Page 27 - Battery Reference Book

1/12 Introduction to battery technology
as in the case of a gas electrode. The oxidized and has an e.m.f. of l.lOV, and by the convention its sign
reduced states are not necessarily ionic. For example, is positive. This means that when the cell operates
an important type of reversible electrode involves the oxidation occurs at the left-hand electrode; that is to
organic compound quinone, together with hydrogen say, metallic zinc atoms are being oxidized to form
ions, as the oxidized state, with the neutral molecule zinc ions in solution, i.e.
hydroquinone as the reduced state. Electrodes of the
kind under consideration, consisting of conventional Zn = Zn2+ + 2e
oxidized and reduced forms, are sometimes called oxi-
dation-reduction electrodes; the chemical reactions At the right-hand electrode there must, therefore, be
taking place at these electrodes are either oxidation reduction of the cupric ions, from the copper sulphate
of the reduced state or reduction of the oxidized state solution, to copper atoms, i.e.
of the metal ion M: cu2+ + 2e = cu
M2+ + M4+ + 2e
The electrons liberated at the zinc electrode travel
depending on the direction of the current. In order that along the external connecting circuit and are available
the electrode may behave reversibly it is essential that for the discharge (reduction) of the cupric ions at the
the system contain both oxidized and reduced states. copper electrode. The complete cell reaction, obtained
The three types of reversible electrodes described by adding the separate electrode reactions, is conse-
above differ formally as far as their construction is quently:
concerned; nevertheless, they are all based on the
same fundamental principle. A reversible electrode Zn + cu2+ = zn2+ + cu
always involves an oxidized and a reduced state,
using the terms ‘oxidized’ and ‘reduced’ in their Since two electrons are involved for each zinc (or
broadest sense; thus, oxidation refers to the liber- copper) atom taking part in the reaction, the whole
ation of electrons while reduction implies the tak- process as written, with quantities in gram-atoms or
ing up of electrons. If the electrode consists of a gram-ions, takes place for the passage of 2F of elec-
metal M and its ions M+, the former is the reduced tricity.
state and the latter is the oxidized state; similarly, The practical convention, employed in connection
for an anion electrode, the A- ions are the reduced with cells for yielding current, is to call the ‘negative’
state while A represents the oxidized state. It can pole the electrode at which the process is oxidation
be seen, therefore, that all three types of reversible when the cell is producing current; the ‘positive’ elec-
electrode are made up from the reduced and oxi- trode is the one at which reduction is the spontaneous
dized states of a given system, and in every case process. The reason for this is that oxidation is accom-
the electrode reaction may be written in the general panied by the liberation of electrons, and so the elec-
form trode metal acquires a negative charge; similarly, the
Reduced state + Oxidized state + ne reduction electrode will acquire a positive charge,
because electrons are taken up from it. According to
where n is the number of electrons by which the the widely used convention, the e.m.f. of a cell is pos-
oxidized and reduced states differ. itive when it is set up in such a way that the negative
A reversible electrode consists of an oxidized and (oxidation) electrode is to the left, and the positive
a reduced state, and the reaction which occurs at (reduction) electrode is to the right.
such an electrode, when it forms part of an oper-
ating cell, is either oxidation (i.e. reduced state +
oxidized state + electrons) or reduction (i.e. oxidized 1.4 Relationship between electrical
state + electrons -+ reduced state). It can be readily energy and energy content of a cell
seen, therefore, that in a reversible cell consisting of
two reversible electrodes, a flow of electrons, and It may be asked what is the relation between the
hence a flow of current, can be maintained if oxida- electrical energy produced in a cell and the decrease
tion occurs at one electrode and reduction at the other. in the energy content of the system, as a result of
According to the convention widely adopted, the e.m.f. the chemical reaction going on therein. Considering
of the cell is positive when in its normal operation oxi- only cells working at constant (atmospheric) pressure,
dation takes place at the left-hand electrode of the cell when a chemical reaction occurs at constant pressure,
as written and reduction occurs at the right-hand elec- without yielding any electrical energy, the heat evolved
trode. If the reverse is the case, so that reduction is is equal to the decrease in the heat content of the
taking place at the left-hand electrode, the e.m.f. of system. In 1851, Kelvin made the first attempt to
the cell, by convention, will have a negative sign. answer the question, by assuming that in the cell the
The Daniel1 cell, represented by whole of the heat of reaction appeared as electrical
energy, i.e. the electrical energy obtained is equal to
Zn 1 MZnS04(soln) MCuS04(soln) 1 cu the decrease in the heat content of the system. This was