3. The Nobel Prizes for Chemistry 1911-2000
A survey of the Nobel Prizes for Chemistry awarded during the 20th century, reveals that the development of this field includes breakthroughs in all of its branches, with a certain dominance for progress in physical chemistry and its subcategories (chemical thermodynamics and chemical change), in chemical structure, in several areas of organic chemistry as well as in biochemistry. Of course, the borders between different areas are diffuse, therefore many Laureates will be mentioned in more than one place.

3.1 General and Physical Chemistry
The Nobel Prize for Chemistry in 1914 was awarded to Theodore William Richards of Harvard University for "his accurate determinations of the atomic weight of a large number of chemical elements". Most atomic weights in Cannizzaros table (see Section 1.2) had already been determined in the 19th century, particularly by the Belgian chemist Jean Servais Stas, but Richards showed that many of them were in error, mainly because Stas had worked with very concentrated solutions, leading to co-precipitation. In 1913 Richards had discovered that the atomic weight of natural lead and of that formed in radioactive decay of uranium minerals differ. This pointed to the existence of isotopes, i.e. atoms of the same element with different atomic weights, which was accurately demonstrated by Francis William Aston at Cambridge University, with the aid of an instrument developed by him, the mass spectrograph. Aston also showed that the atomic weights of pure isotopes are integral numbers, with the exception of hydrogen, the atomic weight of which is 1.008.

For his achievements Aston received the Nobel Prize for Chemistry in 1922.
One branch of physical chemistry deals with chemical events at the interface of two phases, for example, solid and liquid, and phenomena at such interfaces have important applications all the way from technical to physiological processes. Detailed studies of adsorption on surfaces, were carried out by Irving Langmuir at the research laboratory of General Electric Company, and when he was awarded the Nobel Prize for Chemistry in 1932, he was the first industrial scientist to receive this distinction.
Two of the Prizes for Chemistry in more recent decades have been given for fundamental work in the application of spectroscopic methods to chemical problems. Spectroscopy had already been recognized with Prizes for Physics in 1952, 1955 and 1961, when Gerhard Herzberg, a physicist at the University of Saskatchewan, received the Nobel Prize for Chemistry in 1971 for his molecular spectroscopy studies "of the electronic structure and geometry of molecules, particularly free radicals". The most used spectroscopic method in chemistry is undoubtedly NMR (nuclear magnetic resonance), and Richard R. Ernst at ETH in Zürich was given the Nobel Prize for Chemistry in 1991 for "the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy". Ernst's methodology has now made it possible to determine the structure in solution (in contrast to crystals; cf. Section 3.5) of large molecules, such as proteins.

3.2 Chemical Thermodynamics

The first Nobel Prize for Chemistry, that to van't Hoff, was in part for work in chemical thermodynamics, and many later contributions in this area have also been recognized with Nobel Prizes. Already in 1920 Walther Hermann Nernst of Berlin received this award for work in thermochemistry, despite a 16-year opposition to this recognition from Arrhenius [2]. Nernst had shown that it is possible to determine the equilibrium constant for a chemical reaction from thermal data, and in so doing he formulated what he himself called the third law of thermodynamics. This states that the entropy, a thermodynamic quantity, which is a measure of the disorder in the system, approaches zero as the temperature goes towards absolute zero. van't Hoff had derived the mass action equation in 1886, with the aid of the second law which says, that the entropy increases in all spontaneous processes [this had already been done in 1876 by J. Willard Gibbs at Yale, who certainly had deserved a Nobel Prize, but his work had been published in an obscure place]. According to the second law, heat of reaction is not an accurate measure of chemical equilibrium, as had been assumed by earlier investigators. But Nernst showed in 1906 that it is possible with the aid of the third law, to derive the necessary parameters from the temperature dependence of thermochemical quantities.

To prove his heat theorem (the third law) Nernst carried out thermochemical measurements at very low temperatures, and such studies were extended in the 1920s by G.N. Lewis (see Section 1.1) in Berkeley. Lewis's new formulation of the third law was confirmed by his student William Francis Giauque , who extended the temperature range experimentally accessible by introducing the method of adiabatic demagnetization in 1933. With this he managed to reach temperatures a few thousandths of a degree above absolute zero and could thereby provide extremely accurate entropy estimates. He also showed that it is possible to determine entropies from spectroscopic data. Giauque was awarded the Nobel Prize for Chemistry in 1949 for his contributions to chemical thermodynamics.

The next Nobel Prize given for work in thermodynamics went to Lars Onsager of Yale University in 1968 for contributions to the thermodynamics of irreversible processes. Classical thermodynamics deals with systems at equilibrium, in which the chemical reactions are said to be reversible, but many chemical systems, for example, the most complex of all, living organisms, are far from equilibrium and their reactions are said to be irreversible. With the aid of statistical mechanics Onsager developed in 1931 his so-called reciprocal relations, describing the flow of matter and energy in such systems, but the importance of his work was not recognized until the end of the 1940s. A further step forward in the development of non-equilibrium thermodynamics was taken by Ilya Prigogine in Bruxelles, whose theory of dissipative structures was awarded the Nobel Prize for Chemistry in 1977.

3.3 Chemical Change
The chief method to get information about the mechanism of chemical reactions is chemical kinetics, i.e. measurements of the rate of the reaction as a function of reactant concentrations as well as its dependence on temperature, pressure and reaction medium. Important work in this area had been done already in the 1880s by two of the early Laureates, van't Hoff and Arrhenius, who showed that it is not enough for molecules to collide for a reaction to take place. Only molecules with sufficient kinetic energy in the collision do, in fact, react, and Arrhenius derived an equation in 1889 allowing the calculation of this activation energy from the temperature dependence of the reaction rate. With the advent of quantum mechanics in the 1920s (see Section 3.4), Eyring developed his transition-state theory in 1935 and this showed that the activation entropy is also important. Strangely, Eyring never received a Nobel Prize (see Section 1.2).

In 1956 Sir Cyril Norman Hinshelwood of Oxford and Nikolay Nikolaevich Semenov from Moscow shared the Nobel Prize for Chemistry "for their researches into the mechanism of chemical reactions". Among Hinshelwood's major contributions his detailed elucidation of the mechanism for the reaction between oxygen and hydrogen can be mentioned, whereas Semenov's award was for his studies of so-called chain reactions.

A limit in investigating reaction rates is set by the speed with which the reaction can be initiated. If this is done by rapid mixing of the reactants, the time limit is about one thousandth of a second (millisecond). In the 1950s Manfred Eigen from Göttingen developed chemical relaxation methods that allow measurements in times as short as a thousandth or a millionth of a millisecond (microseconds or nanoseconds). The methods involve disturbing an equilibrium by rapid changes in temperature or pressure and then follow the passage to a new equilibrium. Another way to initiate some reactions rapidly is flash photolysis, i.e. by short light flashes, a method developed by Ronald G.W. Norrish at Cambridge and George Porter (Lord Porter since 1990) in London. Eigen received one-half and Norrish and Porter shared the other half of the Nobel Prize for Chemistry in 1967. The milli- to picosecond time scales gave important information on chemical reactions.