8.3 Half-Cell Potentials and Cell Potentials 363
HCI
solutionHCI
solutionAgPt Pt H 2 Pt PtFigure 8.5 A Double Cell Equivalent to the Cell of Figure 8.4.The standard-state cell potential difference of this cell is− 0 .2223 V, the negative of that
of the cell of Figure 8.2. Hydrogen gas at the same pressure is fed into both hydrogen
electrodes and the two HCl solutions are at the same concentration. A wire is connected
between the two hydrogen electrodes and maintains them at the same electric potential.
We can writeE(double cell)φ(R,right cell)−φ(L, left cell)
φ(R,right cell)−φ(L, right cell)+φ(R,left cell)−φ(L,left cell)
(8.3-3)The second equality follows from the fact that the two hydrogen electrodes have the
same value ofφ.E◦(double cell)E◦(right cell)+E◦(left cell)
0 .268 V+(− 0 .2223 V) 0 .046 V (8.3-4)The voltage of the single cell in Figure 8.4 is also 0.046 V, because the state of the left
electrode of the left cell in Figure 8.5 is the same as the state of the left electrode of
the cell in Figure 8.4 and the state of the right electrode of the right cell in Figure 8.5
is the same as the state of the right electrode of the cell in Figure 8.4.
We now adopt the following definition for the standard reduction potential:The
standard-state potential difference of a cell consisting of a hydrogen electrode on the
left and any other electrode on the right is called the standard reduction potential of
the right electrode or of the right half-cell. It is also sometimes called the standard
half-cell potential or standard electrode potential.This convention assigns the value
zero to the standard reduction potential of the hydrogen electrode.
The standard-state potential difference of the cell of Figure 8.4 can now be writtenE◦E◦(right half-cell)−E◦(left half-cell) (8.3-5)There is now a subtraction instead of an addition because the standard reduction poten-
tial of the left cell corresponds to reversing the left cell from its configuration in the