Half-cell

Lore & Background

In a half-cell, the typical anode reaction involves a metal atom in the electrode being dissolved and transported as a positive ion across the double layer, causing the electrolyte to acquire a net positive charge while the electrode acquires a net negative charge. The growing potential difference creates an intense electric field within the double layer, and the potential rises until the field halts the net charge-pumping reactions. This self-limiting action occurs almost instantly in an isolated half-cell. A standard half-cell is defined by a metal electrode immersed in an aqueous solution where the metal ion concentration is exactly 1 molar (1 mol/L) at 298 kelvins (25 °C). For the standard hydrogen electrode (SHE), a platinum electrode is used, placed in an acidic solution with a hydrogen ion concentration of 1M, while hydrogen gas at 1 atmosphere is bubbled through the solution. The electrochemical series, which lists standard electrode potentials and is closely tied to the reactivity series, was established by measuring the potential difference between a metal half-cell and a standard hydrogen half-cell in a circuit connected by a salt bridge. In a Daniell cell, the half-cell reactions are: at the zinc anode, zinc metal oxidizes to Zn²⁺ plus two electrons; at the copper cathode, Cu²⁺ plus two electrons reduces to copper metal. The overall cell reaction is Zn + Cu²⁺ → Zn²⁺ + Cu.

Reader's Guide

In the case of the standard hydrogen electrode (SHE), a platinum electrode is used and is immersed in an acidic solution where the concentration of hydrogen ions is 1M, with hydrogen gas at 1 atm being bubbled through solution. The electrochemical series, which consists of standard electrode potentials and is closely related to the reactivity series, was generated by measuring the difference in potential between the metal half-cell in a circuit with a standard hydrogen half-cell, connected by a salt bridge. The half-cells of a Daniell cell illustrate the anode reaction (Zn → Zn2+ + 2e−) and the cathode reaction (Cu2+ + 2e− → Cu). The significance of the half-cell lies in its role as the building block for all Galvanic cells, enabling the measurement and tabulation of standard electrode potentials that predict the direction of redox reactions.

Did You Know?

• A half-cell contains a conductive electrode and a conductive electrolyte separated by a naturally occurring Helmholtz double layer.
• The standard hydrogen half-cell uses a platinum electrode in 1M acidic solution with hydrogen gas at 1 atm.
• The electrochemical series was generated by measuring potential differences between metal half-cells and a standard hydrogen half-cell.
• In a Daniell cell, the zinc half-cell acts as the anode (Zn → Zn2+ + 2e−) and the copper half-cell as the cathode (Cu2+ + 2e− → Cu).