Development of Physical Chemistry during the Nineteenth Century
Development of Physical Chemistry during the Nineteenth Century
Before the Scientific Revolution of the sixteenth through the eighteenth centuries, all the natural sciences were grouped together under the heading of "natural philosophy." But by 1800 many had become separate disciplines, with distinct subject matters and investigative methods. During the later nineteenth century, however, it became increasingly clear that discoveries in one science often had major consequences for another, and many scientists sought to recover the previous unity between the sciences. One result was the rise of "physical chemistry," which united ideas and techniques from both physics and chemistry.
By the early nineteenth century, physics and chemistry had segregated into two distinct fields. Physics studied the general motions of bodies in space and time according to mathematical laws due to the action of various "forces." Chemistry focused on the discovery, analysis, synthesis, and qualitative description of particular species of matter, chiefly elements and compounds. Heat, light, electricity, and magnetism, once believed to be "imponderable" (weightless) fluids and hence chemical substances, were now thought by many to be only apparent effects of the motions of microscopic particles and invisible forces, and thus a part of physics instead. Attempts to develop a quantitative "force" chemistry, which would explain reactions by mathematical laws of atomic motions due to gravitational or electrical forces of attraction and repulsion, failed.
This situation changed beginning in the 1840s, when the new concept of "energy" began to replace the older one of "force." Like force, energy could be measured quantitatively and its effects expressed in mathematical equations; unlike force, it could be applied far more generally to subjects not involving any apparent mechanical motions or forces, and it more clearly distinguished between measurement of capacity and intensity factors (e.g., heat vs. temperature). By measuring the energy content of a substance or system, as well as changes in that content, physicists could better explain both the generation or absorption of heat and the production and alteration of electrical currents. This made it possible once again for chemists to study the effects of these currents on chemical reactions.
A key step forward was the formulation between 1842 and 1865 of the first two laws of thermodynamics—thermodynamics being the study of motion of heat and its capacity to do "work," such as producing light, electricity, or magnetic current. The first law of thermodynamics states that the energy content of a closed system (one isolated from outside energy sources) is constant, so that forms of energy within it can only be transformed into one another. The second law of thermodynamics states that no process of energy transformation for work is perfectly efficient; some energy is always randomly dissipated as heat, so that the "entropy" or degree of disorder in a closed system always increases. Together these laws mean that energy cannot be created from nothing, and that no transformation process can occur indefinitely without the input of additional outside energy.
The physical laws of thermodynamics initially promoted development of a field called "thermochemistry." In the 1840s Germain Hess (1802-1850) announced two new chemical laws, the law of constant summation of heat and law of thermoneutrality. These laws were quickly extended by Julius Thomsen (1826-1909) and Marcellin Berthelot (1827-1907). Thomsen determined the heats of neutralization for numerous acid-base reactions producing neutral salts, and he used galvanic cells (electrochemical batteries) to calculate the work necessary to decompose various chemical compounds. He also proposed that the evolution of heat accompanying a chemical reaction is a quantitative measure of the chemical affinity (degree of reactivity) of the reactants.
Berthelot introduced the terms "exothermic" and "endothermic" to describe chemical reactions that evolve and absorb heat. He also measured heats of combustion for many reactions and framed three laws governing all chemical processes:
1) The heat change in a chemical reaction equals the amount of work done internally in a chemical system.
2) Assuming no external work is done, the heat evolved or absorbed by a reaction depends only on the initial and final states of the reactants and products.
3) Every chemical change in a closed system yields the products accompanied by the greatest evolution of heat (the "principle of maximum work").
While Thomsen and Berthelot's laws proved to be true only for certain exothermic reactions, they provided chemists with the means for predicting and measuring the courses and products for many reactions.
Electrochemistry also made key contributions to the development of physical chemistry. Humphrey Davy (1778-1829) and Jacob Berzelius (1779-1848) used galvanic cells to decompose compounds into simpler components, by passing electrical currents through water-based solutions and collecting the reaction products at the oppositely charged wire poles. Between 1832 and 1834, Michael Faraday (1791-1867) discovered the two basic laws of electrolysis: 1) the amount of a reaction product collected (as solid or liquid) or evolved (as gas) at an electrode is a product of the strength of the electrical current and the time; and 2) it also depends upon the equivalent weights (ratios of combination) of the reaction products. Faraday also introduced the terms "electrolysis" to describe this process; "electrode" for the poles; "cathode" and "anode" for the positive and negative poles; "ion" (Greek for "wanderer") for the dissociated components that migrate to the opposite poles; and "cation" and "anion" for the ions attracted to the cathode and anode.
Further important advances in electrochemistry occurred in Germany. Between 1853 and 1859 Johann Wilhelm Hittorf (1824-1914) discovered that the concentration at the electrodes of salts in solutions, and the rates of cation and anion migration, differ between the cathode and anode. From 1867 to 1876 Friedrich Kohlrausch (1840-1910) used alternating rather than direct current to perform solution electrolysis, thereby minimizing the decomposition of salts in solutions and obtaining more accurate measurements of effective currents.
