Nernst Equation - Expression

Expression

The two (ultimately equivalent) equations for these two cases (half-cell, full cell) are as follows:

E_\text{red} = E^{\ominus}_\text{red} - \frac{RT}{zF} \ln\frac{a_\text{Red}}{a_\text{Ox}} (half-cell reduction potential)
E_\text{cell} = E^{\ominus}_\text{cell} - \frac{RT}{zF} \ln Q (total cell potential)

where

  • Ered is the half-cell reduction potential at the temperature of interest
  • Eored is the standard half-cell reduction potential
  • Ecell is the cell potential (electromotive force)
  • Eocell is the standard cell potential at the temperature of interest
  • R is the universal gas constant: R = 8.314 472(15) J K−1 mol−1
  • T is the absolute temperature
  • a is the chemical activity for the relevant species, where aRed is the reductant and aOx is the oxidant. aX = γXcX, where γX is the activity coefficient of species X. (Since activity coefficients tend to unity at low concentrations, activities in the Nernst equation are frequently replaced by simple concentrations.)
  • F is the Faraday constant, the number of coulombs per mole of electrons: F = 9.648 533 99(24)×104 C mol−1
  • z is the number of moles of electrons transferred in the cell reaction or half-reaction
  • Q is the reaction quotient.

At room temperature (25 °C), RT/F may be treated like a constant and replaced by 25.693 mV for cells.

The Nernst equation is frequently expressed in terms of base 10 logarithms (i.e., common logarithms) rather than natural logarithms, in which case it is written, for a cell at 25 °C:

E = E^0 - \frac{0.05916\mbox{ V}}{z} \log_{10}\frac{a_\text{Red}}{a_\text{Ox}}.

The Nernst equation is used in physiology for finding the electric potential of a cell membrane with respect to one type of ion.

Read more about this topic:  Nernst Equation

Famous quotes containing the word expression:

    “We can see nothing whatever of the soul unless it is visible in the expression of the countenance; one might call the faces at a large assembly of people a history of the human soul written in a kind of Chinese ideograms.”
    —G.C. (Georg Christoph)

    “You have to begin to lose your memory, if only in bits and pieces, to realize that memory is what makes our lives. Life without memory is no life at all, just as an intelligence without the possibility of expression is not really an intelligence. Our memory is our coherence, our reason, our feeling, even our action. Without it, we are nothing.”
    —Luis Buñuel (1900–1983)

    “No man’s thoughts are new, but the style of their expression is the never-failing novelty which cheers and refreshes men. If we were to answer the question, whether the mass of men, as we know them, talk as the standard authors and reviewers write, or rather as this man writes, we should say that he alone begins to write their language at all.”
    —Henry David Thoreau (1817–1862)