Page:The New International Encyclopædia 1st ed. v. 06.djvu/895

* liLECTRO-CHEMISTRY. 79 ELECTRO-CHEMISTRY. mercuric sulphate and a zinc electrode in a normal sol;ition of the sulphate of zinc, the two solutions separated by a porous partition and the two metals connected by a vire. By direct deteniiination. the electro-motive force of tins cell may be shown to be 1.514 volts: and as the potentialdillercnce between the mercury and its solution amounts to 0.99 volt, it is evident that the potentialditrerence between zinc and the normal solution of its sulphate must be 0.524 volt. In this calculation, the slight potential- difierence between the two solutions has been ne- glected. In calculating the figures of the following table, however, that difference, too. has been taken into account ; and so the figure for zinc, 0.51, is somewhat more precise than our figure 0.524. The table shows the potential-differences between metals and their salt solutions of normal concentration, the plus sigu showing that the solution is electro-positive with regard to its metal, and the minus sign showing that the solution is electronegative -nith regard to its metal. Magnesium + 1.22 Iron + 0.06 Copiier ..—0.60 Zinc -fO.51 Sickel — 0-02 Mercury... ..-0.99 Aluminium + 0.22 Lead — 0.10 SUver .. — 1.01 Cadmium... + 0.19 H.vdrogen — 0.25 The series is .similar to part of the old electro- chemical order of Berzelius. (See historical sec- tion under Chejiistrt. 1 Only, again, while that order was purely qualitative, the modern series represents an exact qualitative expression of the electrical properties of metals and besides refers to a definite concentration of their solutions. Since the potential-difference between a metal and a solution depends upon the solution-tension of the metal and the osmotic pressure of its ions in the solution, and since both the potential- difference and the osmotic pressure can be deter- mined, it ia evidently possible to find also the solution-tensions. The following table shows the results of such calculation.s based on the table given above: Magnesium . Zinc Aluminium.. Cadmium.... . 10' « . ]U>* . 10" . 10" Iron Sickel Lead H.vdrogen.. ...10> ...lO-o ...10-= ..10-* Copper Mercury.... Silver - lo-;; .v. IS-- The figures represent the solution-tensions in terms of atmospheres and may be changed to pounds per square inch by multiplying by 15. The solution-tension of iron, for example, equals a pressure of 15.000 pounds per square inch. The enormous tension of magnesium may serve to indicate how great may be the electrostatic counter-forces long before the amount of metal in solution has become appreciable. Electrolysis. The electro-chemical princi- ples developed above, together with the theory of electrolytic dissociation (see Dissocltion) explain the mechanism of electrolysis without any difficulty. We have seen above that in a vol- taic cell chemical changes are used to produce electrical energy: in electrolysis, on the contrary, an electric current from some outside source is passed through the given system and causes chemical changes to take place in it. The laws of electrolysis may be found stated in the article Electricity. ^ Traxsfor-mation of Energy. The term 'chem- ical alTmity' is now often used, not only in con- VoL. VI —50. nection with compounds, but also in connection with their transformations. It is a fundamental principle in modern exact science, that any change whatever that takes place in a given system without the supply of energy from without is capable of yielding mechanical work. Thus, a weight can be lifted by the expansion of a sufficiently compressed gas, and of course the greater the weight, the greater the work done. The greater the work done, the more the gas is cooled ; for the work is done at the expense of the energy of the gas. If the same gas was allowed to expand the same amount in vacuo (i.e. without having to over- come any resistance ), no work would be done, and therefore no fall of temperature would be observed; in other words, the total energy of the gas would remain unchanged: yet its capacity for doing work would obviousij' be diminished owing to the diminution of pressure. Physicists therefore distinguish between the total energy of s.vstems and their free energy, the latter repre- senting that part of the total energy which can be transformed into mechanical work when the system undergoes a spontaneous change. Evi- dently, the maximum work that can thus be obtained measures the free energy of the system: and as the free energy represents the 'driving power' that causes the change, it is evident that we have obtained an exact measure of that cause when we have measured the maximum work that can be yielded by the change. In the case of chemical transformations, it is that same "driving power' which is now often referred to as the 'chemical affinity of reactions.' In the case of voltaic cells, the free chemical energ- may be entirely transformed into electrical energy. If the cell is perfect, i.e. if it yields really the maximum of electrical energy that can be obtained from its chemical changes, then evi- dently the electrical energy measures the affinity of the chemical changes: for the electrical energii- of a cell can be entirely trans- formed into mechanical work. Xow. we have seen in the introductory section of this article that electrical energy is a product of two factors, quantity of electricity and potential-difference. The quantity of electricity passing through a cell is independent of the nature of the ions, and is proportional to the number of chemical equiva- lents of the ions entering or leaving the solution (Faraday's law), each chemical equivalent (in grams) carrying 96,540 coulombs of electricity. On the other hand, the total potential-difference of a cell is measured by its electro-motive force, and can be readily determined by the use of some 'standard' cell. It is, therefore, clear that the electrical energy of an electrochemical process can be readily ascertained, and that the chemical affinity of any reaction in which acids, bases, or salts can be caused to take part is propoitional to. and hence is measured by. the electromotive force of a voltaic cell based on the reaction. It is further clear that, on the contrary, whenever the maximum mechanical work that can be yielded by a giveh transformation can be ascer- tained without the aid of an electric process, the electro-motive force of a cell based on the tr.ansformation can be calculated beforehand. One more important point claims attention before we dismiss the subject of the energy- changes of voltaic cells. Ve have seen above that the chemical reactions of a cell can also take