Page 214 - Modern physical chemistry

9

Electrochemistry

9. 1 Definitions and Units
ELECTRIC CHARGE IS TRANSPORTED WITHIN A PHASE by the movement of charged
particles. In a metal, these particles are the valence electrons. In a solution of an elec-
trolyte, they include both cations and anions. In a plasma, the principal charged parti-
cles are electrons and cations.
In the body of a phase, electrical neutrality tends to prevail. At a boundary, however,
a charged double layer may build up, negative on one side, positive on the other. A change
in carrier imposed by a boundary is generally effected by chemical reaction.
Current may be introduced into a given phase through electrodes. We call the nega-
tive electrode the catlwde, the positive electrode the anode. At each electrode, chemical
reactions in which electrons are reactants or products occur. The electron conduction
in one electrode is thus changed to ion conduction in the phase and back to electron
conduction at the other electrode.
The equivalent mass of an electrolyte A. Bv that ionizes to give v +N+ and v_W- is
its formula mass divided by v + z+ (or by -v_;J. When it is completely ionized, an equiv-
alent of the electrolyte yields an equivalent of cations carrying one mole positive charge
and an equivalent of anions carrying one mole negative charge. A mole of positive charge
is called thejaraday F.
If Q is the charge carried by n equivalents of an ion, we have
IQI
n=-. [9.1 ]
F
So charge Q passing through an electrode causes n equivalents of chemical reaction
there. By measuring such reaction, one finds that

F = 96,485.3 C mor l [9.2]
Charge is related to force through Coulomb's law (8.51). So transporting charge between
differing points generally requires work. The points are said to be at different electric
potentials. The joules of work needed to take one coulomb from one point to another
gives the volts of potential existing between the points. The unit of potential is the volt.
The coulombs Q of charge passing a given cross section of a conductor per second
yield the electric current I there in amperes. We have
1= Q. [9.3]
t
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