ELECTROLYSIS

• The Process of Electrolysis

  1. When an electric current is passed through a molten ionic compound the compound decomposes or breaks down

  2. The process also occurs for aqueous solutions of ionic compounds

  
• Electrolytes


  Liquids and solutions that are able to conduct electricity
• Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
• Electrolytic cell


  The set-up used in electrolysis consisting of electrodes, electrolyte, anode, anion, cathode, cation
• Ionic compounds in the solid state cannot conduct electricity since they have no free ions that can move and carry the charge
• The ions must be able to move and can only do so in the molten state or when dissolved in a solution, usually aqueous
• Electrolysis


  Ions in the solution start to move towards the electrodes when the cell is turned on and an electric current is passed through an electrolyte

  
• Particles in ionic compounds are in fixed position in the solid state but can move around when molten or in solution

  
• Electron flow in electrochemistry

  Electrons move from the power supply towards the cathode

  
• Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode

  
• Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode

  
• When a metal conducts it is the electrons that are moving through the metal. When a salt solution conducts it is the ions in the solution that move towards the electrodes while carrying the electrons.
• Binary ionic compound


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

  1. The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode

  2. The cathode product will always be the metal and the product formed at the anode will always be 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.
• Aqueous solutions


  Always have water (H2O)
• Electrolysis of aqueous solutions

  1. Water molecules dissociate producing H+ and OH- ions

  2. The ions from the compound plus the ions from the water are involved in the process

  3. Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved

  4. Concentrated and dilute solutions of the same compound give different products

  
• Positive Electrode (anode)


  1. Negatively charged OH- ions and non-metal ions are attracted to the positive electrode

  2. If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen

  3. If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen
• Negative Electrode (cathode)


  1. Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons

  2. Either hydrogen gas or the metal will be produced

  3. If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode

  
• Inert electrodes


  Graphite or other inert materials that don't take part in the electrolysis process
• Electrolysis using inert electrodes

  1. Set up the apparatus

  2. Add the aqueous solution

  3. Add two graphite rods as the electrodes and connect to a power pack or battery

  4. Turn on the power and allow electrolysis to take place

  5. Record the results and repeat for another solution
• Non-inert electrodes


  Electrodes that take part in the electrolysis reactions, used for electroplating and purifying copper
• Purifying copper


  1. The anode is impure copper ore, the cathode is pure copper, the electrolyte is copper(II) sulfate solution

  2. At the anode, copper atoms lose electrons to become ions in the electrolyte

  3. At the cathode, copper ions from the electrolyte gain electrons to form copper atoms

  
• Electroplating


  1. The cathode is the object to be coated, the anode is the metal to coat it with, the electrolyte contains ions of the coating metal

  2. At the anode, metal atoms lose electrons to form ions in the electrolyte

  3. At the cathode, metal ions from the electrolyte gain electrons and are discharged on the surface of the metal that needs coating
• The key difference between inert and non-inert electrodes is that non-inert electrodes take part in the process, inert ones don't.
• You only need to know the half equations for purifying copper if you are higher tier!
• Oxidation


  A substance loses electrons
• Reduction


  A substance gains electrons
• Reactions at electrodes


  1. At the anode, negatively charged ions lose electrons and are oxidised

  2. At the cathode, positively charged ions gain electrons and are reduced
• During electrolysis, use the charge on the ion to help remember whether it loses or gains electrons to become neutral:</b>

  Positive (+) ions gain (+) electrons

  Negative (-) ions lose (-) electrons
• Half equations


  1. Used to demonstrate the oxidation and reduction reactions at each electrode

  2. The number of atoms/ions on each side must be balanced, as well as the charges

  
• Don't forget to make sure the charges on the ions and electrons are balanced.
• Electrolysis of molten lead(II) bromide

  1. Lead ions are discharged at the cathode, gain electrons (reduction) to produce the metal

  2. Bromide ions are discharged at the anode, lose electrons (oxidation) to produce bromine gas
• Lead ion half equation

  Pb2+ + 2e- -> Pb
• Bromide ion half equation

  2Br- - 2e- -> Br2
• Electrolysis of molten aluminium oxide

  1. Aluminium ions are discharged at the cathode, gain electrons (reduction) and the aluminium is collected at the bottom of the cell

  2. Oxide ions are discharged at the anode, lose electrons (oxidation) and oxygen gas is produced
• Aluminium ion half equation

  Al3+ + 3e- -> Al
• Oxide ion half equation

  2O2- - 4e- -> O2
• Metals above hydrogen in the reactivity series

  iron

  zinc

  carbon

  aluminium

  magnesium

  calcium

  lithium

  sodium

  potassium
• metals below hydrogen in the reactivity series

  copper

  silver

  gold