The Foundations of Chemistry

The idea of “electron pressure” helps to explain this process. The negative reduction

potential for the half-reaction

Zn^2  2 e88nZn E^0 0.763 V

says that this reaction is less favorablethan the corresponding reduction to H 2 ,

2H 2 e88nH 2 E^0 0.000 V

Before they are connected, each half-cell builds up a supply of electrons waiting to be

released, thus generating an electron pressure. Let us compare these electron pressures

by reversing the two half-reactions to show production of electrons (and changing the

signs of their E^0 values).

Zn88nZn^2  2 e E^0 oxidation0.763 V

H 2 88n2H 2 e E^0 oxidation0.000 V

The process with the more positive E^0 value is favored, so we reason that the electron

pressure generated at the Zn electrode is greater than that at the H 2 electrode. As a result,

when the cell is connected, the electrons released by the oxidation of Zn flow through

the wire from the Zn electrode to the H 2 electrode,where they are consumed by the reduc-

tion of Hions. Oxidation occurs at the zinc electrode (anode), and reduction occurs at

the hydrogen electrode (cathode).

THE COPPER–SHE CELL

Another voltaic cell consists of an SHE in one beaker and a strip of Cu metal immersed

in 1 Mcopper(II) sulfate solution in another beaker. A wire and a salt bridge complete

the circuit. For this cell, we observe the following (Figure 21-10).

1.The initial cell potential is 0.337 volt.

2.Gaseous hydrogen is consumed. The Hconcentration increases in the solution of

the SHE.

3.The mass of the copper electrode increases. The concentration of Cu^2 ions

decreases in the solution around the copper electrode.

Thus, the following half-reactions and cell reaction occur.

E^0

(oxidation, anode) H 2 88n2H 2 e 0.000 V (by definition)

(reduction, cathode) Cu^2  2 e88nCu 0.337 V

(cell reaction) H 2 Cu^2 88n2HCu E^0 cell0.337 V (measured)

The SHE functions as the anodein this cell, and Cu^2 ions oxidize H 2 to Hions. The

standard electrode potential of the copper half-cell is 0.337 volt as a cathodein the

Cu–SHE cell.

Again, we can think of E^0 oxidationin the two half-cells as “electron pressures.”

Cu88nCu^2  2 e E^0 oxidation0.337 V

H 2 88n2H 2 e E^0 oxidation0.000 V

Now the hydrogen electrode has the higher electron pressure. When the cell is connected,

electrons flow through the wire from the hydrogen electrode to the copper electrode. H 2

is oxidized to 2H(anode), and Cu^2 is reduced to Cu (cathode).

21-13

866 CHAPTER 21: Electrochemistry

What we have informally called
“electron pressure” is the tendency to
undergo oxidation. This is formally
expressed as an oxidation potential;
however, we usually tabulate reduction
potentials (Table 21-2).

Recall that in the Zn–SHE cell the
SHE was the cathode.