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Nernst Equation

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Nernst Equation - Lesson Summary

Cell concentrations are concentration dependent.

The Nernst equation useful to calculate cell potential at non-standard conditions.

Ecell = E°cell - RT/nF lnQ

Ecell = Cell potential at non-standard conditions
E°    = Cell potential at standard conditions
    R = Universal gas constant, 8.314 JK-1mol-1
    T  = Temperature in Kelvin
    n  = Number of electrons transferred in the equation
    F  = Faraday's constant, 96,487 C mol-1
    Q = Reaction quotient

For a reaction under normal conditions,

aA + bB → cC + dD

Q = ([C]c[D]d)/([A]a[B]b)

Ecell = E°cell - RT/nF lnQ

Ecell = E°cell - 0.0592/n lnQ

The cell voltage of a Daniel cell running with 0.1 molar zinc sulphate and 0.5 molar copper (II) sulphate at 25 0C.

n = 2, [Zn] = [Cu] = 1

Ecell = E°cell - 0.0592/n ln([Zn2+]/[Cu2+])

Ecell = E°cell - 0.0592/n ln([0.1]/[0.5])

Ecell = E°cell - 0.0592/n ln 0.2

Ecell = 1.12 V

Appilcations of Nernst Equation:

The Nernst equation can be used to calculate Ecell at different concentrations.

Compared to standard conditions, Ecell will increase as the reactant ion concentration increases (Q decreases).

Compared to standard conditions, Ecell will decrease if the product ion concentration increases (as Q increases).

Concentration cells can be established.

pH meters are also work on the principle of Nernst equation.

The Gibbs free energy can be calculated from Ecell.

The equilibrium constant can be calculated from Ecell.


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