CHAPTER VII THE LAWS OF CHEMICAL COMBINATION: THE ATOMIC THEORY I. Fundamental laws of chemical combination. Having considered two typical elements, and having gained some insight into chemical reactions through a study of the preparation of these elements and the combinations which they form with each other and with other elements, we may now go on one step farther. What generalizations have been reached in regard to the characteristics of chemical action? What theoretical ideas have been developed as to the mechanics of this action? These are the questions which suggest themselves and which we shall now consider. Aside from the question of energy relations, our purely material knowledge of chemical action may be stated in the form of four general laws. 1. The law of conservation of mass. In the earlier stages of the development of chemistry little importance was attached to the relations by weight between reacting substances. In a general way it was assumed that the total weight remained constant, but as heat, light, and phlogiston (the principle of combustion) were all considered to be material, and to escape during action, apparent loss of weight was to be expected. Lavoisier first clearly stated the principle of conservation of mass in 1785, attributing apparent changes to experimental error. Since his time scientists have been accustomed to regard the law a sort of axiom, and few experimental researches have been undertaken with the express purpose of testing it, but experiments carried out for other purposes can be cited in its support. Thus the work of the Belgian chemist Stas (1865) shows that in certain reactions the loss or gain could not have been more than from 2 to 4 parts in 100,000. In 1906 Landolt published the results of a series of experiments carried out at Berlin in critical test of the law. His general plan was to place the materials which were to act on each other (generally in solution) in the two limbs of a glass vessel of the form represented in Fig. 48. The open ends were then sealed off and the vessel weighed. The vessel was inverted and the materials thus brought into contact with each other, and after the reaction the vessel was again weighed. A large number of such experiments were carried out with every refinement of skill and apparatus, and very slight differences between the two weights were detected. These were never more than a few hundredths of a milligram in a total weight of 100 g., that is, about 1 part in 10,000,000. It is questionable whether these slight differences exceed the unavoidable experimental error. Certainly we may state the law in the following form: Within the limits of experimental accuracy no change in the total weight of matter can be detected when chemical action takes place. FIG. 48 2. The law of definite composition. The common experiences of the earlier chemists led them to believe that the composition of a pure compound is quite definite. The question as to whether this is so or not became an important issue in the years 1802-1808, as a result of the views of a distinguished Frenchman, Berthollet. On theoretical grounds Berthollet was led to believe that the composition of a substance is somewhat variable, being dependent on the relative quantities of the several materials present at the time of its formation. For instance, experiment showed that the composition of iron sulfide is, at least approximately, iron 63.55 per cent, sulfur 36.45 per cent. Berthollet thought that such figures were only approximate that if equal parts of iron and sulfur were to be heated together, a larger percentage of sulfur would be present in the product. These views were strongly opposed by a fellow countryman, Proust, who was professor of chemistry at Madrid during most of the controversy. Proust maintained that the composition of a pure compound is perfectly definite, and that when two elements form more than one compound, each has its own exact composition, there being no intermediate gradations. He maintained that apparent variability is due to lack of purity in the compound. Proust's experimental work was very accurate for his time, but his analyses were subject to errors of from 1 to 2 per cent. The advance in experimental exactness has steadily demonstrated the correctness of Proust's conclusions. In 1860 and again in 1866 the Belgian chemist Stas undertook elaborate researches in a critical study of the law of constant composition, кда. Proust made no comment on the relation between the ratios of oxygen to metal in the two cases, and his figures suggest none. Three investigators, Dalton, an English school-teacher (1805), Wollaston, his fellow countryman (1808), and Berzelius, a Swede (1811), — quite independently of each other, observed a striking relationship in such cases, which has come to be known as Dalton's law of multiple proportion, since Dalton first formulated it and was very active in seeking proofs of its validity. He showed that if the composition in such cases is stated not in percentages but in the weights of one element combined with a fixed