The Foundations of Chemistry

By international convention, the standard potentials of electrodes are tabulated for reduc-

tion half-reactions.These indicate the tendencies of the electrodes to behave as cathodes

toward the SHE. Electrodes with positive E^0 values for reduction half-reactions act as

cathodesversus the SHE. Those with negative E^0 values for reduction half-reactions act

as anodes versus the SHE.

Electrodes with Positive E^0 reduction Electrodes with Negative E^0 reduction

Reduction occurs more readilythan the Reduction is more difficultthan the

reduction of 2Hto H 2. reduction of 2Hto H 2.

Electrode acts as a cathodeversus the Electrode acts as an anodeversus

SHE. the SHE.

The more positive the E^0 value for a half-reaction, the greater the tendency for the

half-reaction to occur in the forward direction as written. Conversely, the more

negative the E^0 value for a half-reaction, the greater the tendency for the half-

reaction to occur in the reverse direction as written.

Table 21-2 lists standard reduction potentials for a few elements.

1.The species on the leftside are all cations of metals, hydrogen ions, or elemental

nonmetals. These are all oxidizing agents(oxidized formsof the elements). Their

strengths as oxidizing agents increase from top to bottom, that is, as the E^0 reduction

values become more positive. Fluorine, F 2 , is the strongest oxidizing agent, and Li

is a very weak oxidizing agent.

2.The species on the rightside are free metals, hydrogen, or anions of nonmetals.

These are all reducing agents(reduced formsof the elements). Their strengths as

reducing agents increase from bottom to top, that is, as the E^0 reductionvalues become

more negative. Metallic Li is a very strong reducing agent, and Fis a very weak

reducing agent.

The more positive the reduction potential, the stronger the species on the left is as

an oxidizing agent and the weaker the species on the right is as a reducing agent.

USES OF STANDARD ELECTRODE POTENTIALS

The most important application of electrode potentials is the prediction of the spontaneity

of redox reactions. Standard electrode potentials can be used to determine the spontaneity

of redox reactions in general, whether or not the reactions can take place in electro-

chemical cells.

Suppose we ask the question: At standard conditions, will Cu^2 ions oxidize metallic

Zn to Zn^2 ions, or will Zn^2 ions oxidize metallic copper to Cu^2 ? One of the two

possible reactions is spontaneous, and the reverse reaction is nonspontaneous. We must

determine which one is spontaneous. We already know the answer to this question from

experimental results (see Section 21-9), but let us demonstrate the procedure for predicting

the spontaneous reaction.

21-15

868 CHAPTER 21: Electrochemistry

The oxidizing agentis reduced.

The reducing agentis oxidized.