1.1 What is Chemistry?

23.2. Cell Potential http://www.ck12.org

FIGURE 23.9

(A) The standard hydrogen half-cell is paired with a Cu/Cu^2 +half-cell. H 2 is oxidized, while Cu^2 +is reduced. (B)

The standard hydrogen half-cell is paired with a Zn/Zn^2 +half-cell. Zn is oxidized, while H+is reduced.

The standard reduction potential for the Cu^2 +|Cu half-cell is thus equal to +0.34 V. In a similar way, the reduction
potential for any half-cell can be determined by connecting it to a SHE and measuring the voltage. Notice the line
notation used for the copper half-cell, Cu^2 +|Cu, which indicates the phase boundary between the aqueous copper
cations and the neutral copper metal atoms. This line notation is used as a standard to distinguish between the
components of the electric cell.

When a standard hydrogen half-cell is connected to a standard zinc half-cell (Figure23.9 (B)), the measured voltage
is 0.76 V. However, it is observed that the zinc electrode is oxidized to zinc ions while the hydrogen ion is reduced
to hydrogen gas.

Oxidation: Zn(s)→Zn^2 +(aq)+2e−

Reduction: 2H+(aq)+2e−→H 2 (g)

Overall: Zn(s)+2H+(aq)→Zn^2 +(aq)+H 2 (g)

The SHE is now the cathode, while the zinc electrode is the anode. Now it is the E° for the oxidation half-cell that
is the unknown in the standard cell potential equation.

E◦cell=E◦red−E◦oxid

0 .76 V= 0 .00 V−E◦Zn

E◦Zn= 0 .00 V− 0 .76 V=− 0 .76 V

The standard reduction potential for the Zn^2 +/Zn half-cell is equal to−0.76 V. A negative standard reduction
potential for a particular species means that is easier to reduce H+than to reduce that species. A positive standard
reduction potential for a species means that it reduces more easily than H+. TheTable23.2 lists many reduction