Electrochemical equilibrium

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This article explains electrochemical equilibrium in the presence of redox reactions; for other meanings see electrochemical double layer .

The term electrochemical equilibrium describes the equilibrium that is established in a galvanic element during redox reactions .

motivation

A galvanic element is in electrochemical equilibrium when no current flows through the cell. As long as there is still a voltage between the electrodes, but no current flows, the element is also in electrochemical equilibrium. Discharge reduces this voltage and is finally exhausted - then the cell is in chemical equilibrium . The chemical equilibrium is clearly determined by the composition of the material - on the other hand, there are any number of electrochemical equilibrium states (each with any voltage between the electrodes of the galvanic element).

description

In a redox reaction in a galvanic element, the difference between the electrode potentials (voltage) is the driving force behind the reaction. As long as you can still measure a voltage, the reaction is out of chemical equilibrium . The galvanic element supplies a certain amount of free enthalpy , which depends on that of the standard free enthalpy and the equilibrium constant K.

${\displaystyle \Delta G=\Delta G^{\circ }+RT\,\ln(K)}$

Here R is the general gas constant and T is the temperature in Kelvin .

The free enthalpy results from the converted amount of substance n in mol , the Faraday constant F and the potential difference between the electrodes:

${\displaystyle \Delta G=-n\cdot F\cdot \Delta E}$

The equilibrium constant can be calculated from the difference between the normal potentials using the following relationship :

${\displaystyle K=e^{\frac {n\cdot F\cdot \Delta E^{\circ }}{R\cdot T}}}$

The flow of electrons in a galvanic cell is caused by an electrical voltage between the half-cells : In each half-cell, metal ions constantly pass through the solid / liquid phase boundary in both directions. If the release of metal ions to the liquid phase initially predominates, the metal is negatively charged compared to the liquid phase. This charge counteracts a further transition of positive metal ions into the liquid phase and leads to equilibrium. The same number of ions pass through the phase boundary in both directions per unit of time. An electrochemical double layer of negative and positive charge carriers is created at the phase boundary . Walther Hermann Nernst clearly described these processes as a balance between the dissolution pressure of the metal and the deposition pressure of the ions. After the electrochemical equilibrium has been established, the electrodes of different half-cells are charged differently and a voltage can be measured between them.

literature

• Equilibria . In: Practice of the natural sciences. Chemistry in School , Vol. 61 (2012), Issue 2, ISSN  1617-5638 .
• Wolfgang Asselborn a. a .: Chemistry today. Secondary education . Schroedel Verlag , Hannover 2002, ISBN 3-507-10630-2 .

1. ^ Myland, Jan C .: Fundamentals of electrochemical science . Academic Press, San Diego 1994, ISBN 1-299-53792-8 , pp. 69 ff .

See also

• Electrochemical potential
• Electrochemical gradient
• Electrochemical driving force
• Concentration gradient