Electrolysis: Concepts, Principles, and Applications

Electrolysis is the process of decomposing an ionic compound (either in a molten state or in an aqueous solution) by passing a direct electric current through it. This process involves the conversion of electrical energy into chemical energy.

1. Electrolytes and Non-Electrolytes

• Electrolytes: Substances that conduct electricity in the molten state or in aqueous solution and are decomposed in the process (e.g., NaCl, H2SO4, CuSO4).
• Non-electrolytes: Substances that do not conduct electricity in either the liquid or dissolved state (e.g., sugar, ethanol, benzene, urea).
• Strong Electrolytes: Completely ionized in solution (e.g., HCl, NaOH).
• Weak Electrolytes: Partially ionized in solution (e.g., CH3COOH, NH3).

2. Faraday’s Laws of Electrolysis

• First Law: The mass (m) of a substance discharged at an electrode is directly proportional to the quantity of electricity (Q) passed. Formula: m = zIt, where z is the electrochemical equivalent, I is current in Amperes, and t is time in seconds.
• Second Law: When the same quantity of electricity is passed through different electrolytes, the masses of the substances liberated are proportional to their chemical equivalents.
• Faraday's Constant: 1 Faraday (F) = 96,500 Coulombs. It is the charge carried by 1 mole of electrons.

3. Electrolysis of Specific Substances

• Dilute H2SO4 (Acidified Water): OH- ions are discharged at the anode to produce Oxygen; H+ ions are discharged at the cathode to produce Hydrogen.
• Fused NaCl: Na+ ions are reduced to Sodium metal at the cathode; Cl- ions are oxidized to Chlorine gas at the anode.
• Concentrated NaCl (Brine): H+ ions are preferred over Na+ at the cathode (releasing H2), and Cl- ions are preferred over OH- at the anode (releasing Cl2) due to concentration.
• Aqueous CuSO4 (using Platinum electrodes): Cu2+ is discharged at the cathode; OH- is discharged at the anode releasing O2.

4. Factors Affecting Discharge of Ions

1. Position in Electrochemical Series: Ions lower in the series are discharged more easily (e.g., Cu2+ over H+).
2. Concentration: A higher concentration can favor the discharge of an ion that is otherwise higher in the series (e.g., Cl- in concentrated NaCl).
3. Nature of Electrode: Reactive electrodes can participate in the reaction (e.g., using copper electrodes in CuSO4 solution).

5. Electrochemical Cells and Redox Series

Electrochemical cells convert chemical energy into electrical energy. The standard electrode potential (E°) measures the tendency of a species to be reduced. E°cell = E°cathode - E°anode.

6. Corrosion and Prevention

Corrosion is an electrochemical process where a metal is oxidized (e.g., rusting of iron). Methods of prevention include:

• Painting/Greasing: Barriers against moisture and air.
• Galvanizing: Coating iron with zinc (sacrificial protection).
• Cathodic Protection: Connecting the metal to a more reactive metal (e.g., Magnesium).
• Electroplating: Coating a metal with a thin layer of another metal using electrolysis.