Electrolysis Fundamentals: Product Prediction and Applications

Posted on Mar 26, 2025 in Chemistry

Electricity and Chemistry: Electrolysis Explained

Electrolysis is a method used to decompose chemical compounds using electricity. It involves two electrodes:

  • Cathode: The negative electrode where reduction occurs. Generally, metals or hydrogen are formed here. (Remember: CATions are positive and go to the CAThode).
  • Anode: The positive electrode where oxidation occurs. Typically, non-metals (except hydrogen) are formed here. (Remember: ANions are negative and go to the ANode).

Predicting Electrolysis Products

To predict the products of electrolysis (typically for compounds with two elements):

  • Molten Compounds: If the compound is molten (e.g., molten iron oxide), the two component elements are produced (e.g., molten iron at the cathode and oxygen gas at the anode).
  • Aqueous Solutions: In a solution containing water and an ionic compound:
    • At the cathode, the less reactive of the two positive ions (metal ion vs. H⁺ from water) will be reduced. For example, in aqueous NaCl, hydrogen gas is produced, not sodium metal, because hydrogen is less reactive than sodium.
    • At the anode:
      • If a halide (Cl⁻, Br⁻, I⁻) is present in a concentrated solution, the corresponding halogen (Cl₂, Br₂, I₂) will be produced.
      • If the solution is dilute, or if no halide is present, oxygen (from the oxidation of water or hydroxide ions) will be produced.

Example Reactions:

  • At the anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻
  • At the cathode (reduction): 2H⁺ + 2e⁻ → H₂

Electroplating

Electroplating uses electrolysis to coat an object with a thin layer of metal. The requirements are:

  • An anode made of the metal you want to plate with.
  • A solution (electrolyte) containing ions of the same metal as the anode.
  • The object to be plated placed at the cathode.

Electroplating is used to improve appearance (e.g., coating a watch with gold) or to prevent corrosion.

Materials in Electrical Applications

Conductors

  • Aluminum: Used for electricity cables because it is light, corrosion-resistant, a good conductor, and cheaper/lighter than copper. These cables often have a steel core for added strength.
  • Copper: Used in electrical wires due to its excellent conductivity and ductility. While silver is the best metallic conductor (used in specialized circuits like keyboards and phones), copper is significantly cheaper.

Insulators

  • Plastics and Ceramics: Used as insulators because they do not conduct electricity well and are poor conductors of heat.
  • Plastic: Commonly used for the casing of electrical plugs.
  • Ceramics: These inorganic, non-metallic solids are used to support high-voltage cables on electricity pylons.

Note: The choice of electrode material can sometimes affect the products formed during electrolysis.

Specific Electrolysis Applications

Extraction of Aluminum

Aluminum is extracted by electrolyzing aluminum(III) oxide (Al₂O₃, alumina). Because alumina has a very high melting point, it is dissolved in molten cryolite (Na₃AlF₆). This mixture offers several advantages:

  • Melts at a significantly lower temperature.
  • Acts as a better conductor of electricity.
  • Reduces the overall energy needed and cost, making the process economically viable.

During electrolysis:

  • Aluminum metal is produced at the carbon cathode.
  • Oxygen gas is produced at the carbon anode (which reacts with the carbon, requiring the anodes to be replaced periodically).

Electrolysis of Brine

Brine is a concentrated aqueous solution of sodium chloride (NaCl).

During the electrolysis of brine:

  • Chlorine gas (Cl₂) is produced at the titanium anode. Uses include making PVC, medical drugs, pesticides, sterilizing water, and producing other chemicals.
  • Hydrogen gas (H₂) is produced at the steel or nickel cathode. Uses include making nylon, hydrogen peroxide, hardening vegetable oils, and as a potential fuel.
  • Sodium hydroxide (NaOH) solution is left over in the electrolyte. Uses include manufacturing soaps, detergents, medical drugs, dyes, paper, and ceramics.

An asbestos diaphragm (or a more modern polymer membrane) is often used to allow ions to pass through but keep the gaseous products (H₂ and Cl₂) separate, preventing them from reacting.