Equilibrium

• • The larger the value of Kc, the further the position of equilibrium towards the products
• Units of Kc:

  • The units of Kc depend upon the number of concentration terms on the top and bottom of the equilibrium constant term.
  • To work out the units:
  • substitute units into the expression of Kc
  • cancel common units & show the final units on a single line
• Homogeneous equilibria:

  • A homogeneous equilibrium contains equilibrium species that all have the same state or phase
  • N2(g) + 3H2(g) --> 2NH3(g) homogeneous - all species are gases
• Heterogeneous equilibria:

  • A heterogeneous equilibrium contains equilibrium species that have different states or phases
  • C(s) + H2O (g) --> Co(g) + H2 (g) heterogeneous - mixture of states
• • In homogeneous equilibria, the Kc expression contains concentrations of all species. However, in heterogeneous equilibria, the concentration of solids & liquids are essentially constant. SO any species that are solids & liquids are omitted from the Kc expression.
  • Kc only includes species that are (g) or (aq)
• Mole fraction --> the mole fraction of a gas is the same as its proportion by volume to the total volume of gases in a gas mixture
• For a gas A in a gas mixture:

  mole fraction x(A) = number of moles of A / total number of moles in gas mixture
• Partial pressure --> the partial pressure p of a gas is the contribution that the gas makes towards the total pressure P. The sum of the partial pressures of each gas equals the total pressure
• • partial pressure p(A) = mole fraction of A x total pressure P
  • p(A) = x(A) x P
• Equilibrium position shift:

  • If the concentration of a species is increased, the equilibrium position shifts in the direction that reduces the concentration
  • If the pressure is increased, the equilibrium position shifts towards the side with fewer gaseous moles.
  • If the temperature is increased, the equilibrium position shifts in the endothermic direction
• Equilibrium constant:

  • The magnitude of an equilibrium constant k indicates the extent of a chemical equilibrium
  • k =1 indicates an equilibrium halfway between reactants & products
  • k = 100 indicates an equilibrium well in favour of the products
  • k = 0.01 indicates an equilibrium well in favour of the reactants
• • The value of K gives the exact position of equilibrium
  • When the reactants or products of a reversible reaction are mixed together, the reaction proceeds until the concentrations of the equilibrium species give the value of k when placed in the equilibrium constant expression.
  • At a set temperature, K is constant & doesn't change despite any modifications to concentration, pressure or the presence of a catalyst
  • But k does change if the temperature is changed - a temperature change is the only condition that will cause k to change its value
• Exothermic reactions:

  • The equilibrium constant decrease with increasing temperature
  • Raising the temperature decreases the equilibrium yield of products
• Endothermic reactions:

  • The equilibrium constant increases with increasing temperature
  • Raising the temperature increases the equilibrium yield of products
• Reactions with a catalyst:

  • Catalyst affects the rate of a chemical reaction but not the position of equilibrium
  • Catalysts affect the rate of a chemical reaction but not the position of equilibrium
  • Catalyst speed up both the forward & reverse reactions in the equilibrium by the same factor
  • Equilibrium is reached quicker but the equilibrium position is not changed