Physical Chemistry - chemistry.

Physical Chemistry - chemistry. I INTRODUCTION Physical Chemistry, field of science that applies the laws of physics to elucidate the properties of chemical substances and clarify the characteristics of chemical phenomena. The term physical chemistry is usually applied to the study of the physical properties of substances, such as vapor pressure, surface tension, viscosity, refractive index, density, and crystallography, as well as to the study of the so-called classical aspects of the behavior of chemical systems, such as thermal properties, equilibria, rates of reactions, mechanisms of reactions, and ionization phenomena (see Chemical Reaction; Heat; Heat Transfer; Ionization). 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 vigor with which various substances react with each other. Common examples are the easy 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 were determined independently; 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 (see Catalysis), 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 mutually exactly 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 1850-51. See Atom; Thermodynamics. These developments made it possible to begin to interpret the properties of gases, which represent the simplest states of matter, in terms of the behavior of their individual molecules. In the period 1860-75, 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 (see Gases). 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 toward 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 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 reactions. The application of the first and second laws of thermodynamics to heterogenous 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 1900-30. 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 1926-29, 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. From these beginnings, modern concepts of the structures of atoms and the nature of the bindings between atoms have evolved. III SUBDIVISIONS OF PHYSICAL CHEMISTRY The main subdivisions of the study of physical chemistry are chemical thermodynamics; chemical kinetics; the gaseous state; the liquid state; solutions; the solid state; electrochemistry; colloid chemistry; photochemistry; and statistical thermodynamics. A Chemical Thermodynamics This branch studies energy in its various forms as related to matter. It examines the ways in which the internal energy, degree of organization or order, and ability to do useful work are related to temperature, heat absorbed or evolved, change of state (for instance, from liquid to gas, gas to liquid, or solid to liquid), work done on or by the system in the form of the flow of electrical currents, formation of surfaces and changes in surface tension, changes in volume or pressure, and formation or disappearance of chemical species. B Chemical Kinetics This field studies the rates of chemical processes as a function of the concentration of the reacting species, of the products of the reaction, of catalysts and inhibitors, of various solvent media, of temperature, and of all other variables that can affect the reaction rate. It is an essential part of the study of chemical kinetics to seek to relate the precise fashion in which the reaction rate varies with time to the molecular nature of the rate-controlling intermolecular collision involved in generating the reaction products. Most reactions involve a series of stepwise processes, the sum of which corresponds to the overall, observed reaction proportions (or stoichiometry) in which the reactants combine and the products form; only one of these steps, however, is generally the rate-controlling one, the others being much faster. By determining the nature of the rate-controlling process from the mathematical analysis of the reaction kinetics and by investigating how the reaction conditions (for instance, solvent, other species, and temperature) affect this step, or cause some other process to become the rate-controlling one, the physical chemist can deduce the mechanism of a reaction. C The Gaseous State This branch is concerned with the study of the properties of gases, in particular, the law that interrelates the pressure, volume, temperature, and quantity of a gas. This law is expressed in mathematical form as the "equation of state" of the gas. For an ideal gas (that is, a hypothetical gas consisting of molecules whose dimensions are negligibly small, and which do not exert forces of attraction or repulsion on each other), the equation of state has the simple formula: PV = n RT, where P is pressure, V is volume, n is the number of moles of the substance, R is a constant, and T is the absolute (or Kelvin) temperature. For real gases, the equation of state is more complicated, containing additional variables due to the effects of the finite sizes and force fields of the molecules. Mathematical analysis of the equations of state of real gases permits the physical chemist to deduce much about the relative sizes of molecules, as well as the strengths of the forces they exert on each other. D The Liquid State This field studies the properties of liquids, in particular, the vapor pressure, boiling point, heat of vaporization, heat capacity, volume per mole, viscosity, compressibility, and the manner in which these properties are affected by the temperature and pressure at which they are measured and by the chemical nature of the substance itself. E Solutions This branch studies the special properties that arise when one substance is dissolved in another. In particular, it investigates the solubility of substances and how it is affected by the chemical nature of both the solute and the solvent. It also involves the study of the electrical