Page 18 - Battery Reference Book

Electromotive force 4/3

1 .I Electromotive force produce a current from the solution to the mercury.
This is represented by another arrow, beside which is
A galvanic or voltaic cell consists of two dissimilar placed the potential difference between the electrode
electrodes irnmersed in a conducting material such as and the solution, thus:
a liquid electrolyte or a fused salt; when the two elec-
trodes are connected by a wire a current will flow. Each Z~/N ZnS04/HgzClz in N KCVHg
electrode, in general, involves an electronic (metallic) +
0.281
and an ionic conductor in contact. At the surface of +
separation between the metal and the solution there 1.082
exists a difference in electrical potential, called the
electrode potential. The electromotive force (e.m.f.) Since the total e.m.f. of the cell is 1.082 V, and since
of the cell is then equal to the algebraic sum of the the potential of the calomel electrode is 0.281 V, it
two electrode potentials, appropriate allowance being follows that the potential difference between the zinc
made for the sign of each potential difference as fol- and the solution of zinc sulphate must be 0.801V,
lows. When a metal is placed in a liquid, there is, referred to the normal hydrogen electrode, and this
in general, a potential difference established between must also assist the potential difference at the mercury
the metal and the solution owing to the metal yielding electrode. Thus:
ions to the solution or the solution yielding ions to the
metal. In the former case, the metal will become neg- Z~/N ZnS04/Hg2Clz in N KCVHg
atively charged to the solution; in the latter case, the + +
0.281
metal will become positively charged. 0.801 +
Since the total emf. of a cell is (or can in many 1.082
cases he made practically) equal to the algebraic sum
of the potential differences at the two electrodes, it From the diagram it is seen that there is a tendency
follows that, if the e.m.f. of a given cell and the value for positive electricity to pass from the zinc to the solu-
of the potential difference at one of the electrodes are tion, i.e. the zinc gives positive ions to the solution, and
known. the potential difference at the other electrode must, therefore, itself become negatively charged rel-
can be calculated. For this purpose, use can be made ative to the solution. The potential difference between
of the standard calomel electrode, which is combined zinc and the normal solution of zinc sulphate is there-
with the electrode and solution between which one fore -0.801 V. By adopting the above method, errors
wishes to determine the potential difference. both in the sign and in the value of the potential dif-
In the case of any particular combination, such as ference can be easily avoided.
the following: If a piece of copper and a piece of zinc are placed
in an acid solution of copper sulphate, it is found, by
Z~/N ZnS04/Kg2C12 in N KCI/Hg
connecting the two pieces of metal to an electrometer,
the positive pole of the cell can always be ascertained that the copper is at a higher electrical potential (i.e.
by the way in which the cell must be inserted in the is more positive) than the zinc. Consequently, if the
side circuit of a slide wire potentiometer in order to copper and zinc are connected by a wire, positive
obtain a point of balance, on the bridge wire. To obtain electricity flows from the former to the latter. At the
a point of balance, the cell must be opposed to the same time, a chemical reaction goes on. The zinc
working cell; and therefore, if the positive pole of the dissolves forming a zinc salt, while copper is deposited
latter is connected with a particular end of the bridge from the solution on to the copper.
wire, it follows that the positive pole of the cell in the Zn + CuS04(aq.) = ZnS04(aq.) + Cu
side circuit must also he connected with the same end
of the wire. This is the principle behind many types of electncai
The e.m.f. of the above cell at 18°C is 1.082V and, cell.
from the way in which the cell has to he connected to Faraday’s Law of Electrochemical Equivalents holds
the bridge wire, mercury is found to be the positive for galvanic action and for electrolytic decomposition.
pole; hence, the current must flow in the cell from Thus, in an electrical cell, provided that secondary
zinc to mercury. An arrow is therefore drawn under reactions are excluded or allowed for, the current of
the diagram of the cell to show the direction of the chemical action is proportional to the quantity of elec-
current. and beside it is placed the value of the e.m.f., tricity produced. Also, the amounts of different sub-
-
thus: stances liberated or dissolved by the same amount of
Z~N ZnS04/HgzClz in i~ KCI/Hg electricity are proportional to their chemical equiva-
lents. The quantity of electricity required to produce
1.082 one equivalent of chemical action (i.e. a quantity of
chemical action equivalent to the liberation of I g of
It is also known that the mercury is positive to the hydrogen from and acid) is known as the faraday (F).
solution of calomel, so that the potential here tends to One faraday is equivalent to 96494 ampere seconds