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THE PRESENTATION OF ACIDS AND BASES IN TEXTBOOKS THOMAS S. LOGAN Davidson College, Davidson, North Carolina
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Brgnsted concepts of a d d and base made their appearance in the literature in 1923. These concepts began to appear in introductory textbooks about 1935, and since that time they have more and more supplanted the earlier concepts of Arrhenius. Examination of a number of textbooks published since 1940 shows that a very large majority of introductory textbooks now present acids and bases from this point of view. The definitions of Arrhenius are also often presented as the "older" concepts. However, some very substantial introductory textbooks may be found which use the Arrhenius definitions and which mention the Brgnsted concepts as a generalized extension. The wide adoption of the Brensted concepts a t the introductory level would lead one to expect a similar use in more advanced courses. A number of analytical and physical chemistry textbooks have been examined. While the number a t hand was far from inclusive it may be stated that in these more advanced courses the adoption and application of the Brgnsted concepts is very much less general than is the case a t the introductory level. Perhaps one-half of the qualitative analysis textbooks examined treat acids and bases more or less from the Brgnsted point of view. The quantitative analysis textbooks examined either ignore these concepts or mention them as another point of view in a separate section or footnote. Consistent. application is considered unnecessary. A definite minority of the first-year physical chemistry textbooks examined actually use the Brensted concepts with any degree of consistency. Even. these develop acid-base equilibria and hydrolysis in the tradltional fashion. One of the points made by proponents of the BNnsted system is that its use eliminates the consideration of hydrolysis as a separate phenomenon, with the attendant calculation of hydrolysis constants. This, and basic equilibria also, are incorporated in the treatment of conjugate acids ( 1 , Z ) . Some physical chemistry textbooks ignore the Brgnsted concepts, others mention them in separate sections. No high-school textbooks were examined. However, some fifty introductory students who had studied high-school chemistry were questioned. About ninetyfive per cent of these had studied acids and bases from the older point of view. If this situation is representative the use of the Br@nsteddehitions a t the college level may present a considerable problem of "unlearn-
ing" for students who have studied chemistry in high schools. In reading introductory textbooks which use the Brensted concepts, one encounters sometimes a disinclination to let go of the older ideas. For instance, how should compounds like NaOH, Ca(OH)%,and Al(OH)abe classified? We can find them variously named by Brgnsted proponents as metallic hydroxides, hydrous oxides, alkalies, salts, and bases. Certainly we know what is meant, but could not a more coherent classification be expected? One point is very striking in the qualitative analysis textbooks examined. The authors prefer not to emphasize equations showing hydrated cations. , The general point of view is that ions in aqueoussolution are hydrated, but the hydration numbers are not known with an accuracy that jnstifias definite formulas. Thus, although hydrolysis may be explained on a basis of hydrated cation acids, the tendency is to simplify it to the older concept of the union of cation with hydroxyl ion. This retention of the concept, hydrolysis, as a peculiar situation warranting a particular treatment, is characteristic of textbooks generally in spite of adoption of the Brgnsted concepts. P$ mentioned previously , a strict adherence to these ideas would seem to eliminate hydrolysis as a special case. Certainly, if we agree that acetic acid and ammonium ion are both acids, it seems superfluous to label the action of ammonium ion with water differently from the correspondmg action involvihg acetic acid. . The problem would seem to be to learn the acids. A great majority of the introductory textbooks, which adopt the Brgnsted concepts, use the hydrated cation acid to explain the acidity of aqueous solutions of such compounds as zinc chloride. Brgnsted uses this idea (3, 4), and it seems generally to be considered an adjunct to the Brgnsted concepts (6). Its entrance into textbooks is somewhat more belated than the Br@nsteddefinitions of acid and base. It is interesting to note that this idea predates the Brgnsted definitions by some sixteen years (4, 6), and was, of necessity, originally formulated 'in terms of the older concepts. The topic, hydrolysis, was examined in a number of introductory textbook published in the decade, 193039. Mention of hydrated cation acids was found in only one of these. It may be reasonably concluded that this idea has largely entered introductory text-
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books as an application, or extension, of the BNnsted definitions. Although considerable work has been done on these hydrated cations, treated as acids (6), they are rarely mentioned in introductory physical chemistry textbooks. Writing the formulas of hydrated cations is probably not easy for a beginner. A logical approach to the problem would seem to be through some discussion of Werner's ideas and the coordination number. The term, coordination number, was found in the indexes of about forty per cent of the introductory textbooks using the Br$nsted definitions. The treatment of the amphoteric, or amphiprotic, hydroxide in introductory textbooks is varied. The textbooks using the BrGnsted concepts generally account for the acidity of solutions of such compounds as zinc chloride as due to the acid nature of Zn(HeO)r++. This acid, in a solution of increased pH, would neutralize the base, OH-, forming the precipitate, Zn(HzO)2(OH)2. If this precipitate has acid qualities it would seem logical to suppose the acidity to be due to the remaining water of hydration. Further increase in pH would then cause the precipitate to dissolve in accord with the equation:
