Electrochemistry

• Electrolysis is the process of using electricity to break down or decompose a compound
• Electrolysis takes place in an electrolytic cell which converts electrical energy into chemical energy
• 1. Power supply (drives the movement of charges around a circuit)
  2. electrodes (which are connected to opposite ends of power supply)
  3. Electrolyte (solution where electrodes are immersed)

  
• Metal strips must be cleaned with sandpaper to remove any oxides formed on the surface of metal
• • battery is the power supply as it causes electrons to move from anode to cathode
  • electrons enter battery from anode and leave from cathode
• • electrolyte is an electrically conductive substance in the molten or aqueous state
  • conducts electricity due to mobile ions
  • e.g dilute sulfuric acid, copper(II) sulfate solution, molten sodium chloride
• • electrodes contains delocalised mobile electrons to conduct electricity
  • anode is the positive electrode connected to positive terminal of power source
  • cathode is the negative electrode connected to negative terminal of power source
• electrodes must be electrical conductors that conduct electricity through the movement of delocalised mobile electrons
• inert electrodes do not undergo chemical changes and do not take part in electrolysis reaction

  e.g graphite and platinum
• Metal anodes in reactive electrodes undergo oxidation during the electrolysis reaction

  e.g copper, silver
• charges are carried through the external circuit by electrons and through the electrolyte by ions

  
• At the anode:

  • anions are attracted to the positively-charged anode
  • anions can lose electrons to the anode and become oxidised
• At the cathode:

  • cations are attracted to the negatively charged anode
  • cations gain electrons to the cathode and become reduced
• when anions and cations are oxidised/reduced, they from atoms or molecules
• only anions and cations are discharged at the electrodes
• in solid state, ions in electrolytes are held in fixed positions and are immobile
• in the molten and aqueous state. the mobile ions in electrolyte enable the electrolyte to conduct electricity
• ionic compound must be aqueous or molten before it can be an electrolyte and conduct electricity
• Electrical conductors

  • conduct electricity by flow of delocalised mobile electrons from one end of conductor to the other end
  • substance remains chemically unchanged when an electric current flow through
• electrolytic conductors (electrolytes)

  • conducts electricity by flow of mobile ions
  • substance are broken down to form new substances when an electric current flow through it
• electrolytes undergo redox reactions at the electrodes to form new substances
• a molten binary ionic compound is typically a salt containing only one cation and one anion in the liquid state
• Examples of molten binary ionic compound

  1. Sodium chloride
  2. magnesium bromide
  3. aluminium oxide
• the number of electrons leaving the electrolyte via the anode is always equal to the number of electrons entering the electrolyte from the cathode
• Advantages of graphite

  • high melting point
  • will not melt

  Disadvantages

  • will react with oxygen gas under high temperatures to produce carbon dioxide
  • graphite anodes might have to be periodically replaced
• Advantage of platinum

  1. does not take part in the electrolysis reaction

  Disadvantages

  1. lower melting point than graphite
  2. might melt when used in electrolysis of molten binary ionic compound
  3. mainly used in the electrolysis of aqueous electrolytes
• Selective discharge of cations

  • practically impossible to discharge metals that are above hydrogen
  • the more reactive the metal, the more stable its ion and the harder it is to convert the ion back to metal

  
• selective discharge of anions

  • affected by concentration

  
• Discharge of OH- ions

  4OH–(aq) -> 2H2O(l) + O2(g) + 4e–
• Determining the products of electrolysis of aqueous solutions

  1. Identity the ions present in the electrolyte.
  2. Determine the anion discharged at the anode. Write the half equation involved
  3. Determine the cation discharged at the cathode. Write the half equation involved
  4. write the overall equation for the reaction
  5. Identify the anion and cation left behind to determine products in the electrolyte
• Discharge of H+ ions

  2H+(aq) + 2e- -> H2(g)
• overall equation for electrolysis of water

  2H2O(l)  ->O2(g) + 2H2(g)
• Only reactive anode will react during electrolysis, but not the reactive cathode
• • copper deposited onto the cathode comes mainly from the electrolyte
  • copper anode is constantly oxidising and dissolving into the electrolyte, replenishing the Cu2+ ions which are reduced at cathode
  • thus, the concentration of Cu2+ ions remain unchanged and the colour intensity of the blue electrolyte remains constant
• metal purification

  1. the raw impure copper is the anode while pure copper is the cathode
  2. impure copper anode dissolves and decrease in mass
  3. pure copper deposited onto the cathode
  4. Pure metal increase in mass
  5. impurities present in the impure copper fall to the bottom to form anode slime
• electroplating allows us to coat a thin layer of metal onto an object
• electroplating

  • cathode: object to be coated with metal
  • anode: plating metal
  • electrolyte: aqueous solution of a salt of the plating metal

  
• non-conductive objects can also be electroplated but they will first need to be coated with a layer of graphite before being immersed into electrolyte
• uses of electroplating

  1. enhances the attractiveness
  2. prevent reactive metal from corroding easily
• simple cell is a device that converts chemical energy into electrical energy