Electricity and Chemistry

Electricity and Chemistry

Electrolysis Basics

  • Electrolysis is a chemical reaction caused by passing an electric current through a substance.
  • The substance that undergoes electrolysis is called the electrolyte.
  • The process of electrolysis involves flow of electrons, which cause oxidation at the anode (positive electrode) and reduction at the cathode (negative electrode).
  • In electrolysis, ions are discharged at electrodes leading to products.

Electrolysis of Molten Ionic Compounds

  • Electrolysis of molten ionic compounds leads to the breakdown of the compound into its elements.
  • Positive metal ions are attracted to the cathode where they gain electrons (reduction).
  • Negative non-metal ions are attracted to the anode where they lose electrons (oxidation).

Electrolysis of Aqueous Solutions

  • Electrolysis of aqueous solutions is slightly more complex because water can also be electrolysed.
  • At the cathode, either hydrogen is produced (if the other cation is more reactive than hydrogen) or a metal is produced (if the other cation is less reactive than hydrogen).
  • At the anode, either oxygen is produced (if the solution does not contain halide ions) or a halogen is produced (if the solution contains halide ions).

Electrode Equations

  • Electrode equations represent the reactions occurring at the electrodes during electrolysis.
  • At the cathode, the reduction half-reaction usually takes the form M^(n+) + n e^- -> M, or 2 H^+ + 2 e^- -> H_2.
  • At the anode, the oxidation half-reaction usually takes the form 2 Cl^- -> Cl_2 + 2 e^-, or 2 OH^- -> O_2 + 2 H_2O + 4 e^-.

Electroplating

  • Electrolysis is used in electroplating, where a thin layer of metal is deposited onto the surface of an object.
  • The object to be plated is made the cathode and the plating metal is made the anode in a solution containing a compound of the plating metal.

Quantitative Electrolysis

  • The quantity of product made by electrolysis depends on the number of moles of electrons that pass through the electrolyte.
  • This is summarised by Faraday’s First Law of Electrolysis: the amount of any substance deposited or dissolved at any electrode during electrolysis is directly proportional to the quantity of electricity passed.
  • One mole of electrons represents one Faraday.