seems largely to be the nature of the experimentally determined structure of crystalline hydrates, used in conjunction with Werner's ideas of complex compounds. However, an ion in solution is not an ion in a solid, and it cannot be arbitrarily assigned a hydration number as a fact. Quite often the wording of a a textbook will leave the reader with the impression that it is a fact. In general, little attention is given to the hydration of anions. Much work on hydration of ions has been published. A review of the early work has been made by Washburn (9). To cite a few instances of more recent work, the hydration numbers of ions in solution have been calculated from transference experiments (lo), from salting out experiments (If), from studies of hydrolysis rates in various solutions (If), and are mentioned at times in connection with the measurement of thermodynamic properties (IS, 1.4). The values obtained vary with the methods used. While hydration is admitted, the assignment of definite formulas to hydrated ions in solution is a problem far from solved. It seems possible that here we have at least some of the reasons why the extension of the Brplnsted concepts into the region of definitely hydrated ions does not stand up Zn(H,Oj,(OH), 20HZn(OH).2H*O (1) well in textbooks of analytical and physical chemistry. It is surprising to note the number of textbooks which The question of the "rightness" or the LLwrongness" revert from hydrated cation acids, as discussed above, of these various concepts has been discussed numerous to the simpler picture represented by: times, as has the need of extended or generalized concepts in connection with a system of chemistry which Zn(0H)r 2NaOH NazZn01+ 2H10 (2) is dominantly aqueous. The extended concepts are In terms of such textual treatment, Zn(HzO)&++ func- not difficult, but need of them does not seem to be tions as an acid by losing protons from the water of acutely felt, a t least by some authors, in courses just hydration until the precipitate, Zn(H20)2(0H)2, is beyond the introductory level. It must have been formed. Then this amphoteric substance functions quite a surprise to many a student who had "Brplnsted" further as an acid by losing protons from the hydroxyl drummed into him as a freshman to discover that it groups, the water of hydration no longer being shown. was really not necessary to apply it in a study of analytiOf course, if it is actually a hydrous oxide, ZnO.xH%O, cal or physical chemistry or both. Some interesting water of hydration might be considered as still function- comment along these lines has been quoted anonying in the acid sense, but a beginner may have some mously in the preface of a recent textbook (15). difficulty in visualizing this from equation (2). The Other revisions of ideas have been entering the fundastructures of these aquohydroxy combinations do not mental courses in chemistry during the period since seem to have been settled with finality (7,s). Perhaps 1923. A considerable revision of oxidation-reduction this is the reason for the disinclination to show them. has occurred. The Debye-Hiickel extensions in elecAgain, the very cumbrousness of the full equations may trolyte theory appeared in 1923. This material has be a partial reason. been swiftly incorporated, and one finds little diverIt has been mentioned that authors of analytical text- gence between textbooks a t all levels. The Br$nsted books are not inclined to stress hydrated ions. This concepts havenot been so rapidly received, nor so unanitendency is even more marked in the physical chem- mously. There is some inconsistency in presentation, istry textbook& Here hydration of ions is mentioned and in particular it is strange that the use diminishes only in a general and conservative way, as a rule. An a s the level of the fundamental course rises. This situaexception is the hydronium ion. Its existence is fairly tion may cause one to wonder whether the heavy emgenerally conceded and its use is widespread. How- phasis in introductory textbooks is justified. ever, after it has been used to discuss the action of acids in water, the simple hydrogen ion is quite often substi- LITERATURE CITED tuted for it in textbooks a t all levels. (1) NAIMAN, B., J. CAEM.EDUC.,25,454 (1948). Some of the introductory textbooks state the as(2) HAMMETT, L. P., ibid., 17,131 (1940). sumptions and reasons for assigning hydration numbers BRI~NSTED, J. N., C h m . Rev., 5, 298 (1928). (3) to cations. In other cases, however, the values are (4) BJERRUM, N., ibid., 16,287 (1935). assigned as fact, with no indication of the assumptions made. The basis for assigning these definite formulas (Continued o n page 165)
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MARCH, 1949 THE PRESENTATlbN OF ACIDS AND BASES IN TEXTBOOKS (Continued from page 150)
JOHNSON, W. C., J. CHEWEDUC.,17,135 (1940). HALL,N. F..Chm. Rev.. 19.89 (1936). WHITEHEAD, T. H., ibid., 21, 113 (1937). WEISER,II. B., AND W. A. MILLIDAN,&25.1 I., (1939). WASHBURN, E. W., Tech. Quart., 21,360 (1908). WASHBURN, E. W., J.Am. Chem. Soc., 31,322 (1909). GLASSTONE, S., AND A. POUND. J. Chem. Soc. 127, 2660 (1925).
M., J . chim. phys., 43, 54 (1946); C.A., 40,5984 (12) CORDIER, (1046). (13) SCATCHARD, G., Chem. Rev., 3, 383 (1926). (14) STOKES,R. H., AND B. J. LEVIEN,J. Am. Chm. Soc., 68, 336 (1946). J. H., "Principles of Chemistry." 5th ed.,The (15) HILDEBRAND, Macmillan Co., New York, 1947.
