Electrolysis of Molten Compounds

• A binary ionic compound is one consisting of just two elements joined together by ionic bonding

  • When these compounds undergo electrolysis they always produce their corresponding elements
• To predict the products made at each electrode, first identify the ions

  • The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode
  • Therefore, the cathode product will always be the metal, and the product formed at the anode will always be the non-metal
• Example Method (1)


  1. Add lead(II) bromide into a beaker

  2. Heat it so it will turn molten, allowing ions to be free to move and conduct an electric charge

  3. Add two graphite rods as the electrodes

  4. Connect this to a power pack or battery

  5. Turn on the power pack or battery and allow electrolysis to take place
• Negative bromide ions move to the positive electrode (anode)


  Each loses one electron to form bromine molecules
• There is bubbling at the anode as brown bromine gas is given off
• Positive lead ions move to the negative electrode (cathode)


  They gain electrons to form a grey lead metal which deposits on the surface of the electrode
• Example Method (1):

  • Add lead(II) bromide into a beaker and heat it so it will turn molten, allowing ions to be free to move and conduct an electric charge
  • Add two graphite rods as the electrodes and connect this to a power pack or battery
  • Turn on the power pack or battery and allow electrolysis to take plac
• Example Method (2):

  • Negative bromide ions move to the positive electrode (anode) and each loses one electron to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off
  • Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode
• Diagram showing the electrolysis of lead (II) bromide

  A) power supply

  B) molten lead (II) bromide

  C) negative

  D) positive

  E) loses

  F) 1

  G) positive anode

  H) two

  I) bromine molecule

  J) positive

  K) bromide

  L) lead

  M) negative

  N) gains

  O) 2

  P) negative

  Q) lead metal

  R) metallic bonding

  
• Remember electrodes need to be inert such as graphite or platinum so that they don’t participate in a side reaction with the electrolyte
• Electrolysis of molten lead bromide

  • initially bulb doesn’t glow because the solid compound doesn’t allow electricity to pass through, when the solid becomes molten, the bulb lights
  • lead bromide is breaking up into lead and bromine: lead bromide → lead + bromine or PbBr2 → Pb + Br2
  • lead (cation) is formed at the cathode (the negative electrode) and two electrons have been gained
• Electrolysis of molten lead bromide

  A) bromine vapour

  B) molten lead bromide

  C) liquid

  D) bulb

  E) lead

  F) molten lead

  G) cathode

  
• When substances gain electrons, this is called reduction
• At the same time, each bromide ion must lose electrons toform bromine

  • occurs at the positive electrode akatheanode
• When substances loses electrons, this is called oxidation
• LEOA the lion goes GERCA

  • LEOA - lose electron, oxidation, anode
  • GERC - gain electron, reduction, cathode
• Metal always go to the cathode because of the cations
• Non-metals always go to the anode because of the anions
• Describe and explain how electrolysis is used to make pure copper from a lump of impure copper.

  Impure copper is the anode and pure copper is the cathode with the electrolyte being a copper sulfate solution. At the anode, the copper loses electrons and copper forms positive ions and goes into solution. At the cathode, the copper gains electrons and it is deposited/collected. The impurities/ sludge collect at the bottom because the impurities are not attracted to electrode