Electro Chemistry Study Notes

Chapter 4: Electro Chemistry

Electrochemistry

  • Definition: Electrochemistry is the branch of chemistry that studies the conversion of chemical energy (chemical reactions) to electrical energy (electric current) and vice versa through oxidation-reduction (Redox) reactions.

Oxidation-Reduction (Redox) Reactions

  • Definition: Redox reactions involve the transfer of electrons between reacting substances.
    • Key Characteristics: Electrons are transferred from one reactant to another during the chemical reaction.

Types of Electric Conductors

1. Electronic Conductors
  • Description: Solid substances where electric current is caused by the movement of free electrons.
    • Characteristics: There's no transfer of matter during the conduction of electricity.
    • Examples: All metals, such as Copper (Cu) and Iron (Fe).
2. Electrolytic Conductors (Electrolytes)
  • Description: Liquid or aqueous substances that conduct electric current through the movement of free ions (both +ve and -ve).
    • Characteristics: Involves a transfer of matter.
    • Examples:
    1. Salt solutions (aqueous)
    2. Acidic and alkali solutions
    3. Molten salts (e.g., NaCl in liquid state)

Types of Electrochemical Cells

1. Galvanic Cell (Voltaic Cell)
  • Definition: A type of cell that converts chemical reactions into electric current through spontaneous Redox reactions.
  • Process Description: Composed of two half-cells connected by a salt bridge.
    • Function: Electrons pass via an external wire between half-cells, enabling the generation of electric current.
    • Example: Daniel Cell.
2. Electrolytic Cell
  • Definition: A type of cell that converts electric current into chemical reactions through non-spontaneous Redox reactions.

Experiment: Zinc Rod in Copper Sulfate

  • Procedure: Insert a Zinc (Zn) rod into blue Copper Sulfate (CuSO4) solution:
    • Observations:
    1. Zinc dissolves into the solution, converting into zinc ions (Zn^2+).
    2. Red copper metal precipitates on the surface of the zinc rod.
    3. The blue color of the Copper Sulfate solution diminishes and eventually becomes colorless.
    • Conclusion:
    1. Each Zinc atom (Zn^0) loses two electrons to form Zn^2+ ions.
    2. Copper ions (Cu^2+) gain two electrons to become copper atoms (Cu^0), precipitating on the rod.
    • Oxidation Equation:
      Zn^0(s)
      ightarrow Zn^{2+}(aq) + 2e^-
    • Reduction Equation:
      Cu^{2+}(aq) + 2e^-
      ightarrow Cu^0(s)
    • Net Reaction:
      Zn^0 + Cu^{2+}(aq)
      ightarrow Zn^{2+}(aq) + Cu^0(s)

Components of a Galvanic Cell

  • Half-Cells: Two half-cells isolated in separate containers but connected via a salt bridge.
  • Function of Salt Bridge:
    1. Connects the solutions of the two half-cells indirectly.
    2. Neutralizes excess ions generated by the Redox reactions.
    • Oxidation at Anode:
      • Zinc (Zn) acts as the anode, losing mass as it oxidizes.
    • Reduction at Cathode:
      • Copper acts as the cathode, gaining mass from reduction.

Standard Hydrogen Electrode (SHE)

  • Definition: An electrode formed by passing hydrogen gas at 1 atm on a platinum sheet immersed in 1 M H^+ solution at 25 °C.
  • Potential of SHE: Equals 0 V, but can change based on hydrogen ion concentration and gas pressure.
  • Usage: Acts as a reference electrode for measuring the potential of other electrodes.

Electromotive Series

  • Description: Arrangement of elements based on their oxidation-reduction potentials.
    • More Active Metals: Higher oxidation potentials and serve as good reducing agents.
    • Less Active Metals: Lower oxidation potentials and serve as good oxidizing agents.
    • Applications: Elements higher in the electromotive series can replace those lower in their salt solutions.

Electrolytic Cells

  • Definition: Convert electrical energy into chemical energy through non-spontaneous reactions, often defined by the passage of electric current through an electrolyte.
Examples:
  1. Electroplating: Applications include chrome plating car parts for aesthetics and corrosion resistance.
  2. Purification of Metals: Electrolytic processes used to purify copper to a higher conductivity level.
  3. Aluminum Extraction: Aluminum is extracted from bauxite using electrolytic methods with cryolite and temperature control.

Faraday's Laws of Electrolysis

1. First Law
  • Relation: Quantity of material produced is directly proportional to the quantity of electricity passed through the electrolyte.
2. Second Law
  • Relation: Amount of substance formed is directly proportional to their equivalent mass passed through different electrolytes.
Formulas
  • General law: When one Faraday (96500 Coulombs) passes through, it leads to the dissolution or deposition of gram equivalent mass of the substance.
    • Q=IimestQ = I imes t
    • Substitute values for calculating equivalent mass, current intensity, and dissolved substances.

Applications and Factors in Corrosion

  • Corrosion Processes: Mechanism involves galvanic cell formation causing oxidation of more active metals.
  • Resistance Strategies: Techniques like galvanization (zinc coating) and anodic protection can significantly mitigate corrosion effects.

Summary

  • Galvanic and electrolytic cells represent vital concepts in electrochemistry, showcasing the dual energy conversion between chemical and electrical forms. Understanding these principles facilitates advancements in energy storage, metal processing, and battery technology.