A third source of inspiration for physical chemistry was the kinetic theory of motion by gas particles. Alexander Williamson (1824-1904) proposed in 1850 a dynamical "kinetic" theory of chemical equilibrium, resulting from a balance between ongoing dissociations and reassociations of molecules and elements, instead of the older static theories of a balance between opposing gravitational or electrical forces. Rudolf Clausius (1822-1888), Leopold Pfaundler (1839-1920), August Horstmann (1842-1929), and Peter Waage (1833-1900) and Otto Guldberg (1836-1902) made further advances in this area. The latter pair, working between 1864 and 1879, collaborated to formulate the law of mass action, stating that a reaction in solution reaches an equilibrium point expressed by a mathematical constant.
The formal establishment of physical chemistry as a distinct scientific field was the joint effort of Jacobus van't Hoff (1852-1911), Svante Arrhenius (1859-1927) and Wilhelm Ostwald (1853-1932). In 1884 van't Hoff published his groundbreaking research on the behavior of solute particles in solutions. Drawing upon the work of Williamson, Clausius, Pfaundler, Horstmann, and Guldberg and Waage, he proved theoretically that the properties of solutions could be explained by the kinetic dissociation and association of some of the solute particles into molecules, whose activities obey the law of mass action and whose degrees of dissociation increase with the solution temperature.
That same year Svante Arrhenius completed a doctoral dissertation in which he argued that electrolysis could be explained by assuming that individual ions carry specific units of electrical charge. The amount of current passing through an electrolytic solution thus varied with the degree of dissociation of the solute, as a function of mass action rather than of electrical charge itself. When Arrhenius's skeptical professors gave him only a barely passing grade, he sent copies of his dissertation to several chemists, including van't Hoff and Ostwald, hoping for a more favorable response.
Between 1876 and 1884 Ostwald had shown that physical properties of solutions, particularly heats of reaction and electrical conductivity, could be used to calculate the speed and degree of completion of chemical reactions. Upon reading Arrhenius's dissertation and van't Hoff's book at almost the same time, Ostwald realized that their theories explained his own experimental results, and he established contact with both of them. Arrhenius joined Ostwald's laboratory, and in 1887 Ostwald and van't Hoff founded a new scientific periodical, the Zeitschrift für physikalische Chemie ("Journal of Physical Chemistry"), to propagate their research and views, with Ostwald as chief editor.
Between 1884 and 1887, drawing upon advances by Arrhenius and Friedrich Pfeffer (1845-1920), van't Hoff derived a chemical solution law (PV = iRT) that was analogous to the well-know gas law in physics (PV = nRT), where P = pressure, V = volume, n = number of moles of gas particles, R = the gas constant, and T = temperature. van't Hoff replaced the n with i, where i = the ionic dissociation constant, which differs for each solution. Further important points were van't Hoff's chemical use of both laws of thermodynamics, to explain the conversion of electrical energy into heat and the evolution of heat as signifying a shift in the point of reaction equilibrium, and his refutation of Berthelot's principle of maximum work by demonstrating that such reactions are reversible. In late 1887 and early 1888, Arrhenius and Ostwald made further key advances in this area.
The final area of early research in physical chemistry concerned reaction kinetics and catalysis. Between 1849 and 1854 Ludwig Wilhelmy (1812-1864) used polarized light to study the rate of inversion of the sucrose (table sugar) molecule between two forms, and found it to be related directly to temperature and inversely to concentration in solution. In 1884 van't Hoff established these relations in mathematical form and related them to changes in the internal energy contents of solutions. During the 1890s Ostwald defined a catalyst as "a substance that changes the velocity of a reaction without itself being changed in the process." He demonstrated that catalysts work by lowering the energy barriers necessary for a reaction to occur, but without changing the energy relations between the initial reactants and final products.
Calculation of energy factors thus proved important for studying the activity of chemical systems. Hermann von Helmholtz (1821-1894) and Josiah Willard Gibbs (1839-1903) both derived important formulas related to energy factors in chemical systems. Gibbs also introduced the concept of a chemical potential, or threshold of activity for chemical energy, and formulated the "phase rule" for studying physical changes of state between solid, liquid, and gaseous phases due to chemical reactions. Ostwald furthered the application of energy relations to chemical systems by translating Gibbs's little-known articles into German and republishing them in 1892.
The rise of physical chemistry was not simply a string of unbroken triumphs, however. Detailed explanations of the underlying mechanisms of ionic dissociation, electrolysis, reaction kinetics, and catalysis could not be offered until the advent of modern atomic theory, which made it possible to explain these in terms of molecular structure and the gain and loss of electrons by atoms. Ironically, instead of being a "general science" that would reunite physics and chemistry, as Ostwald hoped, physical chemistry instead developed into an independent discipline bridging the persisting gap between those fields.
Nevertheless, the successes were extraordinary. Arrhenius, van't Hoff, and Ostwald won three of the first nine Nobel prizes in chemistry. Today, physical chemistry ranks beside organic chemistry as one of the two leading areas of chemical research. Its basic concepts of ionic dissociation, energy relations, reaction kinetics, and catalysis are fundamental to all modern chemical research and industrial applications. Its success in melding ideas and discoveries from physics and chemistry inspired the creation of numerous other scientific cross-disciplines, such as biochemistry, biophysics, geochemistry, and geophysics.
JAMES A. ALTENA
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