Equilibrium

• All reversible reactions reach an dynamic equilibrium state.
• The term dynamic means both forward and backward reactions are occurring simultaneously
• Two features of dynamic equilibrium

  1. Forward and backward reactions are occurring atequal rates.
  2. The concentrations of reactants and products stay constant
• We use the expression ‘position of equilibrium’ to describe the composition of the equilibrium mixture.

  If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants.
• We use Le Chatelier’s principle to work out how changing external conditions such as temperature and pressure affect the position of equilibrium.
• 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.
• 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.
• Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate.
• Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure.
• 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.
• 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.
• Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the equipment is expensive (to contain the high pressures)
• A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the

  equilibrium is achieved.

  It 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

  T= 450oC, P= 200 – 1000 atm, catalyst = iron

  Low temp gives good yield but slow rate: compromise temp used

  High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure
• In all cases 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.
• In all cases 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
• Recycling unreacted reactants back into the reactor can improve the overall yields of all these processes. Both methanol and ethanol can be used as fuels.

  If the carbon monoxide used to make methanol in the above reaction was extracted from the atmosphere then it could be classed as carbon neutral.

  It would only be carbon neutral, however, if the energy required to carry out the reaction was not made by combustion of fossil fuels .
• Calculating the moles at equilibrium

  moles of reactant at equilibrium = initial moles – moles reacted

  moles of product at equilibrium = initial moles + moles formed
• Kc only changes with temperature. It does not change if pressure or concentration are altered.A catalyst also has no effect on Kc
• Preparing the equilibrium mixture

  1. Use burettes to prepare a mixture in boiling tube of carboxylic acid, alcohol, and dilute sulfuric acid

  2. Swirl and bung tube

  3. Leave the mixture to reach equilibrium for one week
• Titrating the equilibrium mixture

  1. Rinse a 250 cm3 volumetric flask with distilled water

  2. Use a funnel to transfer the contents of the boiling tube into the flask

  3. Rinse the boiling tube with water and add the washings to the volumetric flask

  4. Use distilled water to make up the solution in the volumetric flask to exactly 250 cm3

  5. Stopper the flask, then invert and shake the contents thoroughly

  6. Use the pipette to transfer 25.0 cm3 of the diluted equilibrium mixture to a 250 cm3 conical flask

  7. Add 3 or 4 drops of phenolphthalein indicator to the conical flask

  8. Set up the burette with sodium hydroxide solution

  9. Add the sodium hydroxide solution from the burette until the mixture in the conical flask just turns pink

  10. Record this burette reading in your table

  11. Repeat the titration until you obtain a minimum of two concordant titres
• Working out initial amount of moles of reactants

  The amount of moles of alcohol and carboxylic acid can be calculated from the densities and volumes of liquids added.Mass = density x volumethen

  Moles = mass / MrThe initial amount of moles of acid catalyst used is usually determined by titrating a separate sample of catalyst with sodium hydroxide