electrolysis

• electrolysis means...


  splitting up with electricity
• electrolysis


  the process in which an electric current is used to produce a chemical reaction, such as the decomposition of water
• what happens during electrolysis?

  an electric current is passed through an electrolyte which causes the ions to move towards the electrodes where they react and the compound decomposes
• why is electrolysis expensive?

  a lot of energy is required to melt the ore and produce the required current
• how can metals be extracted from their ores using electrolysis?

  if a metal is too reactive to be reduced with carbon or reacts with carbon, then electrolysis can be used to extract, however it is very expensive as there is a lot of energy required
• how is a flow of charge through the electrolyte created in electrolysis?

  ions travelling to the electrodes creates a flow of charge
• as ions gain or lose electrons...

  they form the uncharged element and are discharged from the electrolyte
• electrolysis of molten ionic solids form...

  elements
• why can't ionic solids be electrolysed?

  because the ions are in fixed positions and cannot move
• why can molten ionic compounds be electrolysed?

  because the ions can move freely and conduct electricity
• molten ionic compounds are always...

  broken up into their elements
• electrolytes


  a substance that conducts electricity through the movement of ions, but not through the movement of electrons
• electrodes should be made of...

  an inert material, so they don't react with the electrolyte
• metals form ... ions

  positive
• non-metals form ... ions

  negative
• positive metal ions in the electrolyte will...

  move towards the cathode and gain electrons (get reduced)

  - Pb2+ + 2e- → Pb
• negative non-metal ions in the electrolyte will...

  move towards the anode and lose electrons (get oxidised)
• at the negative electrode:

  metals form positive ions so they're attracted to the negative electrode

  - this is reduction (gain of electrons)

  - Al3+ + 3e- → Al
• at the positive electrode:

  non-metals form negative ions, so they're attracted to the positive electrode

  - this is oxidation (loss of electrons)

  - 2O2- → O2 + 4e-
• negative and positive electrode overall equation


  2Al2O3(l) → 4Al(l) + 3O2(g)
• why must the anode be replaced regularly in the electrolysis of aluminium oxide?

  the oxygen molecules produced at the anode react with the graphite (carbon), forming carbon dioxide gas which wears the electrode away

  - C + O2 → CO2
• what is formed at the anode in the electrolysis of aluminium oxide?

  oxygen

  - if the electrodes are made of graphite, then carbon dioxide will be formed as the oxygen will react with the carbon to form carbon dioxide
• what is formed at the cathode in the electrolysis of aluminium oxide?


  aluminium
• what is cryolite used for in the electrolysis of aluminium oxide?


  cryolite is mixed with aluminium oxide to reduce the melting point
• cathode


  the negative electrode at which reduction occurs
• cathode reactions


  positively charged ions gain electrons at the cathode

  - Na+ + e- → Na

  - Pb2+ + 2e- → Pb

  - 2H+ + 2e- → H2
• what is formed at the cathode?

  metals (or hydrogen)

  - because metal ions are positive, they are attracted to the cathode
• anode


  the positive electrode at which oxidation occurs
• anode reactions


  negatively charged ions lose electrons at the anode

  - 2Cl- → Cl2 + 2e-

  - 2O2- → O2 + 4e-
• what is formed at the anode?

  non-metals (other than hydrogen)

  - because non-metal ions are negative, they are attracted to the anode
• metals can be extracted from their ores using...

  electrolysis - if the metal is too reactive to be reduced with carbon
• using electrolysis to extract metals is...

  very expensive as it requires a lot of energy
• how do electrons flow in electrolysis?

  from anode to cathode
• redox reaction


  a chemical reaction involving the transfer of one or more electrons from one reactant to another; also called oxidation-reduction reaction, where both oxidation and reduction are taking place
• if electrons are transferred, it's a ... reaction

  redox
• it may be easier to discharge ions from water than a...

  solute
• electrolysis of aqueous solutions

  - the ions discharge when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved.

  - at the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen

  - at the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced

  - this happens because in the aqueous solution water molecules break down producing hydrogen ions and hydroxide ions that are discharged
• in aqueous solutions, as well as the ions from the ionic compound, there will be...

  hydrogen ions (H+) and hydroxide ions (OH-) from the water

  - H2O(l) ⇌ H+(aq) + OH-(aq)
• which ions are discharged at the electrodes when the solution is electrolysed will depend on...

  the relative reactivity of all the ions in the solution
• what happens at the cathode in electrolysis of aqueous solutions?

  - if H+ ions and metal ions are present, hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen

  - if metal ions form an elemental metal that is less reactive than hydrogen, a solid layer of pure metal will be produced instead, which will coat the cathode