Notes on Chemical Equilibrium Concepts

The Nature of Chemical Equilibrium

• Definition: In equilibrium systems, opposing processes occur simultaneously and at equal rates.
• Examples:
  • Sugar in water: Sugar dissolves and crystallizes at the same rate when equilibrium is reached.
  • Evaporation and condensation in a closed vessel achieve a state where the rates are equal, defining the equilibrium vapor pressure.

Reversible Reactions

• All reactions can theoretically reverse; they are called reversible reactions if products can reform reactants.
• Example: Mercury(II) oxide decomposition:
  • Decomposition: $2HgO(s) \rightleftharpoons 2Hg(l) + O_2(g)$
  • Synthesis: $2Hg(l) + O_2(g) \rightleftharpoons 2HgO(s)$
• Dynamic Equilibrium: Equilibrium is reached when the forward and reverse reaction rates are equal, maintaining constant concentrations of substances involved.

Characteristics of Equilibrium States

• Equilibrium Indicators:
  • The chemical equation at equilibrium is written with double arrows: $2HgO(s) \rightleftharpoons 2Hg(l) + O_2(g)$
• Equilibrium Concentration: The concentrations of reactants and products remain constant during equilibrium.

Different Equilibrium Positions

• Forward Reaction Favored: Equilibrium lies to the right, with higher concentrations of products (e.g., $2SO<em>2(g) + O</em>2(g) \rightarrow 2SO_3(g)$).
• Reverse Reaction Favored: Equilibrium lies to the left, with higher concentrations of reactants (e.g., $H<em>2CO</em>3(aq) + H<em>2O(l) \rightleftharpoons H</em>3O^+(aq) + HCO_3^-(aq)$).
• Balanced Equilibrium: Equilibrium can establish with equal concentrations of reactants and products (e.g., dissociation of sulfurous acid).

The Equilibrium Expression

• General Reaction Form: $nA + mB \rightleftharpoons xC + yD$
• Formation of Equilibrium:
  • Initially, products C and D have zero concentration while A and B are at maximum.
  • As A and B are consumed, the forward reaction rate decreases while the reverse reaction rate increases until equilibrium is reached.
• Equilibrium Condition: After attaining equilibrium, the concentrations of reactants and products remain stable, leading to a defined ratio of their concentrations.