electrolysis of aqueous compound

• Ionic compounds in the AQUEOUS state conduct electricity, as their ions are free to move.

  • Unlike MOLTEN electrolytes, the products formed in aqueous electrolytes are not always the elements in the compound.
  • Because there's MORE THAN TWO DIFFERENT IONS present in an aqueous solution.

  
• Just like acids do, water can partially IONISE to release HYDROGEN IONS (H+) & HYDROXIDE IONS (OH-) into the aqueous solution.

  • H2O -> H+ + OH-

  So the electrolyte can contain the ions from the ionic compound & the ions from water ionising (H+ & OH-).
• The ions in AQUEOUS SODIUM CHLORIDE:

  Aqueous:

  • H+
  • OH-

  Sodium Chloride:

  • Na+
  • Cl-
• All ions of the aqueous solution float around in the electrolyte.

  • When the electricity flows through this, only ONE positive ion can go to the CATHODE & ONE negative ion can go to the ANODE.

  
• To figure out which ions travel to the electrodes to get discharged in an aqueous solution, there's TWO RULES:

  Rules for CATHODES:

  • The LESS REACTIVE positive ions will travel to the CATHODE to be discharged & its element will be formed.

  Rules for ANODES:

  • If a HALOGEN is present, the HALOGEN IONS will travel to the anode to be discharged to form its element.
  • If a halogen is NOT present, the HYDROXIDE IONS will travel to the anode to be discharges & to form oxygen gas, the half equation is:

  4OH- -> O2 + 2H2O + 4e-

  
• Where METALS are formed in electrolysis with AQUEOUS ELECTROLYTES.

  • Observe the metal PLATING the surface of the electrode.

  
• Aqueous electrolyte - sodium chloride (NaCl):

  Product at Cathode:

  • Hydrogen
  • 2H+ + 2e- -> H2

  Product at Anode:

  • Chlorine gas (Cl2)
  • 2Cl- -> Cl2 + 2e-
• Aqueous electrolyte - Copper sulfate (CuSO4):

  Product at Cathode:

  • Copper metal (Cu)
  • Cu2+ + 2e- -> Cu

  Product at Anode:

  • Oxygen gas (O2)
  • 4OH- -> O2 + 2H2O + 4e-
• Aqueous electrolyte - Potassium Iodide (KI):

  Product at Cathode:

  • Hydrogen gas (H2)
  • 2H+ + 2e- -> H2

  Product at Anode:

  • Iodine gas (I2)
  • 2i- -> I2 + 2e-
• Positive electrode:

  • Bubbles
  • fizzing

  Negative electrode:

  • solid

  
• water molecules break down to produce OH- ions, which are attracted to the positive electrode, where OH-ions lose electrons & oxidised.

  
• Silver is produced at the negative electrode (cathode) & not hydrogen, so silver is less reactive than hydrogen.

  
• • 4 ions at start: H+ OH- Na+ Cl-
  • H+ ion discharged at negative electrode
  • Cl- ion discharged at positive electrode
  • So OH- & Na+ remain in solution

  
• At the positive anode:

  It's always the OH-, ion that will be oxidised (lose electrons), unless there's a halide ion (like Cl-, or Br-) present.

  
• At the negative cathode:

  It's always the H+ ion that will be reduced (gain electrons), unless Cu2+ is present (the ion of the least reactive element).

  
• Electrolysis of aqueous solution copper sulfate (CuSO4):

  Ions:

  • Cu2+
  • SO4^2-
  • H+
  • OH-

  At negative cathode:

  • Copper ions get discharged & gain 2 electrons from the cathode to form pure copper.
  • Since this happens at normal temp, the copper will be solid, so it will build up around the electrode.

  At the positive anode:

  • Hydroxide ions get discharge as it lose the extra electron to the anode to form oxygen & water
  • 4OH- -> 2H2O + O2 + 4e-

  
• Electrolysis of aqueous sodium chloride:

  Ions:

  • sodium ions (Na+)
  • Chloride ions (Cl-)
  • Hydrogen ions (H+)
  • Hydroxide ions (OH-)

  At cathode:

  • Hydrogen ions are discharged by gaining electrons & forms hydrogen gas.

  At anode:

  • Chloride ions are discharged by losing electrons & forms chlorine gas.