Chemical Equilibria

• How is a reversible reaction denoted?

  Double arrow symbol: ⇌

  A+B ⇌ C+D
• Define closed system


  no substances are either added to the system or lost from it. 
• define dynamic equilibrium


  an equilibrium can only be established in a closed system and can be set up from either direction. At equilibrium, a natural balance between products (right hand side) and reactants (left hand side) is reached. Chemical equilibria are dynamic as both the forward and reverse reactions continue to occur
• feature 1 of dynamic equilibrium


  rate of forwards reaction is equal to rate of reverse reaction
• feature 2


  concentration of products and reactants remains constant. A reaction in which equilibrium lies to the left- there is a higher concentration of reactants than products. A reaction in which equilibrium lies to the right- higher conc. of products than of reactants
• Le Chatelier's Principle


  if a system in equilibrium is subjected to a change, the position of equilibrium will move in the direction which opposes that change
• effect of changing concentration

  If the concentration of the reactants increases the position of equilibrium shifts to the right to oppose the change and decrease [reactants]

  If the [products] increases equilibrium shifts to the left to oppose the change and decrease [products]
• changes in pressure


  • if the total pressure increases, the equilibrium shifts to the side with fewer moles of gas because fewer moles of gas exert less pressure
  • if the total pressure decreases, equilibrium shifts to the side with more moles of gas because they exert more pressure
  • e.g. Haber process: N2 + 3H2 ⇌ 2NH3. If pressure is increased, yield of ammonia increases as equilibrium shifts to right to oppose change and decrease pressure
• An increase in pressure


  Increases the rate of the reaction
• One of the reactions increases by a larger amount, so equilibria shifts
• An increase in pressure always increases the rate of reaction because the collisions between molecules becomes more frequent. it is just one of the reactions increases by a larger amount so equilbria shifts
• Change in temperature


  • if the temp is increased the equilibrium will move in the direction of the endothermic reaction
  • if the temp is decreased the equilibrium moves in the direction of the exothermic reaction
  • the way the equilibrium responds to the change in temp depends on whether the forwards reaction is exo or endo
  • by convention, delta H always refers to the forward reaction
  • an increase in temp always increases the rate of forwards and reverse reactions but to different extents
• Describe what an increase in temp would do to this reaction

  N2 (g) + 3H2 (g) ⇌ 2NH3 (g) Delta H= -92.4 kJ mol -1

  equilibrium shifts in the direction of the endothermic reaction which is the reverse reaction to oppose the change and absorb the heat. the yield of ammonia decreases
• the effect of a catalyst


  • a catalyst has no effect on the equilibrium position or on the yield as it increases the rate of the forwards reaction and backwards reaction equally
  • a catalyst always increases the rate
• operating conditions required for the Haber process

  in order to balance the yield of ammonia, the rate at which the ammonia is produced and the cost of the plant and energy requirements a compromise temperature and pressure are used

  • p= 200 atm, high pressure requires expensive equipment, they are safety risks - risk of explosion
  • t= 400-450 degrees c, a high yield of ammonia is achieved with a low temp however low temp means slower reaction so a compromise is reached
  • iron catalyst increases rate
  • recycling, save money on raw materials
• Kc


  the equilibrium constant in terms of the concentration of the reactants and products

  aA + bB ⇌ cC + dD

  Kc= conc of products over conc of reactants

  [A], [B], [C] and [D] are the conc of the reactants and products at equilibrium

  a, b, c and d are the balancing numbers of the balanced equation
• A Kc value siginificantly less than 1 means [reactants] > [products]

  so equilibrium lies to left
• A Kc value significantly more than 1 means [products]>[reactants]

  equilibrium lies to right
• units of Kc


  they vary depending on the reaction stoichiometry and must be determined for each Kc expression
• Write an expression for Kc using this reaction

  CH3CH2OH + H2O ⇌ 2CO + 4H2

  Kc= [CO]^2 x [H2]^4/ [CH3CH2OH] x [H2O]

  6 lots of moldm-3 on top of moldm-3 x moldm-3

  = mol4dm-12
• What is the only factor that affects Kc?

  Temperature

  the presence of a catalyst or a change in concentration does not affect Kc. If the temp changes causes the equilibrium to shift right Kc will increase, if the equilibrium shifts left Kc will decrease
• Larger number of products= Kc increases

  larger number of reactants= Kc decreases
• When there is the same number of moles on each side of the equilibrium?


  • Kc has no units - units cancel
  • volume (units cancel)
  • so moles can be used to calculate Kc
• how to calculate equilibrium constant from initial amounts

  when a reversible reaction has reached equilibrium, there will be a mixture of reactants and products.

  to find the equilibrium concentrations you must first determine the number of moles at equilibrium.

  the amount of each chemical present at equilibrium can be determined from the balanced equation and initial moles
• the RICE method


  ratio from equation, initial moles used, change, equilibrium moles