Unit 5: Electrochemistry

1. Electrode Potential

The electrode potential is the potential difference that develops between a metal electrode and the electrolyte solution in which it is immersed when the system reaches equilibrium. It reflects the tendency of the electrode to gain or lose electrons.

When measured under standard conditions (25 °C, 1 atm pressure for gases, 1 M concentration for aqueous species), the value is called the standard electrode potential and is denoted E°. By convention, the standard hydrogen electrode (SHE) is assigned a potential of 0 V, and all other electrode potentials are reported relative to this reference.

2. Types of Electrodes

2.1 Standard Hydrogen Electrode (SHE)

The SHE serves as the primary reference electrode in electrochemistry.

• Construction: A platinum electrode coated with platinum black, immersed in a 1 M H⁺ solution, with hydrogen gas bubbled at 1 atm pressure.
• Reaction: 2H⁺(aq) + 2e⁻ ⇌ H₂(g)
• Standard potential: E°(SHE) = 0.00 V (by definition).

Diagram description: A schematic shows a Pt electrode in a beaker of 1 M HCl, with H₂ gas flowing over the surface, connected to a potentiometer.

2.2 Calomel Electrode (Hg/Hg₂Cl₂/KCl)

The calomel electrode is a widely used secondary reference because of its ease of preparation and stable potential.

• Construction: Mercury (Hg) in contact with mercurous chloride (Hg₂Cl₂) paste, saturated with KCl solution.
• Representation: Hg | Hg₂Cl₂ (s) | KCl (aq)
• Potential values (vs. SHE):
  • Saturated calomel electrode (SCE): E = +0.242 V
  • 1 M KCl calomel: E = +0.280 V
  • 0.1 M KCl calomel: E = +0.334 V

Diagram description: A cross‑section shows a Hg pool at the bottom, a layer of Hg₂Cl₂ paste above it, and the KCl solution filling the remainder of the tube, with a platinum wire providing electrical contact.

3. Electrochemical Series

The electrochemical series (also called the activity series) lists elements ordered by their standard reduction potentials (E°). A more positive E° indicates a stronger oxidizing agent; a more negative E° indicates a stronger reducing agent.

Applications of Electrochemistry

Electrochemistry has numerous practical applications in modern technology and industry:

1. Batteries and Fuel Cells

Electrochemical cells are the basis of all batteries, from simple alkaline cells to advanced lithium-ion batteries. Fuel cells convert chemical energy directly into electricity, powering vehicles and stationary power systems. Understanding electrode potentials helps engineers design more efficient energy storage devices.

2. Electroplating and Surface Coating

Electroplating uses electrolysis to deposit thin layers of metal onto surfaces. This process is used for:

• Corrosion protection (galvanizing steel)
• Decorative finishes (gold and silver plating)
• Wear resistance (chrome plating)
• Electrical conductivity (circuit board manufacturing)

3. Corrosion Prevention

Understanding electrode potentials helps in designing corrosion prevention strategies. Sacrificial anodes, used on ships and underground pipes, work by providing a more reactive metal that corrodes preferentially, protecting the structural metal.

4. Analytical Chemistry

Electrochemical methods like potentiometry, voltammetry, and coulometry are used for precise chemical analysis. These techniques measure electrical properties to determine concentration and identity of chemical species.

Summary of Key Equations

The fundamental equations of electrochemistry include the Nernst equation for cell potential, Faraday's laws for electrolysis calculations, and the relationship between Gibbs free energy and cell potential (ΔG = -nFE). Mastering these equations is essential for solving electrochemistry problems.