TEO. NSEOR MATION or ENERGY. The term 'chem ical affinity' is now often used, not only in eon neetion 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 wit hout is capable of yielding 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; fur 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 cacao (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 g:cs Nvould remain unchanged: yet its capacity fur doing work would obviousiy he diminished owing to the diminution of pressure. Physicists therefore distinguish between the total energy of systems 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 transformation-, 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 energy may he 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 energy of a cell can be entirely trans formed into mechanical work. Now. 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 electromotive force, and can he readily determined by the use of some 'standard' cell. It is, therefore, clear that the electrical energy of an ciectro-chemical process can he readily ascertained, and that the chemical affinity of any reaction in which acids, bases, or salts can he caused to take part is proportional to, and hence is measured by, the electromotive force of a voltaic cell based on the reaction. It is further (dear 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 transformation can he calculated beforehand.
One more important point claims attention before we dismiss the subject of the energy changes of voltaic cells. We have seen above that the chemical reactions of a cell can also take place without producing electrical energy. Thus, the traction of a Daniell cell can be produced by simply immersing a bar of zinc into a solution of copper sulphate. In that case the change is accompanied by the evolution of a definite amount of heat. The question therefore arises, Can all that 'heat of the reaction' be transformed into electrical energy or not At one time this question Rats answered in the affirmative ('Thom son's law'). But Willard Gibbs and Helmholtz have proved thermodynamically, and it has since been demonstrated experimentally. that the elec trical energy of a cell need not V any means equal the heat of the reaction: that. on the con trary, the latter is generally greater than the former. so that even in perfect cells (producing a maximum of electrical energy) a rise of tem perature might generally be observed, See THERMo•CHEMISTRY.