chemical changes - topic 4

• pH scale


  A measure of how acidic or alkaline a solution is
• pH


  The measure of the concentration of H+ ions
• Universal indicator


  A dye that changes colour depending on whether its above or below a certain pH
• Neutralisation Reactions


  H+ and OH- ions make water
• Ionic Equation: H+ + OH-
• Neutralisation of salt production and acids

  Acid + Alkali
• Naming the salt


  • Hydrochloric - Chloride
  • Sulfuric - Sulfate
  • Nitric - Nitrate
• Bases


  Chemicals which can neutralise acids
• Making soluble salts


  1. Pour acid into beaker, heat

  2. Add insoluble base, stir, ensure reaction

  3. Filter excess, add salt and water to evaporating basin

  4. Heat until half evaporated, leave to cool, dry crystals
• 3 main acids


  • Hydrochloric
  • Sulfuric
  • Nitric
• 3 main alkalis


  • Produce OH- ions in aqueous solutions
• Strong acids


  Fully ionise in water, have lower pH
• Strong Acids


  • Hydrochloric
  • Sulfuric
  • Nitric
• Weak acids


  Partially ionise in water
• Weak Acids


  • Carbonic
  • Citric
  • Ethanoic
• As the pH scale decreases by 1 unit


  The concentration of hydrogen ions increases by 10 times
• Concentration of acids


  The amount of acid molecules in a given volume of solution
• A dilute acid will have fewer acid molecules in a given volume than a concentrated acid even if the strength of the acid is the same
• Reactivity of metals


  • Determined by how easily the electron is lost
  • When metals react with acids or water they form positive ions
• Reactivity series of metals

  • Potassium
  • Sodium
  • Calcium
  • Magnesium
  • Aluminium
  • Zinc
  • Iron
  • Copper
  • Silver
• Reacting metals with acids

  Acid + metal
• The more reactive the metal


  The faster rate of reaction (bubble of hydrogen)
• Metal reacting with water

  Metal + water
• Reacting metals with acids - Practical

  1. Add acid to test tube

  2. Add metal, cover with thumb

  3. Test for hydrogen gas after 1 minute

  4. Record observations
• Displacement reactions


  When a more reactive metal displaces the less reactive metal in a compound
• Metal oxide


  Lots of metals react with oxygen to form oxides (oxidation)
• Formation of a metal ore

  • Oxidation: Gain of oxygen, loss of electrons
  • Reduction: Loss of oxygen, gain of electrons
• Carbon can only take oxygen away from metals which are less reactive
• Extracting metals by reduction with carbon

  Metal oxide + carbon
• REDOX Reactions


  O - Oxidation, I - Is, L - Loss, R - Reduction, I - Is, G - Gain
• Electrolysis


  Splitting up with electricity
• Electrolysis of molten compounds

  Metal produced at cathode, non-metal at anode
• Using electrolysis to extract metals

  • Metals more reactive than carbon are extracted by electrolysis of molten compounds
  • Large amounts of energy used to melt compounds and produce electrical current
  • Aluminium extracted by electrolysis of molten aluminium oxide and cryolite
• Electrolysis of aqueous solutions

  Ions discharged depend on reactivity - hydrogen produced at cathode unless metal more reactive, halide or oxygen produced at anode