Lecture Notes: Equilibrium Constants

Introduction to Equilibrium Constants

• Symbol: Represented by the letter K.
• Purpose: Indicates the relative concentrations of reactants and products at equilibrium.

Hypothetical Reaction

• Example: Reactants A and B transform into products C and D.
• Balanced Equation Coefficients: Lowercase letters (a, b, c, d) represent coefficients.

Writing Equilibrium Constant Expression

• Symbol for Concentrations: Kc (c denotes concentration).
• Expression:
  • Numerator: Concentrations of products (C & D) multiplied together, raised to the power of their coefficients.
  • Denominator: Concentrations of reactants (A & B) multiplied together, raised to the power of their coefficients.
• Note: Use equilibrium concentrations.
• Temperature Factor: K is constant at a particular temperature.

Example: Synthesis of Ammonia

• Reaction: Synthesis of ammonia from nitrogen (N2) and hydrogen (H2) gases.
  • Equation:
    • Kc Expression:
      • Numerator: [NH3]^2
      • Denominator: [N2]^1 [H2]^3
• Using Partial Pressures:
  • Symbol: Kp (p denotes partial pressures).
  • Kp Expression for Ammonia:
    • Numerator: (Partial pressure of NH3)^2
    • Denominator: (Partial pressure of N2)^1 x (Partial pressure of H2)^3

Homogeneous vs. Heterogeneous Equilibria

• Homogeneous Equilibrium: All reactants and products in the same phase.
• Heterogeneous Equilibrium: Reactants and products in different phases.
  • Example: Decomposition of calcium carbonate:
    • Calcium carbonate (solid) decomposes into calcium oxide (solid) and carbon dioxide (gas).
• Equilibrium Constant for Heterogeneous Equilibria:
  • Pure solids and liquids are excluded from the equilibrium constant expression due to constant concentration.
  • Example:
    • Kc for Decomposition:
      • Products: Concentration of CO2^1
      • Reactants: Exclude solids (CaCO3 and CaO)
    • Kp for Decomposition:
      • Products: Partial pressure of CO2^1

Reaction Quotient (Q)

• Symbol: Q.
• Form: Similar to the equilibrium constant expression.
• Purpose: Indicates relative concentrations of reactants and products at any time.
• Expressions:
  • Qc: For concentrations
  • Qp: For partial pressures
• Comparison with Equilibrium:
  • If Qc = Kc: Reaction is at equilibrium.
  • If Qc > Kc or Qc < Kc: Reaction not at equilibrium.