Lecture Notes: Equilibrium and Equilibrium Constants

Definition of Equilibrium

• Equilibrium: The rate of the forward reaction equals the rate of the reverse reaction.
  • Forward rate = Reverse rate.
  • Does NOT mean forward rate constant = reverse rate constant.
  • Concentration of reactants ≠ Concentration of products generally.

Types of Equilibrium

• Dynamic Equilibrium (Chemistry focus)

  • Both forward and reverse reactions are ongoing at equal rates.
  • Concentrations remain constant over time.

• Static Equilibrium (Physics focus)

  • Everything is stopped.

Equilibrium Constants

• Equilibrium Constant (K): Ratio of products to reactants, each raised to the power of their coefficients.

  • Capital K (Kc): Used for concentration.
  • Kp: Used for gases, involving pressures.

• Key Points:

  • K does not include solids and liquids as they have an activity of one.
  • A large K value (>1) favors products; a small K value (<1) favors reactants.
  • Textbooks commonly state K > 1000 as favoring products and K < 0.001 as favoring reactants.

Factors Affecting Equilibrium Constants

• Only temperature changes the equilibrium constant.
  • Catalysts do not affect K; they lower activation energy and increase rates.

Relationship Between Kc and Kp

• Equation: Kp = Kc * (RT)^∆n
  • ∆n = change in moles of gas (products minus reactants).
  • Kp = Kc if there is no change in moles of gas.*

Calculations Involving Equilibrium Constants

• Reversing Reactions: K becomes 1/K.
• Changing Coefficients:
  • Halving coefficients: K becomes √K.
  • Doubling coefficients: K becomes K^2.
  • Reversing and halving: K becomes 1/√K.

R Constants

• R in Kp and Kc:
  • Use R = 0.08206 L·atm/mol·K from the ideal gas law.
  • In energy contexts, use R = 8.314 J/mol·K.

Tips for Problem Solving

• Write out the expressions for equilibrium constants and compare when confused.
• Calculate changes step by step using the relationship between reaction modifications and K.