Electrochemistry: Electrolysis Principles

• Electrolysis


  When an electric current is passed through a molten ionic compound the compound decomposes or breaks down
• Covalent compounds


  • Cannot conduct electricity hence they do not undergo electrolysis
• Ionic compounds in the solid state

  • Cannot conduct electricity either since they have no free ions that can move and carry the charge
• Electrode


  A rod of metal or graphite through which an electric current flows into or out of an electrolyte
• Electrolyte


  The ionic compound in a molten or dissolved solution that conducts the electricity
• Anode


  The positive electrode of an electrolysis cell
• Anion


  A negatively charged ion which is attracted to the anode
• Cathode


  The negative electrode of an electrolysis cell
• Cation


  A positively charged ion which is attracted to the cathode
• Metals and hydrogen form positively charged ions and so either a metal or hydrogen gas is formed at the cathode
• Non-metals form negatively charged ions and so non-metals (except hydrogen) are formed at the anode
• PANIC mnemonic


  Positive (is) Anode Negative Is Cathode
• Charge transfer in electrolysis

  1. Power supply provides electrons to cathode

  2. Positive ions (cations) move towards cathode and gain electrons

  3. Negative ions (anions) move towards anode and lose electrons

  4. Electrons move from anode back to power supply
• Electrons are the charge carriers in the external circuit, ions are the charge carriers in the electrolyte
• Binary ionic compound


  A compound consisting of just two elements joined together by ionic bonding
• Electrolysis of molten binary ionic compounds

  1. Positive ion migrates to cathode and forms the metal

  2. Negative ion migrates to anode and forms the non-metal
• Electrodes need to be inert such as graphite or platinum so that they don't participate in a side reaction with the electrolyte
• Brine


  A concentrated solution of aqueous sodium chloride
• Electrolysis of aqueous sodium chloride

  1. Hydrogen gas forms at cathode

  2. Chlorine gas forms at anode

  3. Sodium hydroxide solution remains
• Electrolysis of dilute sulfuric acid

  1. Hydrogen gas forms at cathode

  2. Oxygen gas forms at anode
• If the gas produced at the anode relights a glowing splint, it is oxygen
• If the gas produced at the anode bleaches damp litmus paper, it is chlorine
• If the gas produced at the cathode burns with a 'pop' when lit, it is hydrogen
• Water dissociation


  H2O ⇌ H+ + OH-
• In the electrolysis of aqueous solutions, the water molecules dissociate producing H+ and OH- ions
• Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved
• Concentrated and dilute solutions of the same compound give different products
• Electrolysis of aqueous copper sulfate with graphite electrodes

  1. Copper ions are discharged at cathode, forming copper metal

  2. Hydroxide ions are discharged at anode, forming oxygen gas
• Electrolysis of aqueous copper sulfate with copper electrodes

  1. Copper atoms are oxidised at anode, forming copper ions

  2. Copper ions are reduced at cathode, forming copper atoms
• Products formed for common aqueous solutions


  • See table on page 16
• Oxidation


  A substance loses electrons
• Reduction


  A substance gains electrons
• Ionic half equations show the oxidation and reduction of the ions involved
• OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)