electrolysis of molten compound

• Electrolysis on MOLTEN Sodium Chloride will always form sodium & chlorine aqueous sodium chloride does not.

  
• MOLTEN SODIUM CHLORIDE electrolyte:

  The IONS involve is Na+ & Cl-

  • This means the electrolyte has these ions floating around in it.
  • When the circuit is turned on & electricity flows, the Na+ ions get attracted to the CATHODE & the Cl- ions get attracted to the ANODE.
  • When they reach the anodes, both sets of ions TRANSFER ELECTRONS & turn into ATOMS of ELEMENTS.
  • Na+ IONS turn into Na ATOMS, while Cl- ions turn into Cl ATOMS.
  • To represent these processes, you can write their HALF EQUATIONS.

  
• ELECTROLYSIS HALF EQUATIONS - Step 1:

  Write the ION of the element on the LEFT & the ATOM of the element on the RIGHT.

  
• ELECTROLYSIS HALF EQUATIONS - Step 2:

  Check if any of the elements are DIATOMIC (exist as two atoms).

  • Here is a list of ALL the DIATOMIC elements you need to know.
  • Chlorine is part of the list so it needs to be changed to Cl2.

  
• Diatomic elements:

  • Hydrogen (H2)
  • Nitrogen (N2)
  • Oxygen (O2)

  Group 7 elements:

  • F2
  • Cl2
  • Br2
  • I2
• ELECTROLYSIS HALF EQUATIONS - Step 3:

  BALANCE the equation to make sure you have the same numbers on the left & right.

  
• ELECTROLYSIS HALF EQUATIONS - Step 4:

  Add ELECTRONS to make the charges in the equation balanced.

  • For ANODES, electrons go on the RIGHT.
  • For CATHODES, electrons go on the LEFT.

  
• • Metals MORE reactive than CARBON can NOT be extracted by REDUCTION.
  • These metals need ELECTROLYSIS for extraction.
  • A large amount of energy is required as the metal ore needs to be MELTED to become MOLTEN & ELECTRICITY is needed for the electrolysis.
  • This makes the process EXPENSIVE.
• • There's usually more than one anode submerged in the electrolyte, & the cathode lines the INSIDE of the container.
  • Both electrodes are made of GRAPHITE.
  • The ions present in the liquid include POSITIVE ALUMINIUM IONS (Al3+) & NEGATIVE OXYGEN IONS (O2-).
  • When current flows through the set up, the ALUMINIUM ions are attracted to the CATHODE & the OXYGEN ions are attracted to the ANODE.

  
• To find the half equation:

  • Write ions to atoms
  • check for diatomic atoms
  • balance
  • add electrons

  
• • The Aluminum is produced as a MOLTEN LIQUID, which flows to the bottom of the container due to the HIGH TEMPERATURE.
  • The Oxygen is produced as BUBBLES, which float to the top of the liquid.

  
• During electrolysis of a molten mixture of aluminum oxide & cryolite the GRAPHITE ANODES need to be REPLACED REGULARLY.

  • Because at high temp. oxygen reacts with carbon.
  • So the positive electrode wear away to form carbon dioxide.

  
• Magnesium ions attracted to the electrode, so magnesium ions gain 2 electrons.

  
• Aluminum ions are positive & are attracted to the negative electrode (cathode), where they gain elections.

  • Al3+ + 3e- -> Al

  
• Explain how carbon dioxide at the positive electrodes during electrolysis:

  The positive electrode (anodes) are made of carbon / graphite, oxygen is produced at anode, which reacts with the anodes.

  
• Number of atoms = 10

  
• zinc ions are positive, so they move towards the negative electrode and opposite charges attract.

  
• Cryolite:


  A mineral that lowers melting point.
• To turn aluminum oxide molten:


  • Aluminum oxide is found within solid bauxite ore on earth & can be extracted using electrolysis.
  • Purify the aluminum oxide from the bauxite ore, mined from the ground.
  • Mix the aluminum oxide with cryolite to lower the melting point.
  • So LESS ENERGY is needed to melt it, making the process CHEAPER.
  • Melt it to molten aluminum oxide.

  
• Electrolysis of aluminum oxide:


  • The oxygen 2- ions are attracted to the anode.
  • They’ll transfer their two extra electrons to the anode & discharged to oxygen atoms.
  • Each pair of oxygen can combine an oxygen molecule & float into the air.
  • Electrons pass through the wire to the negative cathode.
  • It attract the positive ions in the electrolyte & donate electrons to them.
  • The positive ions will slowly pool in the bottom of the beaker as molten aluminum metal.

  
• Electrolysis of aluminum oxide:

  2Al2O3(l) ➔ 4Al(l) + 3O2(g) 

  At the anode:

  • O2- -> O2 + 2e-

  But since oxygen forms diatomic molecules, we need 2 oxygen ions at the left & 4 electrons will be released in total.

  At the anode:

  • 2O2- -> O2 + 4e- (oxidised)

  At the cathode:

  • Al3+ + 3e- -> Al (reduced)