Page 24 - Battery Reference Book

Reversible cells 1/B

If the circuit is open, at each junction a potential a cell, if the true thermodynamic e.m.f. is required,
difference arises which just balances the tendency for it is necessary to use a type of measuring equipment
that particular process to occur. When the circuit is that draws a zero or infinitesimally small current from
closed there is an electromotive force in it equal to the cell at the point of balance. The e.m.f. obtained
the sum of all the potential differences. Since each in this way is as close to the reversible value as
potential difference corresponds to the net work of one is experimentally possible. If an attempt is made to
part of the reaction, the whole electromotive force is determine the e.m.f. with an ordinary voltmeter, which
equivalent to the net work or free energy decrease of takes an appreciable current. the result will be in error.
the whole reaction. In practical battery situations, the e.m.f. obtained is
not the thermodynamic value that would be obtained
1.2 Reversible cells for a perfectly reversible cell but a non-equilibrium
value which for most purposes suffices and in many
During the operation of a galvanic cell a chemical instances is, in fact, close to the value that would have
reaction occurs at each electrode, and it is the energy been obtained under equilibrium conditions.
of these reactions that provides the electrical energy One consequence of drawing a current from a cell
of the cell. If there is an overall chemical reaction, which is more than infinitesimally small is that the cur-
the cell is referred to as a chemical cell. In some rent obtained would not be steady but would decrease
cells, however, there is no resultant chemical reaction, with time. The cell gives a steady current only if the
but there is a change in energy due to the transfer of current is very low or if the cell is in action only
solute from one concentration to another; such cells are intermittently. The explanation of this effect, which
called ‘concentration cells’. Most, if not all, practical is termed ’polarization’, is simply that some of the
commercial batteries are chemical cells. hydrogen bubbles produced by electrolysis at the metal
In order that the electrical energy produced by a cathode adhere to this electrode. This results in a two-
galvanic cell may be related thermodynamically to the fold action. First, the hydrogen is an excellent insulator
process occurring in the cell, it is essential that the and introduces an internal layer of very high elec-
latter should .behave reversibly in the thermodynamic trical resistance. Secondly, owing to the electric field
sense. A reversible cell must satisfy the following present, a double layer of positive and negative ions
conditions. If the cell is connected to an external source forms on the surface of the hydrogen and the cell actu-
of e.m.f. which is adjusted so as exactly to balance the ally tries to send a current in the reverse direction or a
e.m.f. of the cell, i.e. SQ that no current flows, there back e.m.f. develops. Clearly, the two opposing forces
should be no chemical or other change in the cell. If eventually balance and the current falls to zero. These
the external e.m.f. is decreased by an infinitesimally consequences of gas production at the electrodes are
small amount, current will flow from the cell, and a avoided, or at least considerably reduced, in practical
chemical or other change, proportional in extent to the batteries by placing between the positive and nega-
quantity of electricity passing, should take place. On tive electrodes a suitable inert separator material. The
the other hand. if the external e.m.f. is increased by separators perform the additional and, in many cases,
a very small amount, the current should pass in the more important function of preventing short-circuits
opposite direction, and the process occurring in the between adjacent plates.
cell should be exactly reversed. A simple example of a primary (non-rechargeable)
It may be noted that galvanic cells can only be reversible cell is the Daniell cell, consisting of a zinc
expected to behave reversibly in the thermodynamic electrode immersed in an aqueous solution of zinc
sense, when the currents passing are infinitesimally sulphate, and a copper electrode in copper sulphate
small; so that the system is always virtually in equi- solution:
librium. If large currents flow, concentration gradi-
ents arise within the cell because diffusion is rela- Zn 1 ZnSO4(soln) j CuS04(soln) j Cu
tively slow; i.n these circumstances the cell cannot be
regarded as existing in a state of equilibrium. This the two solutions being usually separated by a porous
would apply to most practical battery applications partition. Provided there is no spontaneous diffu-
where the currents drawn from the cell would be more sion through this partition, and the electrodes are not
attacked by the solutions when the external circuit is
than infinitesimal. Of course, with a given type of open, this cell behaves in a reversible manner. If the
cell, as the current drawn is increased the departure
from the equilibrium increases also. Similar comments external circuit is closed by an e.1n.f. just less than that
apply during the charging of a battery where current is of the Daniell cell, the chemical reaction taking place
supplied and the cell is not operating under perfectly in the cell is
reversible conditions. Zn + cu2+ = Zn2+ + cu
If this charging current is more than infinitesimally
small, there i,s a departure from the equilibrium state i.e. zinc dissolves from the zinc electrode tQ form zinc
and the cell is; not operating perfectly reversibly in the ions in solution, while copper ions are discharged and
thermodynamic sense. When measuring the e.m.f. of deposit copper on the other electrode. Polarization is