Electromotive Force Series


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Electromotive Force Series

 

(less commonly, activity series), a series of metals and their ions arranged in order of increasing standard electrode potential in electrolytic solutions. A standard hydrogen electrode is usually chosen as the reference electrode, and the electrode potential of hydrogen in the series is given as zero. In the USSR and many other European countries, the electrode potential of a particular metal is taken to have the same sign as the charge of the metal’s electrode relative to the standard hydrogen electrode. In the USA, however, the opposite sign was used until 1975.

The most negative potentials, approximately - 3 volts (V), are characteristic of the alkali metals, followed by the alkaline-earth metals. The most positive potentials, approximately + 1.5 V, are those of the noble metals. The electromotive force series frequently includes nonmetals, ions, and selected chemical compounds. The order in which the most common metals are arranged in the series is Li, K, Ca, Na, Mg, Al, Mn, Zn, Fe, Co, Ni, Sn, Pb, H2, Cu, Hg, Ag, and Au.

The position of an element in the electromotive force series is somewhat arbitrary inasmuch as the value of the electrode potential depends on the temperature and composition of the solution in which the electrodes are immersed and especially on the activity (or concentration) of the ions of a given substance in the electrolyte. The condition of the surface of the electrode (smooth or rough) is an important factor, in particular the presence of a protective oxide layer.

In scientific literature, the electromotive force series is usually given for aqueous solutions at a temperature 25°C, a pressure of 1 atmosphere for gases, and an activity of 1 for the ions involved in the electrochemical reaction. The relative position of substances in the series may change when the concentrations and solvents are changed, especially when the substances are close to one another in the series.

The consequences that can be predicted from the electromotive force series greatly assist laboratory and industrial work: (1) any metal can drive out (replace) from salt solutions all other metals standing to its right in the series; (2) all metals to the left of hydrogen in the series will displace hydrogen from acids; and (3) the farther apart two metals are in the series, the greater will be the electromotive force generated by a galvanic cell constructed from them.

Since the electromotive force series is based on the thermodynamic properties of electrochemical processes, it permits a determination only of the possibility of a process. The realization of these processes, however, depends to a great extent on kinetic factors.

REFERENCES

Kurs fizicheskoi khimii, vol. 2, 2nd ed. Editor in chief, Ia. I. Gerasimov. Moscow, 1973. Chapter 20.
Pauling, L. Obshchaia khimiia. Moscow, 1974. Chapter 15. (Translated from English.)

M. E. ERLYKINA