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
- Saturated calomel electrode (SCE):
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.
| Species (Reduced Form → Oxidized Form) | Standard Reduction Potential, E° (V) |
|---|---|
| Li⁺ + e⁻ → Li | -3.04 |
| K⁺ + e⁻ → K | -2.93 |
| Na⁺ + e⁻ → Na | -2.71 |
| Mg²⁺ + 2e⁻ → Mg | -2.37 |
| Al³⁺ + 3e⁻ → Al | -1.66 |
| Zn²⁺ + 2e⁻ → Zn | -0.76 |
| Fe²⁺ + 2e⁻ → Fe | -0.44 |
| Ni²⁺ + 2e⁻ → Ni | -0.25 |
| Sn²⁺ + 2e⁻ → Sn | -0.14 |
| Pb²⁺ + 2e⁻ → Pb | -0.13 |
| 2H⁺ + 2e⁻ → H₂ | 0.00 (SHE) |
| Cu²⁺ + 2e⁻ → Cu | +0.34 |
| Ag⁺ + e⁻ → Ag | +0.80 |
| Hg²⁺ + 2e⁻ → Hg | +0.85 |
| Cl₂ + 2e⁻ → 2Cl⁻ | +1.36 |
| Br₂ + 2e⁻ → 2Br⁻ | +1.07 |
| O₂ + 4H⁺ + 4e⁻ → 2H₂O | +1.23 |
| F₂ + 2e⁻ → 2F⁻ | +2.87 |