electrolysis

• Electrolysis


  A process where electricity is used to make a chemical change happen that wouldn't happen otherwise
• Electrolysis


  1. Use electricity to break apart a compound into the elements that make it up

  2. Example: Sodium chloride broken apart into sodium metal and chlorine gas

  3. Example: Water broken apart into hydrogen gas and oxygen gas
• Chlorine is a diatomic element, so it always forms groups of two (Cl2) and never just single Cl atoms
• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Sodium chloride (table salt) does not spontaneously break apart into sodium and chlorine gas on its own
• Electrical energy from a battery is required to force the electrolysis process to happen
• Electrolytic cell


  • Contains: Container of molten/liquid sodium chloride, battery, electrodes
  • Anode: Site of oxidation, electrons pulled in
  • Cathode: Site of reduction, electrons pushed out
• Electrolysis of sodium chloride

  1. At cathode: Na+ ions gain electrons to become neutral Na atoms

  2. At anode: Cl- ions lose electrons to become Cl2 gas
• Half-reactions can be written to show the oxidation and reduction processes at the anode and cathode
• Electrolysis of water


  H2O broken apart into H2 gas and O2 gas
• Hydrogen and oxygen are diatomic elements, so they form H2 and O2 molecules
• Electrolysis of water also involves oxidation and reduction processes
• Electrolysis of water


  1. Pair up and form twos to get H2 and O2

  2. Oxidation-reduction process with electrons being transferred

  3. Determine oxidation numbers

  4. Hydrogen is +1 with nonmetals, oxygen is usually -2

  5. H2 and O2 have oxidation number 0
• Hydrogen's oxidation number


  Decreases, indicating reduction and gaining electrons
• Oxygen's oxidation number


  Increases, indicating oxidation and losing electrons
• Electrolysis of water is not a spontaneous process, requires electrical energy from a battery
• Electrolytic cell for electrolysis of water

  • Container with water, test tubes filled with water inverted, electrodes connected to battery, electrolyte (e.g. sulfuric acid) added
• Cathode


  Electrode connected to negative side of battery, site of reduction
• Anode

  Electrode connected to positive side of battery, site of oxidation
• Electrolysis of water

  1. Hydrogen gas produced at cathode

  2. Oxygen gas produced at anode

  3. Water level decreases as gases collect in test tubes
• Ratio of hydrogen gas to oxygen gas is 2:1
• Half-reaction for reduction of hydrogen


  2H2O + 2e- → H2 + 2OH-
• Half-reaction for oxidation of oxygen

  2H2O → O2 + 4H+ + 4e-
• Electrolysis of water involves using electrical energy to split water into hydrogen gas and oxygen gas, which would not happen spontaneously