Digression on the useful work of chemical reaction

It follows from what has been said in 2 4 that the maximum amount of useful work which might be obtained from, say, the combustion of carbon will be obtained when it is converted into that oxide, namely, carbon dioxide, which has the lowest value of G. It is also evident that the maximum work will be obtained if the carbon dioxide is released, at the completion of the combustion, at the same temperature, Tq, and the same pressure, p, as that of the atmosphere. For if it is released at some other temperature, T, then useful work has been lost which could have been obtained by heating or cooling the ccurbon dioxide from T to To. by using it as a heat reservoir to drive a heat engine with the atmosphere as the second reservoir. Similarly, if it leaves the combustion S3 tem at a pressure which is either smaller or greater than Po then useful work has been lost which could have been obtained by allowing the pressure to equalize with that of the atmosphere. Therefore, if we use fuel which is initially at atmospheric pressure and temperature (say 20 the maximum useful work which may be obtained is the decrease of Gf in the process  

In order to achieve direct combination of carbon and oxygen at an appreciable speed it is necessary, of course, to operate at an elevated temperature. However, this does not affect the issue, because it was shown in 2 4 that the maximum useful work is independent of whether or not the system passes through intermediate temperatures different from 20 C. The real difficulty is to achieve reversibility in the reaction process itself. At any chosen operating temperature the partial pressure of oxygen and carbon dioxide above the carbon, at equilibrium, are in a constant ratio as determined by the equilibrium constant 

Whenever the ratio differs from its equilibrium value the reaction proceeds irreversibly and there is a decrease in the output of work. 

For certain types of recM tion it is possible to obtain a direct conversion of chemical energy into electricity and thereby into mechanical work. Processes of heat transfer are thereby avoided and the conversion factor, I be heat engine does not come into the picture. This can be achieved whenever the reaction has an ionic mechanism and can be set up as a galvanic cell. The electrodes act in much the same manner as the semi-permeable membranes of the equilibrium box. That is to say, the reagents can be added to the system, and the recK tion products withdrawn, under reversible conditions. The two great prcu tical advantages of this method of operation are as follows  

X This arises, in the first place, from the logarithmic form of the relation zFE -AO RT In K 

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