Walther Nernst

views updated May 17 2018

Walther Nernst

Walther Nernst (1864-1941) made a significant breakthrough with his statement of the Third Law of Thermodynamics, which holds that it should be impossible to attain the temperature of absolute zero in any real experiment. For this accomplishment, he was awarded the 1920 Nobel Prize for chemistry.

In addition to his important work with thermodynamics, Walther Nernst made contributions to the field of physical chemistry. While still in his twenties, he devised a mathematical expression showing how electromotive force is dependent upon temperature and concentration in a galvanic, or electricity-producing, cell. He later developed a theory to explain how ionic, or charged, compounds break down in water, a problem that had troubled chemists since the theory of ionization was proposed by Svante A. Arrhenius.

Born Hermann Walther Nernst in Briesen, West Prussia (in what is now part of Poland) on June 25, 1864, he was the third child of Gustav Nernst, a judge, and Ottilie (Nerger) Nernst. He attended the gymnasium at Graudenz (now Grudziadz), Poland, where he developed an interest in poetry, literature, and drama. For a brief time, he considered becoming a poet. After graduation in 1883, Nernst attended the universities of Zurich, Berlin, Graz, and Würzburg, majoring in physics at each institution. He was awarded his Ph.D. summa cum laude in 1887 by Würzburg. His doctoral thesis dealt with the effects of magnetism and heat on electrical conductivity.

Nernst's first academic appointment came in 1887 when he was chosen as an assistant to professor FriedrichWilhelm Ostwald at the University of Leipzig. Ostwald had been introduced to Nernst earlier in Graz by Svante Arrhenius. These three, Ostwald, Arrhenius, and Nernst, were to become among the most influential men involved in the founding of the new discipline of physical chemistry, the application of physical laws to chemical phenomena.

The first problem Nernst addressed at Leipzig was the diffusion of two kinds of ions across a semipermeable membrane. He wrote a mathematical equation describing the process, now known as the Nernst equation, which relates the electric potential of the ions to various properties of the cell.

In the early 1890s, Nernst accepted a teaching position appointment at the University of Göttingen in Leipzig, and soon after married Emma Lohmeyer, the daughter of a surgeon. The Nernsts had five children, three daughters and two sons. In 1894, Nernst was promoted to full professor at Göttingen. At the same time, he also received approval for the creation of a new Institute for Physical Chemistry and Electrochemistry at the university.

At Göttingen, Nernst wrote a textbook on physical chemistry, Theoretische Chemie vom Standpunkte der Avogadroschen Regel und der Thermodynamik (Theoretical Chemistry from the Standpoint of Avogadro's Rule and Thermodynamics ). Published in 1893, it had an almost missionary objective: to lay out the principles and procedures of a new approach to the study of chemistry. The book became widely popular, going through a total of fifteen editions over the next thirty-three years.

During his tenure at Göttingen, Nernst investigated a wide variety of topics in the field of solution chemistry . In 1893, for example, he developed a theory for the breakdown of ionic compounds in water, a fundamental issue in the Arrhenius theory of ionization. According to Nernst, dissociation, or the dissolving of a compound into its elements, occurs because the presence of nonconducting water molecules causes positive and negative ions in a crystal to lose contact with each other. The ions become hydrated by water molecules, making it possible for them to move about freely and to conduct an electric current through the solution. In later work, Nernst developed techniques for measuring the degree of hydration of ions in solutions. By 1903, Nernst had also devised methods for determining the pH value of a solution, an expression relating the solution's hydrogen-ion concentration (acidity or alkalinity).

In 1889, Nernst addressed another fundamental problem in solution chemistry: precipitation. He constructed a mathematical expression showing how the concentration of ions in a slightly soluble compound could result in the formation of an insoluble product. That mathematical expression is now known as the solubility product, a special case of the ionization constant for slightly soluble substances. Four years later, Nernst also developed the concept of buffer solutions —solutions made of bases, rather than acids—and showed how they could be used in various theoretical and practical situations.

Around 1905, Nernst was offered a position as professor of physical chemistry at the University of Berlin. This move was significant for both the institution and the man. Chemists at Berlin had been resistant to many of the changes going on in their field, and theoretical physicist and eventual Nobel Prize winner Max Planck had recommended the selection of Nernst to revitalize the Berlin chemists. The move also proved to be a stimulus to Nernst's own work. Until he left Göttingen, he had concentrated on the reworking of older, existing problems developed by his predecessors in physical chemistry. At Berlin, he began to search out, define, and explore new questions. Certainly the most important of these questions involved the thermodynamics of chemical reactions at very low temperatures.

