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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Theory of electrolytic dissociation. History and essence of scientific discovery
Directory / The most important scientific discoveries The history of the emergence of the theory of electrolytic dissociation is associated with the name of the Swedish physical chemist Svante Arrhenius (1859–1927). In 1882 he graduated from the University of Uppsala. In 1895 he became professor of physics at Stockholm University. From 1896 to 1905 Arrhenius was the rector of this university. He is the author of 200 scientific works in the field of chemistry, physics, geophysics, meteorology, biology, physiology. It is interesting that the idea that became the basis of this theory arose on the basis of experiments set up to solve a completely different problem. According to Yu.I. Solovyov, "while still a student at Uppsala University S. Arrhenius, listening to the lectures of his teacher Professor P. T. Kleve, learned that it is impossible to determine the molecular weight of such substances that, like cane sugar, do not pass into a gaseous state. favor", the young scientist decides to determine the electrical conductivity of salts in solutions containing, along with water, a large amount of non-electrolytes. At the same time, he proceeded from the principle that the resistance of the electrolyte solution is the greater, the greater the molecular weight of the solvent. This was the original work plan. But as a result of the first observations, S. Arrhenius loses interest in the conceived topic. He is captivated by a new idea. What happens to an electrolyte molecule in solution? The young scientist was aware that a successful solution of this problem would make it possible to shed bright light on the dark region of solutions. So instead of determining the molecular weight of the dissolved non-electrolyte, S. Arrhenius begins to intensively study the state of the electrolyte molecule in solution. Work in a new direction soon gave excellent results. The data obtained by measuring the electrical conductivity of aqueous solutions of electrolytes of various concentrations allowed S. Arrhenius to draw a bold conclusion: electrolyte molecules dissociate into ions without current, and the degree of dissociation increases with dilution. As it seems to us now, this was a seemingly obvious and simple conclusion from the experimental data. But it was not at all simple for S. Arrhenius, because this conclusion destroyed the hard, "like granite", traditional ideas about the state of the molecules of salts, acids and bases in solution. Arrhenius could not fail to understand that he, a young chemist, was raising his hand against chemical "foundations". But that didn't bother him. In his doctoral thesis (1883), he makes an exceptional conclusion: “The electrolyte activity coefficient indicates the number of ions actually present in the solution, related to the number of ions that would be in the solution if the electrolyte was completely split into simple electrolytic molecules ... Salt splits completely when the amount of water in the solution is infinitely large. However, four years remained before the creation of a full-fledged theory of electrolytic dissociation. Of great importance for the further development of the theory of dissociation was the well-known work of van't Hoff "Chemical equilibrium in systems of gases and dilute solutions" (1885), in which it was found that the actual decrease in the melting point, vapor pressure and osmotic pressure of salts, acids and bases is less than than calculated theoretically according to Raoult's law. These inconsistencies confirmed the provisions of the theory of dissociation, according to which the electrolyte in an aqueous solution decomposes into freely moving ions. In the spring of 1887, Arrhenius worked in Würzburg with F. Kohlrausch. "Shortly before I left Würzburg (March 1887)," Arrhenius recalled, "I received Van't Hoff's work published by the Swedish Academy of Sciences. I looked through it one evening, having finished my daily work at the institute. It immediately became clear to me that the deviation electrolytes in aqueous solution from the van't Hoff-Raoult laws of lowering the freezing point is the strongest evidence of their decomposition into ions.Now I had two ways to calculate the degree of dissociation: on the one hand, by lowering the freezing point, on the other - from conductivity. Both of them gave the same result in the vast majority of cases, and I could talk openly about the dissociation of electrolytes. In a letter to van't Hoff in March 1887, the Swedish scientist wrote: "Both theories are still at the very beginning of their development, and I hope very lively that in the near future not one but several bridges will be thrown between the two areas." And so it happened. In 1887, the famous article by Arrhenius "On the dissociation of substances dissolved in water" appeared. It evoked admiration from some and indignation from others. Here the scientist confidently declares that electrolyte molecules (salts, acids, bases) decompose in solution into electrically charged ions. Arrhenius found a formula for determining the degree of electrolytic dissociation. In doing so, he turned a purely qualitative hypothesis into a quantitative theory that could be tested experimentally. After the main provisions of this theory were created, Arrhenius showed its applicability in various fields of natural science. For the development of the theory of electrolytic dissociation, Arrhenius was awarded the Nobel Prize in 1903. After 1887, the studies of S. Arrhenius, W. Ostwald, N. Nernst, M. Leblanc and other scientists not only confirmed the validity of the main provisions of the theory of electrolytic dissociation, but also significantly expanded the number of individual facts that can be