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Concept
Nernst equation
Summary
In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or full cell reaction) from the standard electrode potential, absolute temperature, the number of electrons involved in the redox reaction, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation respectively. It was named after Walther Nernst, a German physical chemist who formulated the equation.
When an oxidizer (Ox) accepts a number z of electrons () to be converted in its reduced form (Red), the half-reaction is expressed as:
Ox + z → Red
The reaction quotient (Qr), also often called the ion activity product (IAP), is the ratio between the chemical activities (a) of the reduced form (the reductant, aRed) and the oxidized form (the oxidant, aOx). The chemical activity of a dissolved species corresponds to its true thermodynamic concentration taking into account the electrical interactions between all ions present in solution at elevated concentrations. For a given dissolved species, its chemical activity (a) is the product of its activity coefficient (γ) by its molar (mol/L solution), or molal (mol/kg water), concentration (C): a = γ C. So, if the concentration (C, also denoted here below with square brackets [ ]) of all the dissolved species of interest are sufficiently low and that their activity coefficients are close to unity, their chemical activities can be approximated by their concentrations as commonly done when simplifying, or idealizing, a reaction for didactic purposes:
At chemical equilibrium, the ratio Qr of the activity of the reaction product (aRed) by the reagent activity (aOx) is equal to the equilibrium constant K of the half-reaction:
The standard thermodynamics also says that the actual Gibbs free energy ΔG is related to the free energy change under standard state ΔG by the relationship:
where Qr is the reaction quotient and R is the ideal gas constant.
Official source
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