Electrolytic Cells Notes

Electrolytic Cells

• Nonspontaneous redox reactions occur in electrolytic cells, driven by electrical energy.
• Electrolytic cells use electrical energy to cause nonspontaneous chemical reactions.
• External voltage must be greater than the potential produced by the spontaneous reverse reaction.

Comparison with Voltaic Cells

• Electrolytic cells:
  • Connected to a battery or direct current source.
  • Electrical energy from an external source causes nonspontaneous redox reactions.
  • Electrical energy converts to chemical energy.
• Voltaic cells:
  • Source of electrical energy.
  • Spontaneous redox reactions produce electricity.

Electroplating

• Electrolytic process where metal ions are reduced and deposited as solid metal on a surface.
• Requires a solution of the plating metal, an object to be plated (cathode), and a piece of the plating metal (anode).
• Example: Silver electroplating involves the reduction of $Ag^+$ ions at the cathode and oxidation of $Ag$ at the anode.

Rechargeable Cells

• Combine voltaic and electrolytic cell chemistry.
• Discharge: Voltaic cell (chemical energy to electrical energy).
• Recharge: Electrolytic cell (electrical energy to chemical energy).
• Example: Standard 12V automobile battery (lead storage battery)
  • Anode: $Pb(s) + SO<em>4^{2-}(aq) \rightarrow PbSO</em>4(s) + 2e^-$
  • Cathode: $PbO<em>2(s) + 4H^+(aq) + SO</em>4^{2-}(aq) + 2e^- \rightarrow PbSO<em>4(s) + 2H</em>2O(l)$
  • Net reaction: $Pb(s) + PbO<em>2(s) + 2H</em>2SO<em>4(aq) \rightarrow 2PbSO</em>4(s) + 2H_2O(l)$

Electrolysis

• Process of passing a current through a cell with a negative cell potential to cause a redox reaction.
• Used for metal purification and other industrial applications.

Electrolysis of Water

• Breaks down water into hydrogen and oxygen.
• Nonspontaneous process requiring electrical energy.
  • Anode: $6H<em>2O(l) \rightarrow 4e^- + O</em>2(g) + 4H_3O^+(aq)$
  • Cathode: $4H<em>2O(l) + 4e^- \rightarrow 2H</em>2(g) + 4OH^-(aq)$

Aluminum Production (Hall-Héroult process)

• Aluminum extracted from bauxite ore (aluminum oxide).
• Process:
  • Alumina ($Al<em>2O</em>3$) separated from other compounds using sodium hydroxide.
  • Purified alumina dissolved in molten cryolite ($Na<em>3AlF</em>6$) at 970°C.
  • Aluminum ions reduced to aluminum metal.
• Overall reaction: $2Al<em>2O</em>3(l) + 3C(s) \rightarrow 4Al(l) + 3CO_2(g)$
• Significant electrical energy consumption; recycling saves ~95% of the cost.