chemical equilibria

• dynamic equilibrium is


  the rate of the forward reaction is equal to the rate of the reverse reaction (so concs of reactants and products dont change)
• essential condition for an equilibrium mixture:

  equilibrium occurs in a closed system (where reactants and products cant escape)
• le chateliers principle states


  if a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance
• in the equation CH4(g) + H2O(g) <-> CO2(g) + 3H2(g) △H° = +210kJmol-1

  when temperature increases, the equilibrium shifts to the right because the forward reaction is endothermic
• in the equation CH4(g) + H2O(g) <-> CO2(g) + 3H2(g) △H° = +210kJmol-1 when increasing pressure, the equilibrium will shift to the left because the forward reaction produces more moles of gas than the reverse reaction
• in the equation CH4(g) + H2O(g) <-> CO2(g) + 3H2(g) △H° = +210kJmol-1 an industrial chemist might use a high pressure for the production of hydrogen because the high pressure:

  1. increases collusion frequency and hence rate of reaction
• a catalyst has no effect on the position of an equilibrium because a catalyst affects the rate of reaction of the forward and reverse reaction equally
• temperature affects the value of Kc
• a Kc expression for

  2[A] + 3[B] + [C] <-> [D] + 4[E]

  Kc = [D][E]4 / [A]2[B]3[C]
• units for Kc are

  mol-1dm3
• Kc is relevant for homogenous systems in equilibrium
• if Kc is greater than 1, the equilibrium is over to the right
• if Kc is less than 1, the position of the equilibrium is over to the left
• when you decrease the temperature in an endothermic reaction, Kc decreases
• when you increase the temperature in an endothermic reaction, Kc increases
• when you decrease the temperature in an exothermic reaction, Kc increases
• when you increase the temperature in an exothermic reaction, Kc decreases