Equilibrium Constants

• Kc


  Equilibrium constant that indicates the position of equilibrium for a reversible reaction at a certain temperature

  
• Calculating Kc


  1. Know the concentrations of the species present in the reaction at the point of equilibrium

  2. Kc = concentration of products / concentration of reactants

  3. Variation in number of moles raises that species to a power with the same value as the number of moles
• Kc has varying units depending on the chemical reaction
• Kc units can cancel completely, in which case Kc has no units
• Calculating Kc units


  • 2A + 3B ⇌ D + 4E
  • Kc = [(moldm-3)(moldm-3)4] / [(moldm-3)2(moldm-3)3]
  • All units cancel, so Kc has no units
• Temperature changes and Kc

  • Kc is not affected by concentration change or use of a catalyst, but is affected by changing the reaction temperature
  • Equilibrium position changes, resulting in different concentrations of reactants and products
  • For endothermic forward reactions, increase in temperature favours forward reaction and increases Kc
  • For exothermic forward reactions, increase in temperature favours backward reaction and decreases Kc
• Kp


  Equilibrium constant used for gaseous equilibria, where all reactants and products must be in a gaseous state
• Calculating Kp


  1. Use partial pressures of gases in the system

  2. Kp = (partial pressures of products) / (partial pressures of reactants)

  3. Variation in moles raises partial pressure to a power of equal quantity

  
• Kp also has varying units determined by substitution into the Kp equation
• Calculating Kp units


  • A + 3B ⇌ D + 2E
  • Kp = [(kPa)(kPa)2] / [(kPa)(kPa)3] = kPa-1

  
• Temperature changes and Kp

  • Kp can change depending on whether the reaction is endothermic or exothermic
  • For exothermic forward reactions, increase in temperature favours backward reaction and decreases Kp
  • For endothermic forward reactions, increase in temperature favours forward reaction and increases Kp