and hydrogen chloride.
    • High ammonia and hydrogen chloride, low ammonium chloride.

Position of Equilibrium

• Position of Equilibrium: Indicates relative concentrations of reactants and products.
  • Equilibrium lies to the right: More products than reactants.
  • Equilibrium lies to the left: More reactants than products.
• Factors affecting position:
  • Temperature changes can shift equilibrium:
    • Adding heat favors the forward reaction (more products).
    • Cooling favors the backward reaction (more reactants).

Conditions for Equilibrium

• Equilibrium requires a closed system:
  • Sealed environment where reactants/products can’t escape.

Thermodynamics of Reversible Reactions

• Exothermic and Endothermic Directions:
  • Reversible reactions are exothermic in one direction and endothermic in the other.
  • Example: Hydrated copper sulfate ⇌ Anhydrous copper sulfate + Water
    • Forward reaction (decomposition) is endothermic.
    • Backward reaction (reformation) is exothermic.
  • Heating hydrated copper sulfate drives the reaction to the right.
  • Removing heat and adding water shifts equilibrium to the left, releasing energy.

Summary

• Reversible reactions can proceed in both directions, indicated by a double arrow.
• Equilibrium is achieved when the rates of the forward and backward reactions are equal, maintaining constant concentrations.
• The position of equilibrium can shift left or right due to changes in conditions such as temperature.
• Reversible reactions involve both exothermic and endothermic processes.