Principles and Applications of Electrolysis

Fundamentals of Electrolysis

Electrolysis is the process in which the passage of electric current through a solution or a molten electrolyte produces a non-spontaneous oxidation-reduction (redox) reaction.

Key Components of an Electrolytic Cell

The electrolytic cell (or container) is the vessel where the process takes place. It holds the solution or molten electrolyte, in which the electrodes are immersed and connected to a continuous (DC) power source.

The electrodes are the surfaces where the oxidation-reduction half-reactions occur. They tend to be inert. Electrodes are distinguished as follows:

• Cathode: The electrode where reduction occurs. In an electrolytic cell, it connects to the negative pole of the power source.
• Anode: The electrode where oxidation occurs. In an electrolytic cell, it connects to the positive pole of the power source.

Voltaic Cells vs. Electrolytic Cells

Understanding the differences between these two types of electrochemical cells is crucial:

• Voltaic Cell (Galvanic Cell):
  • A chemical reaction produces electrical energy (spontaneous reaction).
  • Typically involves two separate electrolytes (or half-cells).
  • The Anode is the negative pole; the Cathode is the positive pole.
• Electrolytic Cell:
  • Electrical energy produces a chemical reaction (non-spontaneous redox reaction).
  • Usually involves a single electrolyte.
  • The Anode is the positive pole; the Cathode is the negative pole.

Electrolysis Examples and Applications

Electrolysis of Molten Sodium Chloride (NaCl)

Molten NaCl at approximately 800 °C serves as the electrolyte. This is dissociated into ions: NaCl → Na⁺ + Cl^-.

Inert electrodes are inserted into the electrolyte and connected to the DC power supply. The process unfolds as follows:

• Anode Reaction (Oxidation): Anions (Cl^- ions) move toward the anode, where they are discharged by releasing their electrons. Chlorine gas is collected at the anode.
• Cathode Reaction (Reduction): Cations (Na⁺ ions) move toward the cathode, where they are discharged by receiving electrons. Sodium metal is obtained at the cathode. Since sodium is less dense than the melt, it floats in liquid form.

The overall reaction is non-spontaneous, which is confirmed by its negative cell potential.

Electrolysis of Water

Pure water does not contain enough ions to conduct electricity effectively. Thus, to achieve its electrolysis, a small amount of sulfuric acid (e.g., 0.1 M H₂SO₄) is often added. Inert electrodes are immersed in this aqueous solution, at which the following reactions occur:

• Hydrogen gas (H₂) is obtained at the cathode.
• Oxygen gas (O₂) is obtained at the anode.

The volume of H₂ gas produced is twice that of O₂. The H₂O is oxidized, but the sulfate ions (SO₄^2-) from the acid are not discharged because they require a higher potential. Consequently, the H₂SO₄ is not consumed during the process.

Electrolysis of Copper(II) Chloride (CuCl₂) in Aqueous Solution

Electrolysis of various salts dissolved in water usually presents some difficulty in interpretation because the H₂O can interfere with the process. However, in the case of CuCl₂, both ions of the salt are discharged:

• On the cathode, copper metal is deposited, which may form a layer of copper.
• At the anode, chlorine gas is collected.