6 Equilibria

• Two features of dynamic equilibrium:

  1. Forward and backward reactions are occurring atequal rates.
  2. The concentrations of reactants and products stay constant
• The term dynamic means both forward and backward reactions are occurring simultaneously
• If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants.
• Le Chatelier’s principle states that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).
• If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
• If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
• Increase temperature affect on yield of ammonia:

  If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia.

  
• Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and therefore reduce the pressure.
• Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure.
• If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium.
• Industrially high pressures are expensive to produce because of the high electrical energy costs for pumping the gases to make a high pressure and the equipment is expensive to contain the high pressures
• Effect of increasing pressure on yield of methanol:

  If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure . The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol.

  
• Increasing the concentration OH- ions causes the equilibrium to shift to oppose this and move in the forward direction to remove and decrease the concentration of OH- ions. The position of equilibrium will shift towards the right, giving a higher yield of I- and IO-. ( The colour would change from brown to colourless).

  
• Adding H+ ions reacts with the OH- ions and reduces their concentration so the equilibrium shifts back to the left giving brown colour.

  
• A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved.
• A catalyst does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
• Haber process equation:

  A) N2

  B) 3H2

  C) 2NH3

  
• Haber process:

  1. Temperature = 450 degrees celsius
  2. Pressure = 200-1000 atmospheres
  3. Catalyst = iron
  4. Is exothermic
• Contact process to produce sulfur trioxide to react with water to produce sulfuric acid:

  1. Temperature = 450 degrees celsius
  2. Pressure = 1 - 2 atmospheres
  3. Catalyst = V2O5 - Vanadium oxide
• Contact process equations:

  A) SO2

  B) SO2

  C) O2

  D) SO3

  E) Exothermic

  
• Production of methanol from carbon monoxide:

  1. Temperature = 400 degrees celsius
  2. Pressure = 50 atmospheres
  3. Catalyst = chromium and zinc oxides
  4. Is exothermic
• Production of methanol from carbon monoxide equation:

  A) CO

  B) 2H2

  C) CH3OH

  
• Hydration of ethene to produce ethanol:

  1. Temperature = 300 degrees celsius
  2. Pressure = 70 atmospheres
  3. Catalyst = concentrated phosphoric acid
  4. Is exothermic
• Hydration of ethene to produce ethanol equation:

  A) H20

  B) CH3CH2OH

  
• In all reactions, catalysts speeds up the rate, allowing a lower temp to be used (and hence lower energy costs), but have no effect on position of equilibrium
• In all reactions, high pressure leads to too high energy costs for pumps to produce the pressure and too high equipment costs to have equipment that can withstand high pressures.
• Recycling unreacted reactants back into the reactor can improve the overall yields of processes.
• Both methanol and ethanol can be used as fuels.
• Equilibrium equation:

  A) NH3

  B) 2

  C) N2

  D) H2

  E) 3

  
• Kc = equilibrium constant
• What is the unit of Kc here:

  A) no unit

  
• The unit of Kc here is mol-2dm6

  
• moles of reactant at equilibrium = initial moles – moles reacted

  moles of product at equilibrium = initial moles + moles formed
• The larger the Kc the greater the amount of products. If Kc is small we say the equilibrium favours the reactants
• Kc only changes with temperature
• Both the position of equilibrium and the value of Kc will change if temperature is altered
• If temperature is increased the reaction will shift to oppose the change and move in the backwards endothermic direction. The position of equilibrium shiftsleft. The value of Kc gets smaller as there are fewer products.

  
• The position of equilibrium will change if pressure is altered but the value of Kc stays constant as Kc only varies with temperature