Equilibria

• equilibrium = rate of the forward reaction is equal to the rate of the backwards reaction

  • for equilibrium to be established - reversible reaction in a closed system
  • closed system = no matter leaves or enters the system from the environment
  • conc. of reactants & products remain constant

  A) equilibrium reached

  B) constant concentrations at equilibrium

  C) reaction rate

  D) concentration

  
• Le Chatelier's Principle

  'If a change to the system is made when at equilibrium the position of equilibrium will shift towards the reaction that opposes the change until a new equilibrium is established'
• Factors acting on Le Chatlier's Principle

  1. Changing concentration

  • if more added, the equilibrium will shift to remove - favouring the forward/backward reaction

  2. Changing temperature

  • If temperature increased, equilibrium shifts to decrease temp by favouring the backwards/forwards, endothermic reaction

  3. Changing pressure

  • If we increase pressure, equilibrium shifts to decrease pressure by favouring reaction with least no. of moles

  4. Using a catalyst

  • speeds up both forward & backward reactions by same amount therefore no effect on position of equilibrium but is achieved faster
• Equilibrium reactions in industry: The Haber Process pt 1

  1. Ammonia/The Haber Process N2 + 3H2 ↔ 2NH3

  • optimal conditions:
  • high pressure -> 200 atm/20000 kPa
  • low temp -> 670K
  • low conc of ammonia
  • pressure can't be too high -> too expensive & dangerous
  • temperature can't be too low -> RofR decreases too much
  • iron catalyst used
• Equilibrium reactions in industry: ethanol pt 2

  H2C=CH2 + H2O ↔ C2H5OH

  • optimal conditions:
  • high pressure -> 6500 kPa
  • low temp -> 570K
  • excess steam
  • catalyst: phosphoric acid
  • compromises: high pressure can cause ethene to polymerise & too much steam dilutes the catalyst
• Equilibrium reactions in industry: methanol

  CO + 2H2 ↔ CH3OH

  • optimal conditions:
  • high pressure -> 10000kPa
  • low temp -> 500K
  • high conc. of reactants
• Equilibrium constant = mathematical expression that allows us to numerically describe the position of equilibrium by considering the ratio of products & reactants that exist at equilibrium

  • AKA Kc = constant value that represents the ratio of products & reactants that exist when equilibrium established at a given temperature

  A) products

  B) reactants

  C) no. of moles expressed as a power

  
• Factors affecting Kc + units of Kc

  • concentration, pressure & catalysts don't affect Kc
  • e.g. increase in reactants/products -> increase in products/reactants due to Le Chatlier's Principle -> Kc ratio stays the same
  • temperature is the only factor that affects Kc
  • units for Kc can be seen as algebraic fractions
  • replace conc. with mol/dm3 or x

  [A]^2[B]/[C] -> x^2 * x / x = x^2 = mol^2/dm^6

  [C]/[A]^2[B] -> x/x^2 * x = 1/x^2 = 1/(mol1/dm3)^2 = mol-2/dm6 (change signs)