Page 23 - Battery Reference Book

1/8 Introduction to battery technology

almost infinitely small values, to which it was difficult Table 1.1 The thermionic work
to ascribe any real physical meaning. This difficulty functions of the metals
disappears when it is seen that P does not merely
represent a concentration difference, but includes a Metal Thermionic
term representing the difference of energy of the ions work function
in the two phases, which may be large. (VI
The electrode process has also been investigated Potassium 2.12
using the methods of quantum mechanics. The final Sodium 2.20
equations obtained are very similar to those given Lithium 2.28
above. Calcium 3.20
Magnesium 3.68
Work function at the metal-metal junction Aluminium 4.1
Zinc 3.51
When two dissimilar metals are put in contact there is a Lead 3.95
tendency for negative electricity, i.e. electrons, to pass Cadmium 3.68
from one to the other. Metals have different affinities Iron 4.7
for electrons. Consequently, at the point of junction, Tin 4.38
electrons will tend to pass from the metal with the Copper 4.16
Silver
4.68
smaller to that with the greater affinity for electrons. Platinum 6.45
The metal with the greater affinity for electrons will
become negatively charged and that with the lesser
affinity will become positively charged. A potential the other. The old difficulty that no apparent change
difference is set up at the interface which increases occurred at the metal junction which could contribute
until it balances the tendency of electrons to pass from to the electromotive force of a cell thus disappears.
the one metal to the other. At this junction, as at the It should be noted that the thermionic work function
electrodes, the equilibrium potential difference is that is really an energy change and not a reversible work
which balances the tendency of the charged particle to quantity and is not therefore a precise measure of
move across the interface. the affinity of a metal for electrons. When an electric
By measurements of the photoelectric and thermio- current flows across a junction the difference between
nic effects, it has been found possible to measure the energy liberated in the transfer of electrons and
the amount of energy required to remove electrons the electric work done in passing through the potential
from a metal. This quantity is known as its thermionic difference appears as heat liberated at the junction.
work function and is usually expressed in volts, as the This heat is a relatively small quantity, and the junction
potential difference through which the electrons would potential difference can be taken as approximately
have to pass in order to acquire as much energy as is equal to the difference between the thermionic work
required to remove them from the metal. Thus, if 9 functions of the metals.
is the thermionic work function of a metal, the energy Taking into account the above theory, it is now
required to remove one electron from the metal is e@, possible to view the working of a cell comprising two
where e is the electronic charge. The energy required dissimilar metals such as zinc and copper immersed
to remove one equivalent of electrons (charge F) is in an electrolyte. At the zinc electrode, zinc ions pass
thus +F or 96 500qY4.182 cal. The thermionic work into solution leaving the equivalent charge of electrons
functions of a number of metals are given in Table 1.1. in the metal. At the copper electrode, copper ions are
The energy required to transfer an equivalent of deposited. In order to complete the reaction we have to
electrons from one metal to another is evidently given transfer electrons from the zinc to the copper, through
by the difference between their thermionic work func- the external circuit. The external circuit is thus reduced
tions. Thus, if is the thermionic work function of to its simplest form if the zinc and copper are extended
metal 1 and q5z that of metal 2, the energy required to to meet at the metal junction. The reaction
transfer electrons from 1 to 2 per equivalent is
Zn + CuZi(aq.) = Zn2+(aq.) + cu
AE = ($1 - 42)F (1.15)
occurs in parts, at the various junctions:
The greater the thermionic work function of a metal,
the greater is the affinity for electrons. Thus electrons Zinc electrode:
tend to move from one metal to another in the direction Zn = Zn2+(aq.) + 2e(zn)
in which energy is liberated. This tendency is balanced
by the setting up of a potential difference at the Metal junction:
junction. When a current flows across a metal junction, 2e(Zn) = 2e(Cu)
the energy required to carry the electrons over the Copper electrode:
potential difference is provided by the energy liberated
in the transfer of electrons from the one metal to Cu2+(aq.) + 2e(Cu) = ~u