conductivity and colligative properties (the boiling point, freezing point, vapor pressure, and osmotic pressure) of solutions of electrolytes, which are substances that yield ions when dissolved in a polar solvent such as water. F The Solid State This branch deals with the study of the internal structure, on a molecular and atomic scale, of solids, and the elucidation of the physical properties of solids in terms of this structure. This includes the mathematical analysis of the diffraction patterns produced when a beam of X rays is directed at a crystal. By using this method, physical chemists have gained valuable insights into the packing arrangements adopted by various types of ions and atoms. They have also learned the symmetries and crystallographies of most solid substances as well as their cohesive forces, heat capacities, melting points, and optical properties. See Crystal. G Electrochemistry This branch is concerned with the study of chemical effects produced by the flow of electric currents across interfaces (as at the boundary between an electrode and a solution) and, vice versa, the electrical effects produced by the displacement or transport of ions across boundaries or within gases, liquids, or solids (see Electricity). Measurements of electrical conductivity in liquids yield insights into ionization equilibria and the properties of ions. In solids, such measurements provide information about the states of the electrons in crystal lattices and in insulators, semiconductors, and metallic conductors. Measurements of voltages (electric potentials) yield knowledge of the concentrations of ionic species and of the driving forces of reactions that involve the gain or loss of electrons from a variety of reactants. See Electrochemistry. H Colloid Chemistry This branch studies the nature and effects of surfaces and interfaces on the macroscopic properties of substances. These studies involve the investigation of surface tension, interfacial tension (the tension that exists in the plane of contact between a liquid and a solid, or between two liquids), wetting and spreading of liquids on solids, adsorption of gases or of ions in solution on solid surfaces, Brownian motion of suspended particles, emulsification, coagulation, and other properties of systems in which tiny particles are immersed in a fluid medium. See Colloid. I Photochemistry This branch concerns the study of the effects resulting from the absorption of electromagnetic radiation by substances, as well as the ability of substances to emit electromagnetic radiation when energized in various ways. When X radiation interacts with matter, electrons may be ejected from their places in the interiors of atoms, ions, or molecules, and measuring the energies of these electrons reveals much about the nature of the electron arrangement within the atom, ion, or molecule. Similarly, investigation of the absorption of ultraviolet and visible light discloses the structure of the valence, or binding electrons (see Ultraviolet Radiation); absorption of infrared radiation provides information about the vibrational motions and binding forces within molecules; and absorption of microwaves permits scientists to deduce the nature of the rotational motions of molecules, and from this the exact geometries (internuclear distances) of the molecules. The study of the interaction of electromagnetic radiation with matter, when that interaction does not result in chemical changes, is often designated as spectrochemistry, and the term photochemistry is then used only for those interactions that produce chemical changes. Examples of photochemistry (that is, light-induced) reactions are the fading of dyes when exposed to sunlight, the generation of vitamin D in the human skin by sunlight, and the formation of ozone in the upper atmosphere by the ultraviolet radiation from the sun. See Photochemistry. J Statistical Thermodynamics and Mechanics This branch is concerned with the calculation of the internal energy, degree of order or organization (entropy), ability to do useful work (free energy), and other properties, such as the equations of state of gases, the vapor pressures of liquids, the molecular shapes adopted by polymer chains, and electrical conductivities of ionic solutions. These calculations are based on a model of the individual molecule or ion and the mathematical techniques of statistical analysis, which permit the mutual interactions of large numbers of randomly arranged particles to be evaluated. See Statistics. IV CURRENT STATUS Physical chemistry and chemical physics are vigorously active fields of research in chemistry today. Electrochemistry, colloid chemistry, and photochemistry are of great importance in many phases of modern industry. The current computer and communication revolutions, for example, could not have occurred without the special chemicals, crystals, and devices developed in the course of research in these branches of physical chemistry. In the area of fundamental research, as distinguished from applied research, the greatest emphasis today is on the theoretical analysis of spectra of all kinds, ranging from the X-ray region of the electromagnetic spectrum to the radio-wave region (see Spectroscopy; Spectrum). Emphasis is also placed on the application of quantum and wave mechanics to elucidate the principles of molecular binding and structure. Valuable insights into these questions have been gained by studying the properties of substances under conditions of both extremely high and extremely low temperatures and pressures as well as under the influence of strong electrical, magnetic, and electromagnetic fields. See also Chemistry; Physics. Contributed By: Seymour Z. Lewin Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.