Chemical Equilibrium Notes

Chapter 15: Chemical Equilibrium

Key Concepts
  • Dynamic Equilibrium:
    • Definition: A state where the forward and reverse reactions occur at equal rates.
    • Example:
    • Ethanol
      • Vaporization rate = Condensation rate
      • At equilibrium, the amounts of liquid and gas ethanol remain constant.
    • Glucose
      • Dissolution rate = Precipitation rate
      • At equilibrium, solid and aqueous concentrations remain constant.
Equilibrium Constants
  • Equilibrium Constant (K): A ratio expressing the concentrations of products over reactants at equilibrium.
  • Law of Mass Action: The concentration of reactants and products at equilibrium is related to their stoichiometry.
  • Kc vs. Kp:
    • Kc: Concentration-based equilibrium constant (mol/L)
    • Kp: Pressure-based equilibrium constant (atm)
  • Properties:
    1. Overall Reactions: Constants relate to the entire reaction.
    2. Reverse Reactions: K for reverse reaction is 1/K for forward reaction.
    3. Multiplication of Reactions: K for a multiplied reaction is K raised to the power corresponding to the multiple.
Heterogeneous Equilibrium
  • Involves multiple phases (solid, liquid, gas) in equilibrium.
  • Example: CaCO3 (s) ⇌ CaO (s) + CO2 (g)
Quantitative Equilibrium Problems
  • Steps to Solve:
    1. Write the balanced equation.
    2. Write equilibrium expression based on stoichiometry.
    3. Use concentrations or partial pressures.
    4. Substitute values to find K or Q.
Le Châtelier’s Principle
  • Principle: If a system at equilibrium experiences a change (concentration, pressure, temperature), the system shifts to counteract that change.
  • Concentration Changes:
    • Adding reactants: shifts right (to products).
    • Adding products: shifts left (to reactants).
  • Temperature Changes:
    • Increasing temperature in exothermic reactions shifts left.
    • Increasing temperature in endothermic reactions shifts right.
  • Pressure and Volume Changes:
    • Decreasing volume (increasing pressure) favors side with fewer gas moles.
    • Increasing volume (decreasing pressure) favors side with more gas moles.
Example problems

  • Equilibrium Constant Calculation:

    • Given concentrations at equilibrium, substitute into equilibrium expressions to determine Kc or Kp.
    • For example:

    H2 (g) + I2 (g) ⇌ 2HI (g)

    • For Kc: ( Kc = \frac{[HI]^2}{[H2][I2]} )
Reaction Quotient Q
  • Definition: A measure of the current state of the reaction compared to equilibrium.
  • Comparison with K
    • If Q < K, reaction shifts right (towards products).
    • If Q = K, reaction is at equilibrium.
    • If Q > K, reaction shifts left (towards reactants).
Tips for Solving Equilibrium Problems
  1. Write the balanced equation.
  2. Set up the equilibrium expression.
  3. Calculate initial reactant and product amounts.
  4. Calculate Q and determine shift direction.
  5. Use stoichiometry to find changes in concentration.
  6. Substitute back into the equilibrium expression to find unknowns.
  7. Always check if the calculated concentrations are reasonable.