C6 - electrolysis

• Solid ions electrolysis

  • solid ionic compounds cannot conduct electricity
  • Ions are locked in place and not free to move
  • No delocalised electrons
  • Strong electrostatic forces of attraction
• when can electrolysis take place

  • when an ionic compound is melted or dissolved in water, the forces of attraction are broken and the ions are free to move
  • these liquids and solutions can conduct electricity
  • called electrolytes
• electrolysis on molten compound

  • 2 electrodes made of conducting material
  • negative electrode cathode is attached to a negative terminal power pack
  • positive electrode anode is attached to positive terminal of power pack
  • positive ions are attracted to negative electrode and gains electrons from anode to form atoms (reduction)
  • negative ions are attracted to positive electrode to lose electrons to form an atom (oxidation)
• which metals in the reactivity series are extracted using electrolysis

  Metals more reactive than carbon
• electrolysis to extract aluminium

  • Aluminium oxide is mixed with cryolite to lower very high melting point (reduces energy needed and saves money)
  • Apply electric current to molten
  • Cathode and anode are made of graphite
  • Al$^{3+}$ions attracted to cathode each ion gains 3 electrons to form aluminum atom
  • O$^{2-}$are attracted to anode
  • Each oxide ion loses 2 electrons to form oxygen atom
• half equation for reaction at the anode in aluminum electrolysis

  o$^{2-}$ $\rightarrow$ O + 2e- (oxidation)
• why must the anode be replaced regularly (aluminium)

  • oxygen molecules produced at the anode react with graphite forming carbon dioxide
• why is electrolysis expensive

  • melting compound requires great deal of energy
  • alot of energy is needed for electric current
• electrolysis of sodium chloride

  • sodium ion (Na+) and hydrogen ion (H+) are attracted to cathode
  • sodium is more reactive so hydrogen gas is produced at cathode
  • chloride ion (Cl-) and hydroxide ion (OH-) are attracted to anode
  • at anode chlorine gas is produced as chloride ion is a halide
• when is hydrogen produced at the cathode

  if the metal is more reactive than hydrogen
• halide ions

  if aqueous solution contains halide ions then the halogen will be produced at the anode
• reactions in sodium chloride

  cathode - 2H+ + 2e- reduced to H2

  anode- 2Cl- - 2e- oxidises to Cl2
• electrolysis practical method

  1. Add about 50cm' of copper chloride solution to a beaker.
  2. Add the lid and insert electrodes through the holes making sure the electrodes don't touch.
  3. Attach crocodile leads to the electrode and connect the rods to the DC terminals of a low voltage power supply.
  4. Set the power supply to 4V and switch the power supply on.
  5. Using the forceps hold the litmus paper near the positive electrode
  6. After a few minutes turn the power supply off and observe the negative electrode.
  7. Record observations at the electrodes.
• electrolysis practical sodium chloride

  1. Add about 50cm' of copper chloride solution to a beaker.
  2. Add the lid and insert electrodes through the holes making sure the electrodes don't touch.
  3. Attach crocodile leads to the electrode and connect the rods to the DC terminals of a low voltage power supply.
  4. Set the power supply to 4V and switch the power supply on.
  5. Using the forceps hold the litmus paper near the positive electrode
  6. After a few minutes turn the power supply off and observe the negative electrode.
  7. Record observations at the electrodes.