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C6 Electrolysis (using electricity to break down ionic compounds…

- - - - metal is a positive ion
      - non-metal is a negative ion
  - - - prevents short circuit
    - - graphite
      - sometimes platinum
      - does not react with electrolyte or products
    - - Positive Anode Negative is Cathode
      - Anode
        
        positive electode
        
        connected to +ve power supply terminal
      - Cathode
        
        negative electrode
        
        connected to -ve power supply terminal
  - - - gas may be given off
      - metal may be deposited
      - ions gain/lose the no. of electrons to cancel out all their charge
    - - reduction
    - - oxidation
      - some non-metal atoms combine to form molecules
    - - Half equations can represent the changes at each electrode
  - - - e.g. Acids always form H⁺ and -ve ions in an aqueous solution
- - - - ions in fixed position in lattice
      - does not conduct electricity
  - - - liquid at temperature in evaporating dish
      - (l)
    - - solid metal coats the -ve electrode if temperature is lower than melting point
      - melts if the temperature of electrolyte or cell reaches above its melting point
      - (s/l)
    - - a gas may be given off at the +ve electrode
        
        (g)
  - - - must be extracted using electrolysis
    - - purified to separate aluminium oxide
    - - aluminium oxide has to be melted
        
        aluminium oxide melts at 2050°C
        
        requires too much energy
        
        aluminium oxide is mixed with molten cryolite
        
        cryolite is an ionic compound
        
        to lower the melting point of the mixture to 850°C
        
        lots of electricity is used to keep the mixture molten
        
        an aluminium plant uses same amount of electricity as a small town
      - 2Al₂O₃(l) → 4Al(l) + 3O₂(g)
        
        Oxygen is produced at the anode
        
        oxygen ions lose 2 electrons and are oxidised
        
        2O²⁻(l) - 4e⁻ → O₂(g)
        
        oxygen atoms bond in pairs to form molecules
        
        oxygen reacts with the hot carbon anodes
        
        forms carbon dioxide gas
        
        emitted from anodes
        
        C(s) + O₂(g) → CO₂(g)
        
        carbon anodes gradually burn away
        
        needs to be replaced regularly
        
        Aluminium is formed at the cathode
        
        aluminium ions gains 3 electrons and are reduced
        
        Al³⁺(l) + 3e⁻ → Al(l)
        
        molten aluminium is collects at the bottom of the cell
        
        tapped or siphoned off from cell
    - - the lining of the cell is a carbon negative electrode (cathode)
      - positive carbon electrodes (anodes) in the electrolyte
- - - - Anode
        
        negative non-metal ions and OH⁻ ions are attracted
        
        order of discharge
        
        halide (halogen) ions take priority
        
        then hydroxide (OH⁻) ions
        
        lastly, any other negative ions
        
        if no halide ions present, oxygen and water are produced from OH⁻ ions
        
        4OH⁻(aq) - 4e⁻ → O₂(g) + 2H₂O(l)
      - Cathode
        
        positive metal ions and H⁺ ions are attracted
        
        the less reactive element will form
        
        if hydrogen is less reactive than the metal, hydrogen gas is produced
        
        2H⁺(aq) + 2e⁻ → H₂(g)
  - - - contains Na⁺, Cl⁻, H⁺ and OH⁻ ions
    - - NaCl(aq) → H₂(g) + Cl₂(g) + NaOH(aq)
      - Hydrogen gas
      - Chlorine gas
      - Sodium hydroxide solution
    - - chloride ions (Cl⁻) and hydroxide ions (OH⁻) are attracted to anode
      - chloride ions are halide ions so they are discharged
        
        chloride ions lose 1 electrons and are oxidised
        
        2Cl⁻(aq) - 2e⁻ → Cl₂(g)
        
        2 chlorine atoms bond to form a molecule
        
        chlorine gas given off at anode
    - - hydrogen ions (H⁺) and sodium ions (Na⁺) are attracted to cathode
      - hydrogen is less reactive so hydrogen ions are discharged
        
        hydrogen ions gain 1 electron and are reduced
        
        2H⁺(aq) +2e⁻ → H₂(g)
        
        2 hydrogen atoms bond to form a molecule
        
        hydrogen gas given off at cathode
    - - sodium ions (Na⁺) and hydroxide ions (OH⁻) are left in the solution
        
        an alkaline solution of sodium hydroxide (NaOH) is left behind
        
        can be tested using an acid/base indicator
        
        alkaline because it contains OH⁻ ions
  - - - chlorine bleaches damp blue litmus paper
      - a metal may coat the cathode
      - hydrogen 'pops' with light splint
      - oxygen relights glowing splint