weight of the other, then these weights are in the ratio of integer numbers. He showed that in nitrous oxide 1 part of oxygen is combined with 1.648 parts of nitrogen, while in nitric oxide 1 part of oxygen is combined with 0.798 part of nitrogen. The ratio of the two weights of nitrogen is therefore 1.648 0.798 or 2.06: 1, that is, 2:1 within the limits of error. In the case of the two hydrides of carbon (marsh gas and ethylene) Dalton found that the ratios of carbon to hydrogen are respectively 4.3 carbon: 2 hydrogen and 4.3 carbon : 1 hydrogen. He also recalculated some of Proust's results, showing that they supported his generalization, though the deviations are as much as 5 per cent. The results obtained by Berzelius in quite a large number of cases showed the generalization to be true to within possibly 0.3 per cent, In more recent times no definite tests of the law have been undertaken, but analyses of compounds, made with great care for other purposes, have been recalculated to test its accuracy, and within the unavoidable errors of analysis it has been found to be a precise statement of the facts. The composition of the two compounds, water and hydrogen dioxide, affords a good illustration of this law (p. 70), which may be conveniently stated in the following way: When two elements A and B form more than one compound, the weights of the element A, which combine with a fixed weight of the element B, stand in the ratio of small integers to each other. The most usual ratios are 1:1, 1:2, 1: 3, 2: 3, and 2: 5. 4. The law of combining weights. This law, which is often called the law of reciprocal proportion, was formulated by the German chemist Richter as the outcome of his researches between the years 1792 and 1799. He was of a mathematical turn of mind, and was interested in studying the numerical relations between the weights of combining substances. Most of his studies were concerned with those classes of substances known as acids and bases, which act readily upon each other. Richter found that if we take a series of acids, which we may designate as A, B, C, and allow them to act in succession upon a series of bases, designated by X, Y, Z, . . ., then a simple relation may be discovered. Let 1 g. of A, of B, and of C act successively on X, Y, and Z. Then Then a, a, a, are the weights of X, Y, Z respectively, which combine with 1 g. of A; b,, b, b, the weights of the same bases which unite with 1 g. of B, etc. Now Richter found that the ratio That is, the ratio between the several weights of the bases X, Y, Z, which combine with a fixed weight of the acid A, is the same as the ratio in which these three bases combine with any other acid, B, C, D. If with the three bases X, Y, Z this ratio had been determined as 2.2 4.3 6.8, then this generalization of Richter's states that the ratio in which the three combine with any acid is 2.2: 4.3: 6.8. In : : other words, 2.2 g. of the base X has the same value in its action with acids as does 4.3 g. of Y or 6.8 g. of Z. If it should be demonstrated that 1 g. of an acid D combines with 5.3 g. of the base X, then we can calculate what weight of the base Y it will require from the proportion 2.2: 4.3:: 5.3: x. Richter's work did not have any considerable influence upon his contemporaries, owing to a number of causes. His language was obscure and his ideas were expressed partly in terms of the old phlogiston conceptions and partly in accord with the newer oxygen ideas, and so found favor with the adherents of neither theory. He was led away from the really important part of his work by an endeavor to show that the ratios between the combining numbers of the bases are in arithmetical progression, while those of the acids are in geometrical - which is not true at all. Richter's ideas were rediscovered and extended by other workers, notably by Berzelius in 1811. It was found that not only to acids and bases but to every substance a number can be assigned which indicates its relative value by weight in chemical reactions. Evidently it would be a very great task to determine by direct experiment the combining number of each known substance, but by applying Richter's ideas to the elements the matter was very greatly simplified. 10788 Cu Br 79.92 31.78 CI 35.46 The combining weights of the elements. Experiment showed that it is possible to assign to each element a number which is proportional to the weight by which it enters into chemical action. The meaning of Ag this statement is more readily understood by reference to the diagram (Fig. 49), which gives the symbols of six elements, together with their combining weights as determined by experiment. By following the line connecting any two of these elements we may see at a glance the ratio by weight in which they combine. Thus 107.88 g. of silver combines with 35.46 g. of chlorine, with 79.92 g. of bromine, and with 126.92 g. of iodine. Similarly 100.3 g. of mercury combines with 79.92 g. of bromine, and 35.46 g. of chlorine combines with 126.92 g. of iodine. Sometimes an element acts upon a compound in Hg 100.3 126.92 FIG. 49 |