Electrolysis & Redox Reactions

• The process of electrolysis

  • The process of breaking down an ionic substance using electricity.
• The electrolysis of molten Aluminium Oxide :

  • Breaks aluminium oxide down into aluminium and oxygen
• Electrolyte :

  • A ionic substance that is MOLTEN or DISSOLVED IN WATER that can conduct electricity
  • They contain free ions, free to move and carry electrical charge through the liquid.
• Example of electrolyte :

  • molten sodium chloride
  • sodium chloride solution (aqueous)
• General method of electrolysis :

  • Negative ions / anions are attracted to the anode.
  • Negative ions are discharged at the anode and lose electrons to form an atom / molecule.
  • Positive ions / cations are attracted to the cathode.
  • Positive ions gain electrons to form an atom / molecule

  
• Rule for electrolysis

  • Opposites attract
• Negative ions are attracted to :

  • Positive electrode, anode
• Positive ions are attracted to :

  • cathode, negative electrode
• Electrodes should be :

  • Inert (graphite, platinum)
  • Good conductors of electricity
• Redox reactions + Electrolysis 


  • involves oxidation and reduction
  • Oxidation occurs at the positive electrode as the negative ions lose electrons.
  • Reduction occurs at the negative electrode as the positive ions gain electrons.
• OILRIG

  Oxidation

  Is

  Loss

  Reduction

  Is

  Gain

  (of electrons)
• Electrolysis of molten ionic substances

  • Give neutral metal and non-metallic elements.
  • Metal ions are attracted to cathode, are reduced and become neutral.
  • Non-metal ions are attracted to anode, are oxidised and become neutral.
• Electrolysis of lead bromide

  Oxidation = Bromide ions are attracted to positive electrode / anode. They are oxidised / lose / discharge an electron and form the bromine molecule.

  Reduction = Lead ions are attracted to the negative electrode. They are reduced and form a lead atom.

  
• A teacher demonstrates the electrolysis of molten lead bromide + zinc chloride.

  State the products at the negative and positive electrodes.

  Molten lead bromide

  1. negative electrode = lead atoms
  2. positive electrode = bromine molecule

  Zinc chloride

  1. negative electrode = zinc atom
  2. positive electrode = chlorine molecule
• Observation if metal produced at the negative electrode :

  • Solid produced
• Observation if gas produced at the positive electrode :

  • Bubbles
  • Fizzing
• Half equation of Lead at negative electrode

  • Lead gains 2 electrons so :

  Pb^2+ + 2e- --> Pb
• Half equation of bromide ions at positive electrode :

  • Bromide ion loses one electron at anode so :

  2Br- --> Br2 + 2e-
• Opposite charges

  • attract
  • electrodes attract ions of different charges
• Using electrolysis to extract metals

  • Metals can be extracted from molten compounds using electrolysis.
• When is electrolysis used?

  • If the metal is too reactive to be extracted by reduction with carbon.
  • If the metal reacts with carbon.
• Extraction process of electrolysis requires :

  • Large amounts of energy
  • To melt the compounds and produce an electric current
  • Very expensive
• Extracting aluminium by electrolysis :

  • Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide.
• Extracting aluminium by electrolysis : Positive electrode


  • Negatively charged oxygen ions are attracted to the positive electrode because opposite charges attract.
  • Oxygen ions each lose two electrons / oxidised.
  • Neutral oxygen atoms produced and combine to form oxygen molecule.
• Extracting aluminium by electrolysis : Negative electrode

  • Positively charged aluminium ions are attracted to the negative electrode because opposite charges attract.
  • Aluminium ions gain 3 electrons and produces aluminium atoms. Solid is produced at the negative electrode.
• Why is molten cryolite added to aluminium oxide?

  • Cryolite significantly lowers melting point of aluminium oxide.
  • Makes the process cheaper and easier.
• Extracting aluminium by electrolysis : Half equation at positive electrode

  2O2- --> O2 + 4e-
• Extracting aluminium by electrolysis : Half equation at negative electrode

  Al3+ + 3e- --> Al
• Explain why a mixture is used as the electrolyte

  1. Mixture has a lower melting point
  2. So less energy is needed
• Explain why the positive electrode must be continually replaced.

  1. The electrode reacts with oxygen.
  2. The electrode is carbon / graphite.
  3. So carbon dioxide is produced.
  4. Graphite is used up.
• Electrolysis of aqueous solutions

  • In aqueous solutions, there will be some hydrogen ions and hydroxide ions dissolved in the solution.
• When is hydrogen produced at the negative electrode?

  • Hydrogen is only produced at the negative electrode if the metal is more reactive than hydrogen.
  • For example, molten potassium chloride will produce hydrogen at the negative electrode.
• When is hydrogen not produced at the negative electrode?

  • If the metal is less reactive than hydrogen, the metal itself will form at the negative electrode.
  • For example, copper chloride will produce copper metal at the negative electrode.
• When is oxygen ever produced at the positive electrode?

  • If there are no halide ions present (e.g Cl-)
  • Only hydroxide ions are present in the solution
• In aqueous solutions, water molecules..

  Break down producing hydrogen and hydroxide ions that are discharged.
• Difference between electrolysis of MOLTEN ionic compounds and AQUEOUS solutions

  • Aqueous solutions can produce hydrogen or oxygen AS WELL.
  • Molten ionic compounds will just produce the constituent metals and non-metal atoms.
• Describe the products at the positive electrode and negative electrode of an aqueous solution of copper sulfate.

  Aqueous solution SO contains : hydrogen ions, hydroxide ions, copper ions, sulfate ions.

  Negative electrode : Copper metal, so red solid of copper is produced.

  Positive electrode : Oxygen is produced, bubbling
• Describe the products at negative electrode and positive electrode of an aqueous solution of sodium chloride

  IONS present : hydroxide, hydrogen, sodium, chloride

  Negative electrode : Will produce hydrogen gas, bubbles

  Positive electrode : WIll produce chlorine gas molecule
• Half equation when hydrogen is produced at negative electrode

  2H+ + 2e- --> H2
• Half equation when oxygen is produced at positive electrode

  4OH- --> 2O2 + 2H2O + 4e-