Topic 4: Chemical changes

• Oxidation


  When a substance gains oxygen
• Reduction


  When a substance loses oxygen
• Reactivity of metals with water

  • Potassium, sodium and lithium react very rapidly
  • Calcium reacts quite rapidly
  • Magnesium, zinc, iron and copper do not react
• Reactivity of metals with acids

  • Potassium, sodium and lithium react dangerously fast
  • Calcium reacts extremely vigorously
  • Magnesium reacts rapidly
  • Zinc reacts quite rapidly
  • Iron reacts quite slowly
  • Copper does not react
• Reactivity series


  • Potassium
  • Lithium
  • Calcium
  • Magnesium
  • Carbon
  • Zinc
  • Iron
  • Hydrogen
  • Copper
• Acids


  Produce hydrogen ions (H+) when dissolved in aqueous solutions
• Bases


  Chemicals which can neutralise acids
• Neutralisation


  When an acid and alkali react, the hydrogen ions react with the hydroxide ions to produce water
• When acids react with alkalis, a salt and water is formed
• All acids contain hydrogen
• Metals more reactive than hydrogen can displace hydrogen from acids
• When acid reacts with a metal, a salt and hydrogen is produced
• Hydrochloric acid


  Produces salts ending with chloride
• Sulfuric acid


  Produces salts ending with sulfate
• Magnesium reacts very rapidly with acids


  It is quite a lot more reactive than hydrogen so it easily forms magnesium ions (Mg2+)
• Zinc reacts quite rapidly with acids


  It is more reactive than hydrogen so it quite easily forms zinc ions (Zn2+)
• Iron reacts fairly slowly with acids


  It is only slightly more reactive than hydrogen so it less easily forms iron ions (Fe2+)
• Salts

  Contain a positive ion which comes from the base or alkali, and a negative ion which comes from the acid
• When acids react with a metal carbonate, a salt, water and carbon dioxide is produced
• Acid molecules in aqueous solutions

  Ionise or dissociate and release H+ ions
• Strong acids


  Fully ionise or dissociate in aqueous solutions
• Strong acids


  • Hydrochloric, sulfuric and nitric acids
• Weak acids

  Only partially ionise or dissociate in aqueous solutions
• Weak acids


  • Carbonic, ethanoic, and citric acids
• pH scale


  Gives an idea of the concentration of H+ ions produced by an acid
• Strong acids have a lower pH


  They fully ionise or dissociate in aqueous solutions, so they produce a greater concentration of H+ ions
• As the pH scale decreases by 1 unit


  The concentration of H+ ions increases by 10 times
• Concentration of an acid

  Tells the amount of acid molecules in a given volume on a solution
• Dilute acid


  Has fewer acid molecules in a given volume than a concentrated acid
• Solid ionic compounds


  Cannot conduct electricity because the ions are in fixed positions and cannot move
• Melted or dissolved ionic compounds


  Can conduct electricity because the strong forces of attraction are broken and the ions are free to move
• Anode


  Positive electrode in electrolysis
• Cathode


  Negative electrode in electrolysis
• Electrolysis


  1. Positive metal ions are attracted to the cathode

  2. Negative non-metal ions are attracted to the anode

  3. At the cathode, positive metal ions gain electrons to form atoms of that metal (reduction)

  4. At the anode, negative metal ions lose electrons to form atoms of that non-metal (oxidation)
• Metals more reactive than carbon are extracted using electrolysis
• Aluminium oxide


  Formula is Al2O3, has a very high melting point around 2000 degrees celsius
• In the electrolysis of molten aluminium oxide, it is mixed with cryolite which lowers its melting point</b>
• The cathode and anode in electrolysis of aluminium oxide are both made of graphite</b>
• Electrolysis of aluminium oxide

  1. Al3+ ions are attracted to the cathode

  2. O2- ions are attracted to the anode

  3. At the cathode: Al3+ + 3e- → Al (reduction)

  4. At the anode: 2O2- → O2 + 4e- (oxidation)

  5. The anode must be replaced regularly because the oxygen molecules react with the graphite, forming carbon dioxide gas
• Electrolysis is very expensive, because melting the compounds requires a lot of energy, and a lot of energy is required to produce the electric current