Untitled - Kelly Walsh High School

272 CHEMISTRY FOR THE UTTERLY CONFUSED
sign of the standard half-cell potential and add it to the silver half-reaction. We
must multiply the silver half-reaction by two to equalize electron loss and gain,
but the half-cell potential is not:
Ni(s) l Ni2 (aq) 2 e E 0.25 V
2(Ag (aq) e l Ag(s)) E 0.80 V
Ni(s) 2 Ag (aq) l Ni2 (aq) Ag(s) Ecell 1.05 V
18-4 Nernst Equation
Thus far, we have based all of our calculations on the standard cell potential or
standard half-cell potentials—that is, standard state conditions. However, many
times the cell is not at standard conditions—commonly the concentrations are
not 1 M. We may calculate the actual cell potential, E cell, by using the Nernst
equation:
Ecell Ecell –a RT
nF b ln Q Ecell a0.0592 n blog Q at 25C
R is the ideal gas constant, T is the Kelvin temperature, n is the number of electrons
transferred, F is Faraday’s constant, and Q is the activity quotient. The
second form, involving the log Q, is the more useful form. If you know the cell
reaction, the concentrations of ions, and the E cell, then you can calculate the
actual cell potential. Another useful application of the Nernst equation is in the
calculation of the concentration of one of the reactants from cell potential
measurements. Knowing the actual cell potential and the E cell, allows you to
calculate Q, the activity quotient. Knowing Q and all but one of the concentrations,
allows you to calculate the unknown concentration. Another application
of the Nernst equation is concentration cells. A concentration cell is an electrochemical
cell in which the same chemical species are used in both cell compartments,
but differing in concentration. Because the half reactions are the
same, the E cell 0.00 V. Then simply substituting the appropriate concentrations
into the activity quotient allows calculation of the actual cell potential.
When using the Nernst equation on a cell reaction in which the overall reaction
is not supplied, only the half-reactions and concentrations, there are two equivalent
methods to work the problem. The first way is to write the overall redox
reaction based upon E values and then apply the Nernst equation. If the E cell
turns out to be negative, it indicates that the reaction is not a spontaneous one
(an electrolytic cell) or that the reaction is written backwards if it is supposed
to be a galvanic cell. If it is supposed to be a galvanic cell, then all you need to