Electrolytic Processes

3.22 Recall that electrolytes are ionic compounds in the molten state or dissolved in water
  • When an ionic substance is melted or dissolved, the ions are free to move about within the liquid or solution.
  • Passing a current through substances that are molten or solution means that the solution can be broken down into elements. This is electrolysis, and the substance being broken down is the electrolyte.
3.23 Describe electrolysis as a process in which electrical energy, from a direct current supply decomposes electrolytes
  • Electrolysis = process in which electrical energy, from a direct current supply decomposes electrolytes
3.24 Explain the movement of ions during electrolysis, in which: positively charged cations migrate to the negatively charged cathode, and negatively charged anions migrate to the positively charged anode
  • During electrolysis, positively charged ions move to the negative electrode (cathode), and negatively charged ions move to the positive electrode (anode).
  • Ions are discharged at the electrodes producing elements, this process is called electrolysis
3.25 Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including: copper chloride solution, sodium chloride solution, sodium sulfate solution, water acidified with sulfuric acid and molten lead bromide (demonstration)
  • Using the logic above…
  • Electrolysis of:
    • Copper chloride solution
      • Cu+ ions go to cathode
      • Cl- ions go to anode
    • Sodium chloride solution
      • Na+ ions go to cathode
      • Cl- ions go to anode
    • Sodium sulfate solution
      • Na+ ions go to cathode
      • SO4^2- ions go to anode
    • Water acidified with sulfuric acid
      • H+ to cathode
      • SO4^2- to anode
    • Molten lead bromide (demonstration)
      • Pb2+ to cathode
      • Br- to anode
3.26 Predict the products of electrolysis of other binary, ionic compounds in the molten state

*see 3.24

3.27 (HT only) Write half equations for reactions occurring at the anode and cathode in electrolysis
  • This is an example of a half equation; the small number is always the same as the 2 larger numbers within the equation.
  • Electrons are represented by the symbol ‘e-‘

2Cl-  ->  Cl2 + 2e-  OR  2Cl-  -2e-  ->  Cl2

This half equation is an example of an oxidation reaction, which would take place at the anode (positive electrode), it’s an oxidation reaction, because the negatively charge ions LOSE electrons.

The opposite occurs at the cathode (negative electrode) with positive ions GAINING electrons. Therefore, this is a reduction reaction.

3.28 (HT only) Explain oxidation and reduction in terms of loss or gain of electrons
  • Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons) to help you remember learn the phrase: OIL RIG
3.29 (HT only) Recall that reduction occurs at the cathode and that oxidation occurs at the anode in electrolysis reactions
  • ANODE – loss of electrons, oxidation (positive electrode) anions come to it
  • CATHODE – gain of electrons, reduction (negative electrode) cations come to it
3.30 Explain the formation of the products in the electrolysis of copper sulfate solution, using copper electrodes, and how this electrolysis can be used to purify copper
  • Cathode gets coated with copper
  • Bubbles of oxygen are given off at the anode
  • Cu2+ ions are discharged at cathode as Cu atoms
  • At cathode: Cu2+ + 2e- -> Cu
  • At anode: 4OH- – 4e- -> O2 + 2H2O
  • Used to purify copper, to remove the copper from copper sulfate, as it is discharged on the cathode, coating it with copper
3.31 Core practical: Investigate the electrolysis of copper sulfate solution with inert electrodes and copper electrodes