C4 - Chemical Changes

• Soluble Salt Required Practical:

  1. Add metal oxide/carbonate to a solution of warm acid until it is in excess (no more will react)
  2. Filter the excess metal oxide/carbonate to leave a solution of the salt
  3. Gently warm the salt solution so that the water evaporates and salt crystals are formed
• Oxidation


  A substance gains oxygen
• Reduction


  A substance loses oxygen
• Oxidation and reduction always occur together
• Metals reacting with oxygen


  Metals + oxygen → metal oxides
• Metal oxide reduction


  • lead(IV) oxide + carbon → lead + carbon dioxide
• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Reactivity series

  • Potassium, K
  • Sodium, Na
  • Lithium, Li
  • Calcium, Ca
  • Magnesium, Mg
  • Aluminium, Al
  • Carbon, C
  • Zinc, Zn
  • Iron, Fe
  • Tin, Sn
  • Lead, Pb
  • Hydrogen, H
  • Copper, Cu
  • Silver, Ag
  • Gold, Au
  • Platinum, Pt
• In a displacement reaction a more reactive metal will displace a less reactive metal from a solution of its salt
• Magnesium is more reactive than copper


  Magnesium will displace copper from a solution of copper sulfate
• Hydroxide ions, OH-(aq)


  Make solutions alkaline
• Hydrogen ions, H+(aq)


  Make solutions acidic
• pH scale


  Measure of the acidity or alkalinity of an aqueous solution
• A solution with a pH of 7 is neutral
• Aqueous solutions with a pH less than 7 are acidic
• Aqueous solutions with a pH of more than 7 are alkaline
• Strong acid


  Completely ionised (split up into ions) in water
• Weak acid


  Only partially ionised in water
• The pH of a solution is a measure of the concentration of H+ ions
• A pH decrease of one unit indicates that the concentration of hydrogen ions has increased by a factor of 10
• For a given concentration of acid, a strong acid will have a higher concentration of hydrogen ions and, therefore, a lower pH
• Electrolysis


  The use of an electrical current to break down compounds containing ions into their constituent elements
• Electrolyte


  The substance being broken down during electrolysis
• Anode


  Positively charged electrode
• Cathode


  Negatively charged electrode
• Electrolysis of lead bromide


  1. At the cathode: Pb2+ + 2e → Pb

  2. At the anode: 2Br- → Br2 + 2e
• Metals that are more reactive than carbon can be extracted from their ores using electrolysis
• Electrolysis requires lots of heat and electrical energy, making it an expensive process
• Electrolysis of a molten solution:

  •Cathode - the metal

  •Anode - the non-metal
• Electrolysis of an aqueous solution:

  •Cathode - whichever is less reactive out of hydrogen and the metal

  •Anode - oxygen, unless a halogen is present, when the halogen would be formed
• Acid + base -> salt + water
• Acid + metal oxide -> salt + water
• Acid + metal hydroxide -> salt + water
• Acid + metal carbonate -> salt + water + carbon dioxide
• Acid + metal -> salt + hydrogen
• Metal + water -> metal hydroxide + hydrogen
• Electrolysis Required Practical:

  1. Set up a beaker half-filled with the solution to be electrolysed
  2. Dip two inert electrodes into the solution and connect them to a power pack
  3. Run a current through the solution
  4. Observe the products formed at each electrode