Attempting to extend the Gibbs-Helmholtz equation and the Thomsen-Berthelot principle of maximum work to temperatures close to absolute zero—the temperature at which there is no heat—Nernst eventually concluded that it would be possible to reach absolute zero only by a series of infinite steps. In the real world, that conclusion means that an experimenter can get closer and closer to absolute zero, but can never actually reach that point. Nernst first presented his "Heat Theorem," as he called it, to the Göttingen Academy of Sciences in December of 1905. It was published a year later in the Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen. The theory is now more widely known as the Third Law of Thermodynamics. In 1920, Nernst was awarded the Nobel Prize in chemistry in recognition of his work on this law.

The statement of the Heat Theorem proved to be an enormous stimulus for Nernst's colleagues in Berlin's chemistry department. For at least a decade, the focus of nearly all research among physical chemists there was experimental confirmation of Nernst's hypothesis. In order to accomplish this objective, new equipment and new techniques had to be developed. Nernst's Heat Theorem was eventually integrated into the revolution taking place in physics, the development of quantum theory. At the time he first proposed the theory, Nernst had ignored any possible role of quantum mechanics. A few years later, however, that had all changed. In working on his own theory of specific heats, for example, Albert Einstein had quite independently come to the same conclusions as had Nernst. He later wrote that Nernst's experiments at Berlin had confirmed his own theory of specific heats. In turn, Nernst eventually realized that his Heat Theorem was consistent with the dramatic changes being brought about in physics by quantum theory. Even as his work on the Heat Theorem went forward, Nernst turned to new topics. One of these involved the formation of hydrogen chloride by photolysis, or chemical breakdown by light energy. Chemists had long known that a mixture of hydrogen and chlorine gases will explode when exposed to light. In 1918, Nernst developed an explanation for that reaction. When exposed to light, Nernst hypothesized, a molecule of chlorine (Cl₂) will absorb light energy and break down into two chlorine atoms (2Cl). A single chlorine atom will then react with a molecule of hydrogen (H₂), forming a molecule of hydrogen chloride and an atom of hydrogen (HCl + H). The atom of hydrogen will then react with a molecule of chlorine, forming a second molecule of hydrogen chloride and another atom of chlorine. The process is a chain reaction because the remaining atom of chlorine allows it to repeat.

In 1922, Nernst resigned his post at Berlin in order to become president of the Physikalisch-technische Reichsanstalt. He hoped to reorganize the institute and make it a leader in German science, but since the nation was suffering from severe inflation at the time, there were not enough funds to achieve this goal. As a result, Nernst returned to Berlin in 1924 to teach physics and direct the Institute of Experimental Physics there until he retired in 1934.

In addition to his scientific research, Nernst was an avid inventor. Around the turn of the century, for example, he developed an incandescent lamp that used rare-earth oxide rather than a metal as the filament. Although he sold the lamp patent outright for a million marks, the device was never able to compete commercially with the conventional model invented by Thomas Alva Edison . Nernst also invented an electric piano that was never successfully marketed.

The rise of the Nazi party in 1933 brought an end to Nernst's professional career. He was personally opposed to the political and scientific policies promoted by Adolf Hitler and his followers and was not reluctant to express his views publicly. In addition, two of his daughters had married Jews, which contributed to his becoming an outcast in the severely anti-Semitic climate of Germany at that time.

Walther Nernst was one of the geniuses of early twentieth-century German chemistry, a man with a prodigious curiosity about every new development in the physical sciences. He was a close colleague of Einstein, and was a great contributor to the organization of German science— he was largely responsible for the first Solvay Conference in 1911, for example. In his free time, he was especially fond of travel, hunting, and fishing. Nernst also loved automobiles and owned one of the first to be seen in Göttingen. Little is known about his years after his retirement. Nernst died of a heart attack on November 18, 1941, at his home at Zibelle, Oberlausitz, near the German-Polish border.