substantiated by the theory. In 1888, Walter Friedrich Nernst (1864–1941), professor of physical chemistry at Göttingen and Berlin, winner of the 1920 Nobel Prize in Chemistry for discovering the third law of thermodynamics, comparing the diffusion rate of ions with the speed of movement of ions during electrolysis, showed that these numbers coincide . In 1889, based on the theory of osmotic pressure and the theory of electrolytic dissociation, Nernst developed the osmotic theory of the occurrence of galvanic current. According to this theory, when the concentration of metal ions (electrode) is higher than their concentration in solution, the ions go into solution. When the concentration of ions is higher in the solution, they are deposited on the electrode and give up their charge. But in both cases, double electric layers meet on the way of ions. Their charge inhibits the precipitation of ions or the dissolution of a given metal. "In these simple provisions," Ostwald remarked, "the whole theory of precipitation is contained, and all the phenomena of both a decrease and an abnormal increase in solubility find their explanation and can be predicted in advance in each individual case." Wilhelm Friedrich Ostwald (1853–1932) was born in Riga into the family of a German craftsman-cooper. The boy studied at a real gymnasium, and then entered the University of Dorpat. After completing his chemical education, Ostwald was left there as an assistant to A. Ettingen (1875). In 1878, Ostwald defended his doctoral thesis "Volume-chemical and opto-chemical research", in which he began to systematically apply physical methods to solve chemical problems. In 1881 he became a professor at the Riga Polytechnic School. Ostwald was engaged in the measurement of chemical affinity, conducted calorimetric studies, and studied chemical dynamics. The problems of the theory of solutions and electrochemistry came to the fore in Ostwald's work already at the beginning of his research activity. In 1885-1887, Ostwald published a two-volume "Textbook of General Chemistry", where he outlined the main provisions of the doctrine of ions, which most chemists then refused to recognize, and emphasized the importance of physical chemistry as an independent science. The appearance of this textbook and the founding, together with Arrhenius and van't Hoff in 1887, of the "Journal of Physical Chemistry" not only ensured the independence of the new scientific discipline, but also prepared the way for the penetration of physics into all areas of chemistry. Investigating the electrical conductivity of acids at various dilutions, Arrhenius established as early as 1884–1886 that the electrical conductivity of acids increases with dilution - asymptotically approaching a certain limiting value. He found that for solutions of weak acids (succinic, etc.) and bases, the increase in molecular electrical conductivity with dilution is much more noticeable than for strong acids, such as sulfuric, etc. In 1888, he proposed a method for determining the basicity of acids by the electrical conductivity of their solutions and showed that the rate of a chemical reaction in solutions depends only on the dissociated part of the solute (on the concentration of ions). In the same year, Ostwald derived a relationship for binary weak electrolytes, which he called the law of dilution. In this particular case of the law of mass action, the relationships between the dissociation constant of the electrolyte, electrical conductivity, and solution concentration are formulated. The new law became the basis for the chemistry of aqueous solutions. In one of his works, Ostwald gave a mathematical formulation of the law of dilution. "W. Ostwald's law of dilution," writes Yu. it is inapplicable. It took numerous studies by scientists of the late XNUMXth and early XNUMXth centuries to explain the reason for the non-subordination of strong electrolytes to the law of dilution. The fruitfulness of the theory of electrolytic dissociation was especially clearly manifested in the fact that it was successfully used to explain the mechanism of many chemical reactions and the nature of various compounds, such as complex. In 1889, a scientist, considering the results of analyzes of mineral waters, noticed a discrepancy between these data and the theory of electrolytic dissociation. Since all these salts are electrolytes, Ostwald believes that they are dissociated into ions. This was the reason for him to revise the material of analytical chemistry and create the textbook "Scientific Foundations of Analytical Chemistry" (1894), which played an important role in the development of modern analytical chemistry. The theory of electrolytic dissociation was able to combine both the theory of solutions and the electrochemical theory. As Arrhenius suggested, both streams merged into one. "After the foundation of the mechanical theory of heat," Ostwald wrote in 1889, "there has not been a single series of ideas in the physical sciences as comprehensive as the theory of solutions by van't Hoff and Arrhenius." Objections to the theory were based mainly on the fact that the one proposed by Arrhenius was suitable only for explaining the properties of weak electrolytes. To overcome this shortcoming, Arrhenius carried out numerous experiments, seeking to prove the applicability of the theory to all electrolytes. But these brilliant foundations of the theory of electrolytic dissociation were further developed in the works of the next generation of scientists. The theory of electrolytic dissociation was subsequently improved thanks to the work, first of all, of N. Bjerrum, P. Debye and E. Hückel. They developed the ideas expressed earlier by I. Van Laar that the unusual behavior of strong electrolytes can be explained by the action of Coulomb forces. Author: Samin D.K.
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