ON CHEMICAL AFFINITY BY A. KOREVAAR
When we peruse the chemical literature and notice the meaning of the word affinity, it soon becomes clear that two different things are called by this name. In the first place, we know the classic definition of affinity coming from the Greek philosophers and which denotes a force, acting between combining atoms. In the second place, modern chemistry shows us the thermodynamical conception of affinity, created by van’t Hoff, which defines affinity as the maximum external work done by the chemical reaction a t constant temperature and volume. We, therefore, find the chemists using two quite different definitions of chemical affinity. It is to be expected that such a state of affairs will cause a confusion in our science and this is really the case. The different text-books on chemistry define affinity quite differently. One maintains the classical definition, a second gives the thermodynamical one; and others mix both definitions up in a wonderful way, some even advocating removing this vague idea from chemical science. This last would not be right. Neither the classical nor the thermodynamical conception is indefinite; but we must always remember that the word affinity denotes two quite different ideas. At present this is not generally known. This appears clearly from the confusion on this point, existing in our text-books, as shown by the following examples: Lewis, in his “A System of Physical Chemistry,’’ Vol. 11, page 339 (1920), gives the definition by van’t Hoff ; Holleman, in his text-book of inorganic chemistry, page 47 (1906), the classical conception. Ostwald in his “Grundlinien der anorganischen Chemie,” page 88 (1904), discusses the classical conception and advises not to use this idea, because he thinks it in contradiction with certain phenomena which, on page 104 appear to be the reversible reactions.
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J. W. Mellor, in his “Modern Inorganic Chemistry,” page 94, 1912, deals first with the classical conception, but does not stick to it further on, while R. de Forcrand in his “Cours de Chimie,” 1919, does not speak a t all about affinity. Ostwald’s attitude in his “Der Werdegang einer Wissenschaft,” 1908, is quite remarkable. I n this book, Chapter VI deals with affinity. I n fact, in the beginning the author writes about affinity, but soon he enters into thermodynamical considerations without giving an analysis or description of the idea “affinity,” and the reader gets the impression that the author does not know what he shall say on this point. The cause of all this is quite clear. The authors are not conscious that tm7o different ideas are called by the same name “affinity. ” We hope that this short explanation may cause the different authors to revise their texts on this point and that they will not only rigidly separate the classical from the thermodynamical conception; but also that they will call these differept ideas by different names. The best were perhaps to continue to call the classical conception by the name “affinity” and to denote the thermodynamical one by the maximum external work of the reaction. 0. Sackurl has contributed to a great extent to the existence of the above-mentioned confusion. His book on chemical affinity is well known and justly admired, as it gives a clear and simple treatment of the maximum external work and its applications to chemistry. But this author has totally forgotten that the classical conception of affinity has still a right to exist. Sackur begins in the first chapter with a historical development of the idea of affinity and we might expect that, in doing this, he would have discovered the confusion of ideas in question. But the author has failed to do so and therefore it is worth while to trace the historical line once more and show how it has been possible that chemical Dr. 0. Sackur: Die chemische A s n i t a t und ihre Messung. Vieweg (1908).
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science gave birth t o two different definitions of chemical affinity. We may say that the conception of affinity as a force between the smallest particles, or in the middle ages, when the atomic theory was denied on the authority of Aristotle, as a force between the elements, remained invariably the same till the end of the 18th century. It is true that the nature of these forces was differently explained. The Greeks had an antropomorphistical conception of affinity and this character was preserved till the 17th century. The force, driving different substances together, was compared with love between two human beings. With the rise of the mechanical contemplation of nature in the 17th and 18th centuries the ideas on the nature of chemical force became more sober. Lemery (16i5) thought the atoms were provided with little hooks for sticking to each other. Newton classified the chemical force under the gravitation of the smallest particles ; but according t o him it was not necessary that this force was, as with the visible masses, inversely proportional to the distance, This latter restriction was dropped by Buffon (1770) who saw no difference between chemical force and gravitation. In all of these theories the driving force of a reaction was considered acting between the smallest particles of the combining substances. The difficulties arose when Berthollot proved experimentally that the progress of a reaction not only depends on the nature of the reacting substances but also on their quantity, or, as Guldberg and Waage proved afterwarde, on their concentration. These researches led, in 1864, to the formulation of the law of mass action. Guldberg and Waage’s views are of the greatest importance for our purpose and so we will pay some further attention to them. Guldberg and Waage published their views in three treatises which are collected in Ostwald’s Klassiker, No. 104, under the title : “Untersuchungen uber die Chemischen Affiniti-iten.” The treatises are dated 1864, 1867 and 1879, and we are not surprised that the ideas of the authors have
developed in that course of time. In €act, we shall see that their conception of chemical affinity has undergone radical changes. Their views may be summed up as follows: A substance A and a substance B combine, by the operation of cohesive forces between the molecules, to a compound AB. When a substance C reacts with AB according to the equation AB
+ C = =1C + B,
