Inelectrochemistry, ahalf-cell is a structure that contains a conductiveelectrode and a surrounding conductiveelectrolyte separated by a naturally occurringHelmholtz double layer. Chemical reactions within this layer momentarily pumpelectric charges between the electrode and the electrolyte, resulting in apotential difference between the electrode and the electrolyte. 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 intenseelectric field within the double layer, and the potential rises in value until the field halts the net charge-pumping reactions. This self-limiting action occurs almost instantly in an isolated half-cell; in applications two dissimilar half-cells are appropriately connected to constitute aGalvanic cell.
A standard half-cell consists of a metal electrode in anaqueous solution where the concentration of the metal ions is 1molar (1 mol/L) at 298kelvins (25 °C).[1] In the case of thestandard 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 1atm being bubbled through solution.[2] Theelectrochemical series, which consists ofstandard electrode potentials and is closely related to thereactivity 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 asalt bridge.
The standard hydrogen half-cell:
The half-cells of aDaniell cell:
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