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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 ...
The Nernst–Planck equation is a continuity equation for the time-dependent concentration of a chemical species: where is the flux. It is assumed that the total flux is composed of three elements: diffusion, advection, and electromigration. This implies that the concentration is affected by an ionic concentration gradient , flow velocity , and ...
at constant temperature and pressure, the thermodynamic voltage (minimum voltage required to drive the reaction) is given by the Nernst equation: = / = / where is the Gibbs energy and F is the Faraday constant. The standard thermodynamic voltage (i.e. at standard temperature and pressure) is given by:
Here n e is the number of electrons (in moles), F is the Faraday constant (in coulombs/mole), and ΔE is the cell potential (in volts). Finally, Nernst divided through by the amount of charge transferred to arrive at a new equation which now bears his name: =
Related to the Faraday constant is the "faraday", a unit of electrical charge. Its use is much less common than of the coulomb, but is sometimes used in electrochemistry. [4] One faraday of charge is the charge of one mole of elementary charges (or of negative one mole of electrons), that is, 1 faraday = F × 1 mol = 9.648 533 212 331 001 84 × ...
: Faraday constant: Reaction quotient; This equation describes how the changes in applied potential will alter the concentration ratio. However, the Nernst equation is limited, as it is modeled without a time component and voltammetric experiments vary applied potential as a function of time.
Solving the Nernst equation for the half-reaction of reduction of two protons into hydrogen gas gives: 2 H+ + 2 e− ⇌ H2. In biochemistry and in biological fluids, at pH = 7, it is thus important to note that the reduction potential of the protons ( H +) into hydrogen gas H. 2 is no longer zero as with the standard hydrogen electrode (SHE ...
If the concentrations are the same, = and the Nernst equation is not needed under the conditions assumed here. The value of 2.303 R / F is 1.9845 × 10 −4 V/K , so at 25 °C (298.15 K) the half-cell potential will change by only 0.05918 V/ ν e if the concentration of a metal ion is increased or decreased by a factor of 10.