Vojtech Fried Brooklyn College. CUNY Brooklyn, New York 11210
Our experience with nature teaches us that all processes occurring in nature are spontaneous. They proceed in certain directions only: in the direction of greater stability of the system. All the spontaneous processes must meet the requirement' -dGp,= > ad
(1)
accordine to which the decrease of the Gibbs free enerev .. a t constant temperature and pressure is greater than is the useful work (work diffrrmt from PdV) done by the svstem. More work can be obtained during reversihie processes, when - d G p , ~= itW? however, these non-spontaneous processes are so slow that they cannot be used effectively and economically to produce energy in the form of work. As eqn. (1)indicates, spontaneous processes may serve, a t least theoretically, as sources, of energy (work). The spontaneous flow of heat from a higher temperature source to a lower temperature sink (heat engine) is a good and well-known example of it. Here we would like to deal with one of the most frequently occurring natural processes, mixing. Mixing is a natural process, during which the Gibbs free energy is lowered and energy in the form of work is given off. If one would be able to collect all the energy given off during all the processes of mixing in nature, and find a way of using it, mankind would never have to face an energy crisis. Mixing is, however, a relatively fast process (except in solids) and thus cannot serve as a lastine" source of enerw. As lone as there is a concentration gradient between the two or more systems to be mixed, mixing will continue s~ontaneouslvand enerav will be aiven off. As soon as the concentration gradients drop to zero, the driving force for mixing disappears,. the . process becomes non-spontaneous, and icabiliiGo give off energy ceases. What is r&lly needed here are two solutions of different concentrations, that are able to keep a constant concentration gradient over a long period of mixing. The delta of ariver is a good example of two such solutions. The constantly flowing river into the sea keeps its relatively low mineral (salt) concentration fixed and the sea keeps its relatively high salt concentration fixed. The concentration gradient between the sea and river remains fixed over a long period of time. The mixing may continue "forever" and may serve as a lasting source of energy.
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According to thermodynamics the Gibbs free energy ol mixing is AG,i,= n R T l n ~ = n ~ T l n ~ az
CZY~
(2:
where a is activity, e, concentration, and y, the activity coefficient; 1and 2 refer to the river and sea, respectively; cz > el The smaller the el/cz ratio the more pronounced is the Gibbr free energy lowering and the more energy given off during the process of mixing. Setting yllyz = 1and cllcz 10, the Gibbt free energy of mixing per mole of solution is roughly -550C J a t 17°C. Since yllyz # 1and since the process of mixing ir irreversible. the euerw eiven off in the form of work will be less than is the ~ i b b ~ j r energy ee of mixing. In any way, due to the fact that laree masses of water mix. the enerev aiven ofl during the processof mixing will be extremely large. To make this tremendous amount of energy able to serve mankind, the river-sea system can be changed into a concentration cell. Two chloride ion sensitive electrodes (or some other ion sensitive electrodes), one immersed in the river and the other in the sea mav serve as two half-cells of the con. centration cell. In such a-cell, part of the Gihbs free energy ol mixing can he transformed into electrical work, -AG,i, = nFE = we,.To construct a cell that would be able to transform the Gihhs free energy of mixing into electrical work is not an easy task, and perhaps from the practical point of view not even possible. However, the idea is worth considering, especially now, when new sources of energy are desperate15 needed. While this paper has been pending publication, I was tolc by Dr. R. Weintraub that the idea of a delta cell is known and has been tested. The tropical ocean heat enzine, in which the warm uppel Inyrr and the cold deep water of thp tr