Nernst, Walther Hermann

views updated May 18 2018

Nernst, Walther Hermann

GERMAN CHEMIST
1864–1941

Walther Hermann Nernst, born in Briesen, Prussia (now Wabrzezno, Poland), was a pioneer in the field of chemical thermodynamics in a wide range of areas. His most outstanding contributions were his laws for electrochemical cells and his heat theorem, also known as the third law of thermodynamics, for which he was awarded the Nobel Prize in chemistry in 1920.

Nernst first studied physics before he became an assistant in 1887 to German physical chemist Friedrich Wilhelm Ostwald at the University of Leipzig, then the only institute for physical chemistry in Germany. In 1891

he was appointed associate professor at the university in Göttingen and, three years later, convinced officials there to create an institute for physical chemistry modeled on the Leipzig center. He served as its director until his move in 1905 to Berlin, where he once again established an institute renowned worldwide.

During his Leipzig period, Nernst performed a series of electrochemical studies from which, at the age of twenty-five, he arrived at his well-known equations. These equations described the concentration dependence of the potential difference of galvanic cells, such as batteries, and were of both great theoretical and practical importance. Nernst started with the investigation of the diffusion of electrolytes in one solution. Then he turned to the diffusion at the boundary between two solutions with different electrolyte concentrations; he determined that the osmotic pressure difference would result in an electric potential difference or electromotive force (emf). Next he divided both solutions into two concentration half-cells, connected to each other by a liquid junction, and measured the emf via electrodes dipped into both solutions. The data supported his first equation where the emf was proportional to the logarithm of the concentration ratio. Finally, he investigated galvanic cells where a redox reaction (e.g., Zn + 2Hg⁺ → Zn²⁺ + 2Hg) was divided such that oxidation (Zn → Zn² + 2e ⁻) and reduction (2Hg⁺ + 2e ⁻ → 2Hg) occurred at the electrodes in two half-cells. By combining this with Helmholtz's law, which related thermodynamics to the emf of electrochemical cells, and van't Hoff's equation, which related chemical equilibria to thermodynamics, Nernst derived his second equation for galvanic cells. Supported by many measurements, the equation described the emf of galvanic cells as a function of the concentration of all substances involved in the reaction.

Nernst's formulation of the third law of thermodynamics was originally an ingenious solution to a crucial practical problem in chemical thermodynamics, namely, the calculation of chemical equilibria and the course of chemical reactions from thermal data alone, such as reaction heats and heat capacities. Based on the first two laws of thermodynamics and van't Hoff's equation, chemical equilibria depended on the free reaction enthalpy ΔG, which was a function of both the reaction enthalpy ΔH and the reaction entropy ΔS according to the Gibbs-Helmholtz equation:

The problem was that, although enthalpy values could be calculated from thermal measurements, entropy values required data at the absolute zero of temperature, which was practically inaccessible. Guided by theoretical reasoning and then supported by a huge measurement program at very low temperatures, Nernst in 1906 suggested his heat theorem. According to a later formulation, it stated that all entropy changes approach zero at the absolute zero.

The theorem not only allowed the calculation of chemical equilibria, it was also soon recognized as an independent third law of general thermodynamics with many important consequences. One such consequence was that it is impossible to reach the absolute zero. Another consequence was that one could define a reference point for entropy functions, such that the entropies of all elements and all perfect crystalline compounds were taken as zero at the absolute zero.

Nernst made numerous other important contributions to physical chemistry. For example, his distribution law described the concentration distribution of a solute in two immiscible liquids and allowed the calculation of extraction processes. He also formulated several significant theories, such as those on the electrostriction of ions, the diffusion layer at electrodes, and the solubility product. In addition, he established new methods to measure dielectric constants and to synthesize ammonia, on which the German chemist Fritz Haber later successfully followed up.

see also Electrochemistry; Haber, Fritz; Ostwald, Friedrich Wilhelm; Physical Chemistry.

Joachim Schummer

Bibliography

Barkan, Diana (1999). Walther Nernst and the Transition to Modern Physical Science. Cambridge, U.K.: Cambridge University Press.

Hiebert, Erwin N. (1978). "Nernst, Hermann Walther." In Dictionary of Scientific Biography, Vol. XV, Supplement I, ed. Charles C. Gillispie. New York: Scribners.

Mendelssohn, Kurt (1973). The World of Walther Nernst: The Rise and Fall of German Science. London: Macmillan.

Chemistry: Foundations and Applications Schummer, Joachim