Nernst Equation

Definition - What does Nernst Equation mean?

The Nernst equation defines the relationship between cell potential to standard potential and to the activities of the electrically active (electroactive) species. It relates the effective concentrations (activities) of the components of a cell reaction to the standard cell potential.

The equation can be used to calculate the cell potential at any moment during a reaction at conditions other than the standard state. In corrosion studies, the equation is used to analyze concentration cells and in the construction of Pourbaix diagrams.

Corrosionpedia explains Nernst Equation

The Nernst equation is a mathematical description of ideal pH electrode behavior and correlating chemical energy as well as the electric potential of a galvanic cell or battery. It shows the relationship between the potential of a half cell or full cell at any point in time and the standard electrode potential, activity, temperature, reaction quotient of the species used and the underlying conditions.

When a redox reaction occurs in a galvanic cell, the concentration of the reactants decreases as they are consumed while the concentration of the products increase due to more product formation, and the cell potential decreases until equilibrium is reached and a zero cell potential is reached.

During the reaction, the Nernst equation can be used to determine the cell potential at any instant and conditions different from the standard state.

    Ecell = E0cell - (RT/nF)lnQ

Where:

    Ecell = cell potential under nonstandard conditions (V)
    E0cell = cell potential under standard conditions
    R = gas constant, which is 8.31 (volt-coulomb)/(mol-K)
    T = temperature (kelvin), which is generally 298°K (77°F/25°C)
    n = number of moles of electrons exchanged in the electrochemical reaction (mol)
    F = Faraday's constant, 96500 coulombs/mol
    Q = reaction quotient, which is the equilibrium expression with initial concentrations rather than equilibrium concentrations

The equation can be rearranged to give ln Kc = nFE/RT where Kc is the equilibrium constant at the equilibrium state. The equilibrium potential is dependent on temperature and concentration of reaction partners.

The Nernst equation used in for:

  • Accurate determination of equilibrium constants
  • Determining voltage and concentration of a component of an electrochemical cell
  • Calculating the potential developed by a concentration cell (in corrosion)
  • Construction of Pourbaix diagram showing the equilibrium potential between a metal and its various oxidized species as a function of pH

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