AQA Chemistry Unit 2: 9 Equilibria

• Equilibrium mixture: The mixture formed when a reversible reaction proceeds in a closed container until no further change occurs. The backward and forward reactions are proceeding at the same rate

Setting up an Equilibrium

• Dynamic Equilibrium: Forward and Backward reactions are occurring at the same rate so the mixture composition doesn't change

Conditions

• Can only be reached in a closed system
• The final equilibrium position will be the same
• It is a dynamic process where the forwards and backwards reactions occur at the same time
• When the density, concentration, colour and pressure are the same equilibrium has been reached

Chemical Equilibria

• Equalibrium can be formed of any proportions of reactant and products
• Proportions can change due to conditions like temperature, pressure and concentration

The Equilibrium Mixture

• Changing the proportions of reactants to products is a way of being able to obtain a greater yield of product
• This is changing the position of equilibrium
• If the proportion of product is increased the equilibrium position shifts to the right
• If the proportion of reactants is increaased the equlibrium position shifts to the left

Le Chatelier's Principle

• If a system at equilibrium is disturbed, the equilibrium position moves to oppose it

Changing Concentrations

• If you increase the concentration of a reactant then the equilibrium position shifts to reduce this concentration
• The equilibrium position moves to the right increasing the yield of product

Temperature Increase

• Equilibrium position shifts to decrease the temperature
• The position will move in the endothermic direction
• It will shift to the right

Increase in Pressure

• The position shifts to decrease pressure
• Equilibrium position shifts from the higher moles to the lower moles side of the reaction
• For: CO + 2H2 -> CH3OH
• The right side has less pressure so it shifts to the right

Catalyst

• No effect on position of equilibrium
• Work by producing alternative route for reaction with lower activation energy
• They allow equilibrium to be reached quicker

• Raw Materials:
  • Nitrogen (from air), coal (to condense to liquid then for fractional distillation), water and natural gas
• Hydrogen: Achieved by H20 + CH4 -> CO + 3H2
• Conditions:
  •  20,000kPa and 670K (15% conversion to ammonia so gases are recycled back into the process)
  • Iron Catalyst (allows continuous flow of gases but becomes poisoned after 5 years)
• Reasons:
  • High temperature would give lower yield
  • Low temperature would have slow rate of reaction
  • Higher pressure is more dangerous and uses expensive equipment
  • Lower pressure gives low yield
• Uses:
  • 80% is used in fertiliser, rest makes nylon, explosives, drugs and dyes
• Equation:
  • N2(g) + 3H2(g) ->2NH3(g), -92Kjmol-

• Raw Materials:
  • Ethene from crude oil obtained by fractional distillation and cracking
  • Reactants are all gaseous
• Conditions:
  • Made by hydration of ethene using catalyst: phosphoric acid absorbed on silica
  • 570K and 6500kPa (give conversion of 5%, unreacted ethene is reused)
• Reasons:
  • Lower temperature reduces rate of reaction
  • High pressure causes ethene to polymerise to polyethene and increases the costs of the building
  • Too much steam dilutes the catalyst
• Uses:
• • Cosmetics, drugs, detergents, inks, motor fuel and alcohol
• Equations:
• • Fermentation: C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)
  • Ethene: H2C=CH2(g) + H2O(g) -> CH3CH2OH(g)
  • -46Kjmol-

• Raw Materials:
  • Excess hydrogen used to make more methanol
  • Carbon monoxide 
• Conditions:
  • 500K and 10000kPa (produces around 5-10% yield)
• Reasons:
  • Higher temperature used for faster rate of reaction
  • Lower pressure used to reduce building costs
• Uses:
  • Chemical feedstock (starting material for making other chemicals)
  • Manufacture of methanal which is used for plastics like Bakelite
  • Forms terylele and perspex or as a motor fuel
• Equation:
  • CO(g) + 3H2(g) -> CH3OH(g)
  • -91Kjmol-1