The standard cell potential (E°) applies to cells where all substances are present in their standard state (1 mol L–1 for solutes and 100 kPa for gases). The cell potential (E) for cells having substances in nonstandard concentrations or pressures can be calculated using the the Nernst equation (1).
(1)
RT/F = 0.0257 V at 298 K
z = the number of electrons transferred from reductant to the oxidant in the cell reaction
Q is reaction quotient for the cell reaction into which the nonstandard concentrations and pressures are substituted.Q depends on the equation for the cell reaction according to the guidelines below.
Using the Nernst equation to predict the direction of change in cell potential when the concentration of one of the substances appearing in Q is changed.
An increase in concentration of a product (C or D) leads to a more negative cell potential.
This change makes the negative term in the Nernst equation larger. This makes the cell potential more negative.
Looked at from a different perspective:
This change reduces the tendency for the forward reaction to occur because the concentrations of products are higher.
An increase in the concentration of a reactant (A or B) leads to a more positive cell potential.
This change makes the negative term in the Nernst equation smaller, and thus the cell potential is more positive.
Looked at from a different perspective:
This change increases the tendency for the forward reaction to occur to consume the added A and B.
| E = E° – | RT | ln Q |
| zF |
(1)
RT/F = 0.0257 V at 298 K
z = the number of electrons transferred from reductant to the oxidant in the cell reaction
Q is reaction quotient for the cell reaction into which the nonstandard concentrations and pressures are substituted.Q depends on the equation for the cell reaction according to the guidelines below.
The general expression is given for the reaction below where A, B, C and D are all solutes or gases.
aA + bB
cC + dD
a, b, c and d are the numbers required to balance the equation
!!!Note that some substances appearing in the reaction equations do NOT appear in the reaction quotient.
Solids and pure liquids do not appear because, provided some is present, the amount of these does not affect the concentrations of solutes in solutions in which they are in contact.
Solvents, like H2O for reactions in aqueous solution, do not appear because they are present in large excess, and their concentration does not change as a consequence of reaction
aA + bB
cC + dDa, b, c and d are the numbers required to balance the equation
| Q = | [C]c[D]d |
| [A]a[B]b |
!!!Note that some substances appearing in the reaction equations do NOT appear in the reaction quotient.
Solids and pure liquids do not appear because, provided some is present, the amount of these does not affect the concentrations of solutes in solutions in which they are in contact.
Solvents, like H2O for reactions in aqueous solution, do not appear because they are present in large excess, and their concentration does not change as a consequence of reaction
Using the Nernst equation to predict the direction of change in cell potential when the concentration of one of the substances appearing in Q is changed.
An increase in concentration of a product (C or D) leads to a more negative cell potential.
This change makes the negative term in the Nernst equation larger. This makes the cell potential more negative.
Looked at from a different perspective:
This change reduces the tendency for the forward reaction to occur because the concentrations of products are higher.
An increase in the concentration of a reactant (A or B) leads to a more positive cell potential.
This change makes the negative term in the Nernst equation smaller, and thus the cell potential is more positive.
Looked at from a different perspective:
This change increases the tendency for the forward reaction to occur to consume the added A and B.