Equilibrium

• Equilibrium constant K


  Expression that describes the relationship between the concentrations of reactants and products at equilibrium
• Calculating K


  1. Write the equilibrium equation

  2. Substitute equilibrium concentrations into the K expression

  3. Calculate the numerical value of K
• Units of K


  • Depend on the number of concentration terms in the K expression
  • Cancel common units to get the final units
• Homogeneous equilibria


  • Contain equilibrium species that all have the same state or phase
  • K expression contains concentrations of all species
• Heterogeneous equilibria


  • Contain equilibrium species that have different states or phases
  • Concentrations of solids and liquids are constant and omitted from K expression
  • K only includes species that are (g) or (aq)
• Calculating equilibrium quantities and K

  1. Determine equilibrium amounts of all species

  2. Calculate equilibrium concentrations

  3. Write K expression, substitute values, and calculate K
• Mole fraction


  Proportion by volume of a gas in a gas mixture
• Calculating mole fractions


  1. Mole fraction of A = number of moles of A / total moles in mixture

  2. Sum of mole fractions must equal 1
• Partial pressure


  • Contribution of a gas to the total pressure in a gas mixture
  • Partial pressure of A = mole fraction of A x total pressure
• Equilibrium constant K_p


  Expressed in terms of partial pressures instead of concentrations
• Temperature change


  Only condition that causes K to change value
• Le Chatelier's principle explains how equilibrium position shifts in response to changes
• At constant temperature, K is constant and does not change despite changes in concentration, pressure, or presence of catalyst
• Le Chatelier's principle


  Explains why equilibrium shifts in response to changes in conditions
• Equilibrium constant (K)


  Indicates the extent of a chemical equilibrium
• K=1 indicates an equilibrium halfway between reactants and products
• K=100 indicates an equilibrium well in favour of the products
• K=1x10^-2 indicates an equilibrium well in favour of the reactants
• At a set temperature, K is constant and does not change despite any modifications to concentration, pressure, or the presence of a catalyst
• K does change if the temperature is changed - a temperature change is the only condition that will cause K to change its value
• If the forward reaction is exothermic

  The equilibrium constant decreases with increasing temperature
• If the forward reaction is exothermic


  Raising the temperature decreases the equilibrium yield of products
• If the forward reaction is endothermic

  The equilibrium constant increases with increasing temperature
• If the forward reaction is endothermic

  Raising the temperature increases the equilibrium yield of products
• Changes in concentration or pressure do not affect the value of the equilibrium constant K
• Equilibrium shift due to concentration changes

  1. Ratio of reactants to products changes

  2. Concentrations change to restore the ratio to the K value

  3. New equilibrium is established
• Equilibrium shift due to pressure changes

  1. Ratio of reactants to products changes

  2. Partial pressures change to restore the ratio to the K value

  3. New equilibrium is established
• Catalysts affect the rate of a chemical reaction but not the position of equilibrium
• Catalysts speed up both the forward and reverse reactions in the equilibrium by the same factor
• Equilibrium is reached quicker but the equilibrium position is not changed by the presence of a catalyst