Chemistry, Physical

I

Introduction

Chemistry, Physical, field of science that relates chemical structure to the physical properties of substances. The term physical chemistry is usually applied to the study of properties such as vapour pressure, surface tension, viscosity, refractive index, and density, as well as to the study of the so-called classical aspects of the behaviour of chemical systems, such as thermal properties, equilibria, rates of reactions, mechanisms of reactions, and ionization phenomena. In its more theoretical aspects, physical chemistry attempts to explain spectral properties of substances in terms of fundamental quantum theory; the interaction of energy with matter; the nature of chemical bonding; the relationships correlating the number and energy states of electrons in atoms and molecules with the observable properties shown by these systems; and the electrical, thermal, and mechanical effects of individual electrons and protons on solids and liquids.

II

Historical Development

The earliest phase of the development of physical chemistry as a specialized field of study was devoted to investigating the problem of chemical affinities, or the widely varying extents and degrees of vigour with which various substances react with each other. Common examples are the corrosion of iron compared to gold, and the fact that oxygen supports combustion but nitrogen does not.

A

The 19th Century

It was first assumed that rapid reactions were those that proceeded to completion. It was soon realized, however, that these two factors were independent; the degree of completeness of a reaction is determined by its so-called equilibrium constant, a concept introduced in 1864 by the Norwegian chemists Cato Maximilian Guldberg and Peter Waage, whereas the rate of a reaction is determined by the intimacy of contact between the reactants, the presence or absence of a catalyst, and other variables.

The British chemist John Dalton proposed his atomic theory in 1803 and it was placed on a firm footing in 1811 when the Italian physicist Amedeo Avogadro made clear the distinction between atoms and molecules of elementary substances. About the same time, the concepts of heat, energy, work, and temperature began to be clarified and made more precise. The first law of thermodynamics, according to which heat and work are interconvertible, was first clearly stated by the German physicist Julius Robert von Mayer in 1842. The second law of thermodynamics, according to which spontaneous processes occur with an increase in the degree of disorder in the system, was enunciated by the German mathematical physicist Rudolf Julius Emanuel Clausius and the British mathematician and physicist William Thomson, later Lord Kelvin, in 1850-1851.

These developments made it possible to begin to interpret the properties of gases, which represent the simplest states of matter, in terms of the behaviour of their individual molecules. In the period 1860-1875, Clausius, the Austrian physicist Ludwig Boltzmann, and the British physicist James Clerk Maxwell showed how to account for the ideal gas law in terms of a kinetic theory of matter. From this beginning have flowed all the subsequent insights into the kinetics of reactions and the laws of chemical equilibrium.

Important contributions to the field of physical chemistry were made towards the end of the 18th century by the French chemist Comte Claude Louis Berthollet, who studied the rate and reversibility of reactions, and the Anglo-American physicist Benjamin Thompson, Count Rumford, who attempted to deduce the mechanical equivalent of heat. In 1824 the French physicist Nicolas Léonard Sadi Carnot published his studies of the correlation between heat and work, which established him as the founder of modern thermodynamics, and in 1836 the Swedish chemist Jöns Jakob Berzelius assessed the role played by catalysts in accelerating chemical reactions. The application of the first and second laws of thermodynamics to heterogeneous substances in 1875 by the American mathematical physicist Josiah Willard Gibbs and his discovery of the phase rule laid down the theoretical basis of physical chemistry. The German physical chemist Walther Hermann Nernst, who in 1906 enunciated the third law of thermodynamics, also made a lasting contribution to the study of physical properties, molecular structures, and reaction rates.

The Dutch physical chemist Jacobus Hendricus van't Hoff, generally regarded as the father of chemical kinetics, initiated the foundation of stereochemistry in 1874 with his work on optically active carbon compounds and three-dimensional and asymmetrical molecular structures. Three years later, he related thermodynamics to chemical reactions and developed a method for establishing the order of reactions. In 1889 the Swedish chemist Svante August Arrhenius investigated the speeding of chemical reactions with increase in temperature and enunciated the theory of electrolytic dissociation, known as Arrhenius's theory.

B

The 20th Century

The development of chemical kinetics has continued into the 20th century with the contributions to the study of molecular structures, reaction rates, and chain reactions by physical chemists such as Irving Langmuir of the United States, Jens Anton Christiansen of Denmark, Michael Polanyi of Great Britain, and Nikolay Semenov of the Soviet Union, and important basic research continues today.

In 1923 the American chemist Gilbert Newton Lewis further clarified the principles of chemical thermodynamics enunciated by Gibbs.

The great watershed period for the development of physical chemistry was 1900-1930, with the gradual elucidation of quantum theory. In 1897 the German physicist Max Planck had proposed that energy in certain systems is “quantized”, or occurs in discrete units or packages, just as matter occurs in discrete units, the atoms. In 1913, the Danish physicist Niels Bohr showed how the concept of quantization served fully to explain the spectrum of atomic hydrogen. In 1926-1929, the Austrian physicist Erwin Schrödinger and the German physicist Werner Heisenberg developed the picture of the wave function, a mathematical expression incorporating the wave-particle duality of electrons, and showed how to calculate useful properties from this formula.