chapter 19

• homogeneous equilibria


  contains equilibrium species that all have the same state or phase e.g

  N2(g) + 3H2(g) -> 2NH3(g) are all gases
• heterogeneous equilibria


  contains equilibrium species that have different states or phases e.g

  C(s) + H2O(g) -> CO(g) + H2(g) are all in different states
• in homogeneous equilibria, the Kc expression contains concentrations of all species
• in heterogeneous equilibria, the concentration of solids and liquids are constant
• equilibria involving gases are usually expressed in Kp, because in gases it is easier to measure pressure than concentration
• concentration and pressure are proportional to each other and Kp has a direct relationship with Kc
• mole fraction of a gas


  the same as its proportion by volume to the total volume of gases in a gas mixture
• mole fraction for gas A = number of moles of A / total number of moles in mix
• partial pressure


  contribution that the gas makes towards the total pressure, the sum of the partial pressures of each gas equals the total pressure
• partial pressure of A = mole fraction of A x total pressure
• sum of mole fractions = 1
• sum of partial pressures = total pressure
• Kp is written the same way as Kc but with partial pressures replacing concentration terms
• p is the equilibrium partial pressure
• equilibrium constant K of 1 = indicates equilibrium halfway between reactants and products
• equilibrium constant K of 100 = indicates equilibrium in favour of products
• equilibrium constant K of 0.01 = indicates equilibrium in favour of 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 forward reaction is exothermic, the equilibrium constant decreases with increasing temperature, and raising the temperature decreases the equilibrium yield of products
• if forward reaction is endothermic, the equilibrium constant increases with increasing temperature, and raising the temperature increases the equilibrium yield of the products
• value of equilibrium constant K is unaffected by changes in concentration or pressure
• doubling of pressure results in doubling of the partial pressures and concentration of all gases
• 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