Concentration Cells and Electrolysis Notes

Concentration Cells

  • Definition: An electrochemical cell consisting of two half-cells made of the same material but differing in their ion concentrations.

  • Key Characteristics:

    • Operates on the principle that nature seeks equilibrium by equalizing concentrations.
    • Electron flow occurs from the lower concentration (anode) to the higher concentration (cathode).
    • The voltage produced is typically small.

  • Equilibrium and Voltage Calculation:

    • The potential can be calculated using the Nernst Equation to determine voltage as the cell seeks to reach equilibrium when concentration levels equalize.

  • Concentration Cell Corrosion:

    • Occurs when different areas of a metal surface are exposed to varying concentrations of the same solution. There are three main types:
    1. Metal Ion Concentration Cells:
      • High concentration of metal ions under faying surfaces and lower concentration at adjacent crevices results in electrical potential.
      • The area with high metal ion concentration acts as the cathode (protected) and low concentration as the anode (corroded).
    2. Oxygen Concentration Cells:
      • Variability in oxygen diffusion creates concentration differences.
      • Corrosion occurs at low-oxygen areas where the metal surface is anodic.
    3. Active-Passive Cells:
      • For metals with passive oxide films protecting against corrosion, damage that exposes active metal leads to pitting.
      • A potential difference develops between the passive (large cathode area) and exposed active (small anode area) leading to corrosion.

Electrolysis

  • Definition: A process involving an electric current passing through an ionic substance, either molten or dissolved, resulting in chemical reactions at the electrodes.

  • Process Overview:

    1. An ionic compound is dissolved or melted to free its ions in a liquid.
    2. An external power supply applies current between inert electrodes submerged in the ionic solution.
    3. Ions migrate to electrodes:
    • Cations: Move towards cathode.
    • Anions: Move towards anode.
    1. At the electrodes, ions gain or lose electrons, forming desired elements or compounds.

  • Characteristics of Electrolysis:

    • Electrolytic Cells: Commonly used in electrolysis.
    • Measurements: Often involve amperage (current) and are quantified in amperes (A).
    • Unit Relation: 1 A = 1 Coulomb/s.
    • Stoichiometry: Crucial for calculating mass, time, and current related to electrolysis reactions.
    • Applications: Includes galvanizing and electroplating.

Example Problems

  • Q1: How many Faradays are transferred with a current of 121 amps over 16 hours?
  • Q2: How many grams of copper can be deposited from a CuSO4 solution using 3.0 A for 2.0 hours?