A little bit of engineering (Несколько текстов для зачёта), страница 9

All these advances led in the 18th century to the discovery of new metals and their compounds and reactions. Qualitative and quantitative analytical methods began to be developed, and the science of analytical chemistry was born. Nonetheless, as long as the part played by gases was believed to be only physical, the full scope of chemistry could not be recognized.

The chemical study of gases, generally called “airs,” became important after the British physiologist Stephen Hales developed the pneumatic trough to collect and measure the volume of gases released from various solids by heating in a closed system and collecting over water. The pneumatic trough became a valuable device for the collection and study of gases uncontaminated by ordinary air. The study of gases advanced rapidly and led to a new level of understanding of various different gases.

The initial understanding of the role of gases in chemistry occurred in Edinburgh in 1756, when British Chemist Joseph Black published his studies on the reactions of magnesium and calcium carbonates (see Carbonates). When these compounds were heated, they gave off a gas and left a residue of what Black called calcined magnesia, or lime (the oxides). The latter reacted with “alkali” (sodium carbonate) to regenerate the original salts. Thus, the gas carbon dioxide, which Black called fixed air, took part in chemical reactions (was “fixed,” as he said). The idea that a gas could not enter a chemical reaction was overthrown, and soon a number of new gases were recognized as being distinct substances.

The British physicist Henry Cavendish isolated “flammable air” (hydrogen) in the next decade. He also introduced the use of mercury instead of water as the confining liquid over which gases were collected, making it possible to collect water-soluble gases. This variant was used extensively by the British chemist and theologian Joseph Priestley, who collected and studied almost a dozen new gases. Priestley's most important discovery was oxygen, and he quickly realized that this gas was the component of ordinary air that was responsible for combustion and made animal respiration possible. However, he reasoned that combustible substances burned more energetically in this gas, and metals formed calxes more readily, since it was devoid of phlogiston. Hence, the gas accepted the phlogiston present in the combustible substance or the metal more readily than ordinary air, which was already partially filled with phlogiston. He named this new gas “dephlogisticated air” and defended that belief to the end of his life.

Meanwhile chemistry had been making rapid progress in France, particularly in the laboratory of Lavoisier. He was troubled by the fact that metals gained weight when heated in the air when presumably they were losing phlogiston.

In 1774 Priestley visited France and told Lavoisier about his discovery of dephlogisticated air. Lavoisier quickly saw the significance of this substance, and the way was opened for the chemical revolution that established modern chemistry. He used the name “oxygen,” meaning acid former.

Lavoisier showed by a series of brilliant experiments that air contains 20 percent oxygen and that combustion is due to the combination of a combustible substance with oxygen. When carbon is burned, fixed air (carbon dioxide) is produced. Phlogiston therefore does not exist. The phlogiston theory was soon replaced by the view that oxygen from the air combines with the components of the combustible substance to form oxides of the component elements. Lavoisier used the laboratory balance to give quantitative support to his work. He defined elements as substances that could not be decomposed by chemical means and firmly established the law of the conservation of mass. He replaced the old system of chemical names (which was still based on alchemical usage) with the rational chemical nomenclature used today, and he helped to found the first chemical journal. After his death on the guillotine in 1794, his colleagues continued his work in establishing modern chemistry. A little later the Swedish chemist Jöns Jakob Berzelius proposed symbolizing atoms of the elements by the initial letters or pairs of letters from their names.

By the beginning of the 19th century the precision of analytical chemistry had improved to such an extent that chemists were able to show that the simple compounds with which they worked contained fixed and unvarying amounts of their constituent elements. In certain cases, however, more than one compound could be formed between the same elements. At the same time the French chemist and physicist Joseph Gay-Lussac showed that the volume ratios of reacting gases were small whole numbers (which implies the interaction of discrete particles, later shown to be atoms). A major step in explaining these facts was the chemical atomic theory of the English scientist John Dalton in 1803.

Dalton assumed that when two elements combined, the resulting compound contained one atom of each. In his system, water could be given a formula corresponding to HO. He arbitrarily assigned to hydrogen the atomic weight of 1 and could then calculate the relative atomic weight of oxygen. Applying this principle to other compounds, he calculated the atomic weights of other elements and drew up a table of the relative atomic weights of all the then known elements. His theory contained many errors, but the idea was correct, and a precise quantitative value could then be assigned to the mass of each atom.

The major weaknesses in Dalton's theory were that he did not account for the law of multiple proportions and made no distinction between atoms and molecules. Thus, he could not distinguish between the possible formulas for water HO and H₂O₂, nor could he explain why the density of water vapor, with its assumed formula HO, was less than that of oxygen, assumed to have the formula O. The solution to these problems was found in 1811 by the Italian physicist Amedeo Avogadro. He suggested that the numbers of particles in equal volumes of gases at the same temperature and pressure were equal and that a distinction existed between molecules and atoms. When oxygen combined with hydrogen, a double atom of oxygen (a molecule in our terms) was split, each oxygen atom then combining with two hydrogen atoms, giving the molecular formula of H₂ O for water and O₂ and H₂ for molecules of oxygen and hydrogen.

