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Electrolysis | AQA GCSE Chemistry Notes | Topic 4

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  • Oxidation is the gain of oxygen, reduction is the loss of oxygen.
  • Also, oxidation is the loss of electrons, reduction is the gain of electrons (OILRIG).
  • Reactivity series: potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen, copper.
  • A more reactive metal can displace a less reactive metal from a compound.
  • We want to extract metals but metals are found as compounds e.g. sodium oxide or copper oxide.
  • If the metal is less reactive than carbon, react with carbon.
  • e.g. copper oxide + carbon --> copper + carbon dioxide
  • If the metal is more reactive than carbon, use Electrolysis.
  • Molten: so the ions are free to move
  • Cryolite: lower the melting point
  • Heat: when it is molten, the ions are free to move
  • Graphite electrode: inert (unreactive), but can react with O2 to form CO2.
  • Opposite charges attract so positive ions move to the negative electrode and negative ions move to the positive electrode.
  • Half equations show the ions turning into the element.
  • Na+ + e- --> Na
  • Be careful if the element is diatomic; the element only exists in pairs e.g. Cl2, Br2, O2, N2,
  • Diatomic molecules: anything with ending -gen, plus anything in group 7 which are called halogens.
  • 2Cl- - 2e- --> Cl2
  • If it is electrolysis of an aqueous solution, then it is dissolved in water so there are H+ and OH- ions present (the ions of water).
  • E.g. aqueous sodium chloride: Na+, Cl-, H+, OH-
  • Na+ and H+ would both like to go to the negative electrode.
  • You pick whichever is less reactive.
  • Cl- and OH- would like to go to the positive electrode.
  • You pick the halogen, and if there isn't one, you pick oxygen.
  • Reactivity series: potassium, sodium, lithium, calcium, magnesium, CARBON, zinc, iron, tin, lead, HYDROGEN, copper.
  • Common Ions: NH4+, OH-, NO3-, SO42-, CO32-
  • Common Acids: HCl, HNO3. H2SO4.
  • Acids: low pH (closer to 0). Always contain H+ ions.
  • Alkalis or bases have a high pH (closer to 14).
  • Neutral: pH is 7.
  • Salts: when the H in the acid is replaced by the metal.
  • acid + metal --> salt + hydrogen
  • acid + metal oxide --> salt + water
  • acid + metal hydroxide --> salt + water
  • acid + metal carbonate --> salt + water + carbon dioxide
  • Soluble salts: react acids with solid insoluble bases until no more reacts, then filter off excess solid to produce solution.
  • Salt solutions can be crystallised to produce solid salts.
  • Neutralisation reactions: H+ + OH- --> H2O.
  • Strong acids: completely ionise in aqueous solution.
  • Weak acids: partially ionise in aqueous solution.
  • If pH decreases by one, the hydrogen ion concentration of multiplies by 10.

Topic 4 - Chemical Changes AQA GCSE Chemistry Notes for Triple Science Students Only

  • Titration calculations (see video)
  • Concentration = moles/ volume