C4 - Chemical changes

• Oxidation - When a substance gains oxygen

  Reduction - When a substance loses oxygen
• The reactivity series of metals shows the metals in order of their reactivity

  Metals above H2 in reactivity series react with acid to produce H2

  The more reactive the metal is, the quicker and more violent reaction with acid occurs

  Metals below H2 don’t react with acids

  Not all metals above H2 react with water - mostly Group I and II metals

  Aluminium is the borderline case
• A displacement reaction is a reaction where a more reactive metal displaces a less reactive metal from a compound
• Unreactive metals are found in their natural state
• Metals less reactive than carbon can be extracted by reduction with carbon

  Carbon displaces the metal in a metal oxide, gets oxidised to carbon oxides

  Metal from the metal oxide gets reduced to the pure metal
• Metals more reactive than carbon are extracted by electrolysis
• Oxidation – loss of electrons

  Reduction – gain of electrons
• General equation for a reaction between metals and acids:

  Metal + acid → salt + hydrogen

  Type of reaction: Redox reaction, also a displacement reaction
• Metals above hydrogen in the reactivity series will react with acid
• General equation for a neutralisation reaction:

  Base + acid → salt + water
• General equation for the reaction between metal carbonate and acid:

  Metal carbonate + acid → salt + water + carbon dioxide
• General equation for the reaction between metal oxides and acids:

  Metal oxide + acid → a salt + water
• A redox reaction is a reaction where both oxidation and reduction occurs
• Magnesium has lost electrons and thus has been oxidised (Mg to Mg2+)

  The hydrogen in HCl has gained electrons and thus has been reduced (H+ to H2)
• How a soluble salt is formed:

  1. React the excess acid with some insoluble chemical (e.g. metal oxide)
  2. Filter off the leftovers
  3. Crystallise the product
• Acids produce hydrogen ions, alkalis produce hydroxide ions in aqueous solutions
• Bases are compounds that neutralise acids

  Acids produce hydrogen ions in aqueous solutions

  Alkalis are soluble bases - produce hydroxide ions in aqueous solutions
• The pH scale is a measure of acidity/alkalinity of a solution

  A pH of 7 shows a neutral solution
• General equation for a neutralisation reaction in a short, ionic form
• General equation for a neutralization reaction in a short, ionic form:

  H⁺ + OH⁻ → H₂O
• Strong acid is completely ionized in aqueous solution; weak acid is only partially ionized in aqueous solution
• As the concentration of H⁺ increases, the pH decreases
• Concentrated acid has more moles of acid per unit volume than dilute acid

  • Concentration is not the same as strength of an acid
  • Strength refers to whether the acid is completely ionized in water (strong) or only partially (weak)
• As the pH is decreased by one unit, the hydrogen ion concentration increases by a factor of 10
• Name of salts:

  • LiNO₃: Lithium nitrate
  • K₂CO₃: Potassium carbonate
  • MgBr₂: Magnesium bromide
  • BaSO₄: Barium sulfate
• Electrolysis is the passing of an electric current through ionic substances that are molten or in solution to break them down into elements; ions are discharged at electrodes to produce these
• Electrolyte is the liquid/solution which conducts electricity
• Cathode is the negative electrode, anode is the positive electrode
• During electrolysis:

  • Reduction occurs at the cathode
  • Oxidation occurs at the anode
• In aqueous electrolysis, the less reactive element discharges at the cathode. Oxygen is produced unless there is a less reactive metal, in which case the said metal is produced. Oxygen is produced at the anode unless the solution contains halide ions, in which case halogen molecules are produced
• Aluminium is manufactured through the electrolysis of aluminium oxide and cryolite. Lots of energy is needed to produce the current in electrolysis which makes this process expensive
• Half equations in the extraction of aluminium:

  • Al³⁺ + 3e⁻ → Al (cathode)
  • 2O²⁻ → O₂ + 4e⁻ (anode)
  • Oxygen reacts with C of the anode producing CO₂
• Cryolite is used in the manufacturing of aluminium to lower the melting point of aluminium oxide, reducing energy costs
• Half equations in electrolysis of the aqueous Na₂SO₄:

  • 2H⁺ + 2e⁻ → H₂ (cathode)
  • 4OH⁻ → 2H₂O + O₂ + 4e⁻ (anode)
• Half equations in electrolysis of the molten and aqueous KCl:

  • K⁺ + e⁻ → K (cathode)
  • 2Cl⁻ → Cl₂ + 2e⁻ (anode)
  • 2H⁺ + 2e⁻ → H₂ (cathode)
  • 2Cl⁻ → Cl₂ + 2e⁻ (anode)
• Half equations in electrolysis of the aqueous CuBr₂:

  • Cu²⁺ + 2e⁻ → Cu (cathode)
  • 2Br⁻ → Br₂ + 2e⁻ (anode)