Topic 4 - chemical changes

• Reactions of acids with metals

  Acid + metal -> salt + hydrogen
• Redox reactions


  One substance is reduced and another substance is oxidised
• Identifying redox reactions

  Look at electrons gained and lost (following OIL RIG)
• Magnesium oxidation


  Mg -> Mg2+ + 2e-
• Hydrogen reduction


  2H+ + 2e- -> H2
• Neutralisation of acids and salt production

  Acid + alkali -> salt + water
• Neutralisation of acids by metal carbonates

  Acid + metal carbonate -> salt + water + carbon dioxide
• Salts produced from acids

  Depend on the acid used and the positive ions in the base/alkali/carbonate
• The charges on the positive ion from the base/alkali/carbonate and the negative ion from the acid must add up to zero
• Making soluble salts


  Add insoluble substance to acid, 2) Excess solid sinks when neutralised, 3) Filter and evaporate to crystallise
• pH scale


  Measures acidity or alkalinity of a solution, pH 7 is neutral, pH < 7 is acidic, pH > 7 is alkaline
• Neutralisation reaction

  H+(aq) + OH-(aq) -> H2O(l)
• Titration


  Wash burette, 2) Fill burette with acid, 3) Add alkali to flask, 4) Add indicator, 5) Add acid until end-point, 6) Repeat for precision
• Titration calculations


  1 dm3 = 1000 cm3, Moles = volume x concentration
• Strong acid


  Completely ionised in aqueous solution (e.g. HCl, HNO3, H2SO4)
• Electrolysis


  The process of breaking down a molten or dissolved ionic substance into its elements by passing an electric current through it
• Weak acid


  Partially ionised in aqueous solution (e.g. CH3COOH, citric acid, carbonic acid)
• The process of electrolysis

  1. Ionic substance is melted or dissolved

  2. Ions are free to move about

  3. Current is passed through the molten or dissolved substance

  4. Substance is broken down into its elements
• Cathode


  The negative electrode where positively charged ions move to and are discharged
• Strength of acid


  Lower pH, higher H+ concentration
• Anode


  The positive electrode where negatively charged ions move to and are discharged
• Electrolysis of molten ionic compounds

  1. Metal is produced at the cathode

  2. Non-metal is produced at the anode
• Strong/weak is not the same as concentrated/dilute
• Metals extracted by electrolysis

  • Metals more reactive than carbon
  • Large amounts of energy used to melt compounds and produce electrical current
• Metals extracted by electrolysis

  • Aluminium
• Aluminium extraction by electrolysis

  • Molten mixture of aluminium oxide and cryolite used
  • Carbon used as positive electrode (anode)
  • Positive electrodes need to be continually replaced
• Electrolysis of aqueous solutions


  1. Hydrogen produced at negative electrode unless metal less reactive than hydrogen

  2. Halide ion or oxygen produced at positive electrode depending on ions present
• Half equation


  Representation of reactions at electrodes, with small number same as 2 larger numbers, and electrons represented as e-
• Writing half equations for reactions at electrodes

  1. Negative electrode: Positive ion + e- -> Element (reduction)

  2. Positive electrode: Negative ion -> e- + Element (oxidation)
• Metal oxides


  Metals + oxygen -> metal oxides
• Oxidation


  Gain of oxygen
• Reduction


  Loss of oxygen
• Reactivity series


  • When metals react, metal atoms form positive ions
  • Reactivity of a metal is related to its tendency to form positive ions
  • Metals can be arranged in order of their reactivity
• Metals in order of reactivity

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
  • Zinc
  • Iron
  • Copper
• Reactions of metals with water

  • Potassium: violent
  • Sodium: very quick
  • Lithium: quick
  • Calcium: more slow
• Reactions of metals with dilute acid

  • Calcium: very quick
  • Magnesium: quick
  • Zinc: fairly slow
  • Iron: more slow
  • Copper: very slow
• Non-metals hydrogen and carbon are often included in the reactivity series
• Displacement


  A more reactive metal can displace a less reactive metal from a compound
• Gold is very unreactive and is found in the Earth as the metal itself
• Most metals are found as compounds that require chemical reactions to extract the metal