Reversible Reactions and Equilibrium

Introduction

• Discussion on reversible reactions, equilibrium, and the position of equilibrium.

Reversible Reactions

• One-Way Reaction: Reactants turn into products and do not revert back.
• Reversible Reaction: Indicated by a double arrow, showing that products can revert to reactants.
  • Forward Reaction: E.g., Ammonium chloride breaks down into ammonia and hydrogen chloride.
  • Backward Reaction: Ammonia and hydrogen chloride combine to reform ammonium chloride.

Equilibrium

• Definition: When the forward and backward reactions occur at the same rate.
• Concentrations at Equilibrium:
  • Concentrations of reactants and products remain constant.
  • Molecules continue reacting, but net concentrations do not change.
• Misconception Clarification: Constant concentrations do not mean they are equal.
  • Equilibrium can have more reactants or more products.

Position of Equilibrium

• Right: More products present.
• Left: More reactants present.
• Factors Affecting Position:
  • Temperature changes can shift equilibrium.
    • Adding heat favors forward reaction (shifts right).
    • Cooling favors backward reaction (shifts left).

Closed System Requirement

• Equilibrium needs a closed system where no reactants or products can escape.

Thermodynamics of Reversible Reactions

• Exothermic and Endothermic:
  • Reversible reactions are exothermic in one direction and endothermic in the other.
  • Example: Thermal decomposition of hydrated copper sulfate.
    • Forward reaction: Endothermic.
    • Backward reaction: Exothermic.

Example: Thermal Decomposition of Hydrated Copper Sulfate

• Forward Reaction (Endothermic):
  • Heat applied: Blue hydrated copper sulfate crystals turn into white anhydrous powder.
• Backward Reaction (Exothermic):
  • Remove heat and add water: Reforms blue crystals, releases absorbed heat.

Recap

• Reversible reactions: Double arrow, can happen in both directions.
• Equilibrium: Rates of both directions equal, constants maintained.
• Position of equilibrium: Shifts according to conditions, affects reactant/product concentration balance.

Conclusion

• Understanding reversible reactions and equilibrium is crucial for predicting and controlling chemical reactions under different conditions.
• Equilibrium can only be achieved in a closed system and involves thermodynamic principles of exothermic and endothermic processes.