2.6 Chem- Reversible reactions and important chemicals

• Reversible reaction


  A chemical reaction where the products can react to produce the original reactants
• Reversible reaction


  1. Forward reaction

  2. Backwards reaction
• The direction of a reversible reaction can be changed by changing the conditions
• If a reversible reaction is endothermic one way


  It is exothermic in the opposite direction
• The same amount of energy is transferred each way in a reversible reaction
• Haber process


  Used to manufacture ammonia, which is used to produce nitrogen-based fertilisers
• Haber process


  • Raw materials are nitrogen and hydrogen gas
  • Purified gases passed over an iron catalyst at high temperature (450°C) and high pressure (200 atm)
• Haber process


  1. Nitrogen and hydrogen react to form ammonia

  2. Ammonia breaks down again into nitrogen and hydrogen

  3. Ammonia liquefies and is removed

  4. Remaining nitrogen and hydrogen are recycled
• Dynamic equilibrium


  The forward and backward reactions keep going once equilibrium is reached
• Fewer moles of gas on the product side

  Increase pressure to move equilibrium to the right so more ammonia is produced
• Forward reaction is exothermic


  Low temperature would favour the forward reaction and mean more ammonia was produced
• The actual conditions used in the Haber process are a compromise between maximising reaction rate and the yield of ammonia
• Test for ammonia gas

  Ammonia gas is alkaline so will turn moist red litmus paper blue
• Test for ammonium ions

  1. Ammonium ions react with hydroxide ions to form water and ammonia gas

  2. Ammonia gas can then be detected with red litmus paper
• Nitrogenous fertilisers


  Made from neutralisation reactions with ammonia
• Production of ammonium sulfate

  1. Ammonia + sulfuric acid → ammonium sulfate

  2. Ammonium hydroxide + sulfuric acid → ammonium sulfate + water
• Production of ammonium nitrate


  1. Ammonia + nitric acid → ammonium nitrate

  2. Ammonium hydroxide + nitric acid → ammonium nitrate + water
• Advantages of fertilisers


  • Increases crop yields and growth
  • Increases profits for farmers
• Disadvantages of fertilisers


  • Can cause eutrophication
  • Many stages required in their manufacture
  • Changes the pH of the soil
  • Can cause baby blue syndrome
• Sulfuric acid


  A strong acid that completely dissociates to release H+ ions
• The contact process for manufacturing sulfuric acid


  1. Sulfur is burned in air to form sulfur dioxide

  2. Sulfur dioxide reacts with oxygen to form sulfur trioxide (reversible reaction)

  3. Sulfur trioxide is reacted with water to form sulfuric acid
• The contact process


  • Conditions required: Vanadium(V) oxide catalyst, 450°C and 2 atm pressure
• Uses of sulfuric acid


  • Production of fertilisers
  • Manufacture of chemicals
  • Petroleum refining
  • Processing metals
  • Rayon production
  • Lead-acid storage battery electrolyte
• Concentrated sulfuric acid as a dehydrating agent


  Removes water from other compounds
• Dehydration of sugar with concentrated sulfuric acid


  Concentrated sulfuric acid removes 6 water molecules per glucose molecule, releasing the water as steam and forming a black mass of carbon
• Dehydration of hydrated copper(II) sulfate with concentrated sulfuric acid


  Removes the water, forming anhydrous copper(II) sulfate which is a white powder