then, AC is principally produced by cohesive forces between A and C. But the forces between A and B, and perhaps also between B and C, have all exerted their influence on the reaction, so that the force that finally produces AC is the result of the cooperation of different forces. This resultant force k is the coefficient of affinity. According to these investigators, the driving force of a reaction is not only proportional to the coefficient of affinity, but also to the concentrations of the reacting substances. While formerly only the affinity was considered to be the driving force of a reaction, according to the new conception, the affinity is only a part of the driving force. T h e a f i n i t y has to be distinctly distinguished front ihe drieiwg force of a reaction, they are quite dijeerent thiwgs. Guldberg and Waage expressed the driving force of a reaction by the product of the concentrations of the reacting substances and the coefficient of affinity k . When these concentrations are denoted by p and q, then the driving force = kpq. The authors originally thought that in determining k , they had found a measure for the affinity, for h was the resultant of the forces, acting between the molecules; but on this point they changed their views fundamentally. This appears from their treatise of 1879, where the idea of mobile equilibrium is developed in the following way: To explain a reaction according to the equation: A
+ B = A’ + B’,
it is insufficient to accept cohesive forces between the mole-’ cules, but it is also necessary to keep account with the motion of the atoms and molecules. The equilibrium is not a state
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of repose, i t is a mobile equilibrium, the reactions in both directions going equally fast. The authors give a sketch of a kinetical deduction of the law of mass action. If p and q are the number of molecules of A and B in the unit of volume, then the number of collisions between the molecules A and B is represented by the product pq. When each collision caused a reaction, the reaction velocity would be = Qzpq, Q being a velocity coefficient, dependent on temperature. Of p molecules of A, only a part u will be in a condition to cause a reaction during a collision. Of q molecules of B, a part b is in a similar condition. So that up molecules of A will combine with bq molecules of B, and the reaction velocity is: Qapbq or k p q when k = Qab. I t is clear that this formulation is based on a quite different conception of k . At present this constant k is the product of a velocity coefficient Q and two constants a and b, which are perhaps best called activity coefficients. For the present it is impossible to determine these constants separately. Concerning their views on chemical affinity, we conclude from their treatises the following: Guldberg and Waage certainly adopt the classical idea of affinity, viz., the affinity being a force acting between the molecules. This is clear from their former treatise and appears in the mathematical treatment of the problem from the meaning of k . Their later considerations about the mobile equilibrium do not contradict the classical definition of affinity, but the meaning of k has become much more complicated and their conception of affinity is not clearly defined, The great importance of Guldberg and Waage's work does not lie in the sphere of affinity, and their great success is due to the formulation of the law of mass action, which became the basis of chemical statics and dynamics. At present, the names of these investigators are always connected with the law of mass action but never with chemical affinity, though this was originally the object of their researches. Though they did not do it themselves, and though, as far as we know, others did not do it either, we may draw from
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Guldberg and Waage‘s treatises a conclusion of importance. We have learned to make a sharp distinction between affinity as a force, acting between the smallest particles and the driving force of a reaction. This is clearly expressed in their original formula : Driving fcrce
=
kpq.
The driving force of a reaction is quite different from the affinity. Though we are convinced that k is in some way connected with the affinity, it appears from the later treatise of Guldberg and MTaage that this connection is not a simple one. It is true that they succeeded in determining k , but they did not know the exact meaning of k . This is the reason that all determinations of affinity have failed. In 1883, van’t Hoff showed that the driving force of a reaction can be measured by the maximum external work a t constant temperature and constant volume which is done by the system in passing from the initial state to the eqiiilibriurn point. This thought has been of the greatest importance for chemical science, but it was a great pity that van’t Hoff called the driving force a reaction, affinity1 and in consequence, considered the maximum external work of a reaction a measure for affinity. In the preceding pages we have emphasized the difference between the driving force and affinity. By calling the driving force, affinity, van’t Hoff has made the clear distinction between these two different ideas very vague. This was unnecessary. The maximum external work is a sharply defined idea, which does not need any other name, and it was a pity, for it caused a confusion, which, as we have seen, remains in our text-books up to the present day. After this digression we return to Sackur’s historical treatment of affinity. Sackur did not understand the confusion of affinity and driving force. According to him, the thermodynamical conception of affinity has gradually developed Vorlesungen uher theoretische und physikalische Chemie ,
I,
171 (1901).
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from the classical one. If this were true, the modern formulation would have made the ancient one superfluous, for then the modern one, by a broader development of the idea, would have opened an new insight given us a new viewpoint which would have included the old one completely. This did not happen and Sackur writes unjustly that van’t Hoff succeeded in “eine gluckliche Dejinition der A HInitatskraft aufzustellen, die allen bisherigen Erfahrungen Rechnung tragt u n d gleichxeitig die quantitative Messung in mechanischen iwassen ermoglicht. ” No, van’t Hoff has mixed up the ideas of driving force and affinity; but, by calling the driving force affinity, the classical idea of affinity does not become superfluous. Therefore it is not true that van’t Hoff’s affinity, as Sackur says: “allew bisherigen Erfahrungen Rechnung tragt.” It is better that the maximum external work of a reaction remains the maximum external work. Then, it is clear to everybody that the affinity is not meant. That is the question and as Sackur did not understand it, his much read and otherwise excellent book is also responsible for the confusion which prevails in this sphere of chemical science. The IIa,aue, Dei ember, 1920