Unfortunately, Avogadro's ideas were overlooked for nearly 50 years, and during this time great confusion prevailed among chemists in their calculations. It was not until 1860 that the Italian chemist Stanislao Cannizzaro reintroduced Avogadro's hypotheses. By this time chemists had found it more convenient to take the atomic weight of oxygen, 16, as the standard to which to relate the atomic weights of all the other elements instead of taking the value 1 for hydrogen, as Dalton had done. The molecular weight of oxygen, 32, was then used universally and, expressed in grams, was called the gram molecular weight of oxygen, or more simply, 1 mole of oxygen. Chemical calculations were standardized, and fixed formulas written.

The old problem of the nature of chemical affinity remained unsolved. For a time it appeared that the answer might lie in the newly discovered field of electrochemistry. The discovery in 1800 of the voltaic pile, the first true battery, gave chemists a new tool, which led to the discovery of such metals as sodium and potassium. It seemed to Berzelius that positive and negative electrostatic forces might hold elements together; at first his theories were generally accepted. As chemists prepared and studied more new compounds and reactions in which electrical forces did not seem to be involved (the nonpolar compounds), the problem of affinity was shelved for a time.

The most striking advances in chemistry in the 19th century were in the field of organic chemistry (see Chemistry, Organic). The structural theory, which gave a picture of how atoms were actually put together, was nonmathematical, but employed a logic of its own. It made possible the prediction and preparation of many new compounds, including a large number of important dyes, drugs, and explosives that gave rise to great chemical industries, especially in Germany.

At the same time, other branches of chemistry made their appearance. Stimulated by the advances in physics then being made, some chemists sought to apply mathematical methods to their science. Studies of reaction rates led to the development of kinetic theories that had value both for industry and for pure science. The recognition that heat was due to motion on the atomic scale, a kinetic phenomenon, led to the abandonment of the idea that heat was a specific substance (termed caloric) and initiated the study of chemical thermodynamics (see Thermodynamics). Continuation of electrochemical studies led the Swedish chemist Svante August Arrhenius to postulate the dissociation of salts in solution to form ions carrying electrical charges. Studies of the emission and absorption spectra of elements and compounds became important to both chemists and physicists (see Spectroscopy; Spectrum). In addition, fundamental research in colloid and photochemistry was begun. By the end of the 19th century, studies of this type were combined into the field known as physical chemistry (see Chemistry, Physical).

Inorganic chemistry also required organization. The number of new elements being discovered continued to grow, but no method of classification had been developed that could bring order to their reactions. The independent development of the periodic law by the Russian chemist Dmitry Ivanovich Mendeleyev in 1869 and the German chemist Julius Lothar Meyer in 1870 eliminated this confusion and indicated where new elements would be found and what their properties would be (see Elements, Chemical; Periodic Law).

At the end of the 19th century chemistry, like physics, seemed to have reached a stage in which no striking new fields remained to be developed. This view changed completely with the discovery of radioactivity. Chemical methods were used in isolating new elements such as radium, in the separation of the new class of substances known as isotopes, and in the synthesis and isolation of the new transuranium elements. The new picture of the actual structure of atoms obtained by physicists solved the old problem of chemical affinity and explained the relation between polar and nonpolar compounds. See Nuclear Chemistry.

The other major advance for chemistry in the 20th century was the foundation of biochemistry. This began with the simple analysis of body fluids; methods were then rapidly developed for determining the nature and function of the most complex cell constituents. By midcentury biochemists had unraveled the genetic code and explained the function of the gene, the basis of all life; the field had grown so vast that its study had become a new science, molecular biology. See also Genetics.

Recent advances in biotechnology and materials science are helping to define the frontiers of chemical research. In biotechnology, sophisticated analytical instruments have made it possible to initiate an international effort to sequence the human genome. Success in this project will likely completely change the nature of such fields as molecular biology and medicine. Materials science, an interdisciplinary combination of physics, chemistry, and engineering, is guiding the design of advanced materials and devices. A recent example is the discovery of high-temperature superconductors, ceramic compounds that lose their resistance to the flow of electricity above 77K (-196° C/-321° F; see Superconductivity). Characterization of surfaces is being advanced by the invention of the scanning tunneling microscope, which can provide images of certain surfaces with atomic-scale resolution. See Microscope; Superconductivity.

Even in conventional fields of chemical research, new, more powerful analytical tools are providing unprecedented detail of chemicals and their reactions. For example, laser techniques are providing snapshots of gas-phase chemical reactions on the femtosecond (a millionth of a billionth of a second) time scale. From the soot produced by graphite electrodes has been isolated a new form of carbon, called buckminsterfullerene, that has the shape of a soccerball, and the chemical formula C₆₀. This compound and its chemistry have been characterized with astonishing rapidity using the vast array of analytical techniques currently available. Certain alkali metal salts of this compound have even been found to be superconducting.

The growth of chemical industries and the training of professional chemists had an interestingly shared history. Until about 150 years ago chemists were not trained professionally. Chemistry was advanced by the work of those who were interested in the subject, but who made no systematic effort to train new workers in the field. Physicians and wealthy amateurs often hired assistants, only some of whom continued their masters' work.