Chemical changes

• Metal oxides:

  • Metals + oxygen -> metal oxides
  • Known as oxidisation reactions because the metals gain oxygen
  • Reduction = loss of oxygen & oxidization = gain of oxygen [OIL RIG]
• Reactivity series:

  • When metals react with other substances, metal atoms for positive ions
  • The reactivity of a metal is related to its tendency to form positive ions
  • Metals can be arranged in order of reactivity in a reactivity series: Potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper can be put in order of their reactivity from their reactions with water and dilute acids
  • Non-metals hydrogen and carbon are often included in the reactivity series
  • A more reactive metal can displace a less reactive metal from a compound

  
• Reactivity series:

  
• Extractions of metals and reduction:

  Gold, since it is very unreactive, is found in the earth as the metal itself but most metals are found as compounds that require chemical reactions to extract the metal.

  Metals are less reactive than carbon can be extracted from their oxides by reduction with carbon.
• Reactions of acids with metals:

  Acids react with some metals to produce a salt and hydrogen:

  • Acid + metal -> salt + hydrogen

  These are redox reactions - this means that one substance is reduced and another substance is oxidised.

  You should be able to identify which substances are which by looking at electrons gained and lost.
• Neutralisation of acids and salt production:

  Alkalis and bases neutralise acids to produce salts and water

  • acid + alkali -> salt + water
  • acid + base -> salt + water

  Acids are neutralised by metal carbonates to produce salts, water and carbon dioxide

  • acid + metal carbonate -> salt + water + carbon dioxide

  The salt produced...

  • in alkali and base reactions depend on the acid used: HCl produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates
  • it also depends on the positive ions in the base, alkali, or carbonate
• Soluble salts:

  They can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides, or carbonates:

  1. Add the chosen solid insoluble substance to the acid then the solid will dissolve.
  2. You know the acid has been neutralised when excess solid sinks to the bottom, so keep adding until this happens.
  3. Filter out excess solids leaving the salt solution, then evaporate some water, then leave the rest to evaporate slowly.

  This is called crystallisation.
• Acids produce H+ ions in aqueous solutions.

  Alkalis produce OH- ions in aqueous solutions.
• How to carry out a titration:

  1. Wash the burette using dilute hydrochloric acids and then water.
  2. Fill the burette to 100cm3 with acid with the meiniscus' base on the 100cm3 line.
  3. Use a pipette to add 25cm3 of alkali into a conical flask, drawing alkali into the pipette using a pipette filler.
  4. Add a few drops of a suitable indicator to the conical flask.
  5. Add acid from the burette to alkali until the end-point is reacted.
  6. The titre is the difference between the first and second readings on the burette.
  7. Repeat the experiment and calculate the mean to gain more precise results.
• The process of electrolysis:

  When an ionic substance is melted or dissolves, the ions are free to move about within the liquid or solution.

  Passing a current through substances that are molten or solution means that the solution can be broken down into elements. This is electrolysis, and the substance being broken down is the electrolyte.

  During electrolysis, positively charged ions move to the negative electrode [cathode] and negatively charged ions move to the positive electrode [anode].

  Ions are discharged at the electrodes producing elements.
• Electrolysis of molten ionic compounds:

  When a simple ionic compound is elecrolysed in the molten state using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode.
• Using electrolysis to extract metals

  1. Metals more reactive than carbon are extracted by electrolysis of molten compounds

  2. Large amounts of energy are used to melt the compounds and produce electrical current
• Aluminium extraction


  Electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode [anode]
• Aluminium oxide has a very high melting point, so it is mixed with cryolite to lower the melting point and make it cheaper to extract
• Aluminium extraction process


  Anodes need to be continually replaced because oxygen is formed, which reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away
• Representations of reactions at electrodes as half equations:

  This is an example of a half equation; the small number is always the same as the 2 larger numbers within the equation and electrons are represented by the symbol e-

  Writing half equations for the reactions at each electrode:

  • Negative electrode: X+ -> X, so ionic equation must be: X+ + e- -> X, electrons gained, so positive, so positive ions are reduced
  • Positive electrode: X- -> X, so ionic equation must be: X- -> e- + X, electrons are lost, so negative ions are oxidised