Physical Chemistry , 1st ed.

The two independent, physical systems that contain the reactions are called

half-cells.The half-cell that contains the oxidation reaction is called the anode,

and the half-cell containing the reduction reaction is called the cathode.The

two half-cells together make up a system that, for a spontaneous reaction, is

called a voltaic cellor galvanic cell.All batteries are voltaic cells, although their

redox chemistry and construction may not be as simple as the battery illus-

trated in Figure 8.3. (The zinc/copper voltaic cell is called a Daniell cell after

the English chemist John Daniell, who developed it in 1836. At the time, it was

the most reliable source of electricity.) Figure 8.4 shows a detailed diagram of

a modern voltaic cell.

Systems in which nonspontaneous reactions are forced to proceed by the

intentional introduction of electrons are called electrolytic cells.Such cells are

used for electroplating metals onto jewelry and metalware, among other uses.

Keep in mind that the calculated value ofGfor an electrochemical process

represents the maximum amount of electric work that the reaction can do. In

reality, less than that maximum is actually extracted. This is a consequence of

the less than 100% efficiency of all processes.

8.4 Standard Potentials

Recall that E, the electromotive force, is originally defined as the difference be-

tween the reduction potential and the oxidation potential. Do we know the ab-

soluteelectromotive force for any individual reduction or oxidation process?

Unfortunately, we don’t. The situation is very much like internal energy, or any

other kind of energy. We understand that there is some absolute amount of

energy in a system, we accept the fact that we can never know exactly how

much energy there is in a system, but we do know that we can follow changes

in the energy of a system. It is the same thing with E.

In order to keep track of the energies of a system, we defined certain stan-

dards, like the heats of formation of compounds, with the recognition that the

heats of formation of elements in their standard states are exactly zero. We do

something similar for electromotive forces. The conventions we use for defin-

ing standard potentialsare as follows:

• We consider the separate half-reactions rather than balanced redox reac-
  tions. This way, any redox reaction can be constructed by algebraically
  combining the appropriate two (or more) half-reactions.


• Typically, we speak of the potential for a half-reaction as that half-reaction
  written as a reductionreaction. When combining two (or more) reac-
  tions, at least one must be reversed to express it as an oxidation reaction.
  When reversing a reaction, the standard potential changes sign.


• For standard potentials, the standard thermodynamic conditions of pres-
  sure and concentration are presumed, and are usually given at the com-
  mon reference temperature of 25°C. That is, if we are using a standard
  potential for a half-reaction, it is assumed that the reaction is occurring
  at 25°C, a fugacity of 1 for gaseous species, and an activity of 1 for dis-
  solved species. (A common approximation is 1 atm or 1 bar for gases and
  1 M for dissolved species.)


• The standard potential for the reduction half-reaction
  2H^ (aq) 2e^ →H 2 (g) (8.23)
  is defined as 0.000 V. This is the reaction of the standard hydrogen elec-
  trode,or SHE (Figure 8.5). All other standard potentials are defined with
  respect to this half-reaction.

8.4 Standard Potentials 215

Figure 8.4 A modern battery is more compli-

cated than a simple Daniell cell, but the electro-

chemical principles are the same.

Figure 8.5 The standard hydrogen electrode.

The half-reaction occurring in this electrode has

been assigned a standard reduction potential of

exactly 0.000 V.

Seal

Carbon rod (cathode)

Moist paste containing

MnO 2 (s), NH 4 Cl (aq)

and an inert filler.

Zinc can (anode)

Insulation

Wire connection

to other half cell

Pt electrode

Glass tube

H^ (aq)

aH^ 1.00

H 2 gas
H 2 1.00 atm