TEACHING the NEW CONCEPTS of ACIDS and BASES in GENERAL CHEMISTRY' H. T. BRISCOE
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Indiana University, Bloomington, Indiana
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0 OTHER portion of the course in general chemistry receives so varied a treatment as the portion dealiig with acids and bases an$ closely related topics, such as neutralization and hydrolysis. Instructors are fairly well agreed in their opinions of what should be presented and how it should be presented in other portions of the course, such as those that deal with valence, atomic, and molecular weights, the structure of the atom, the properties of chlorine, and the uses of nitric acid. But when differentinstructors come to the study of electrolytes, and especially to the study of acids and bases, there is a sharp division of opinion. Some believe that acids and bases should be treated entirely from the point of view of the classical theories of Arrhenius and Ostwald. Acids ionize when dissolved in water, or certain other solvents, to liberate hydrogen ions or protons; bases ionize to liberate hydroxyl ions. Frequently, the newer ideas are presented very briefly, and the student is left with the impression that there may be some basis for them, but as yet they are untried Contribution to the Symposium on Theories and Teaching of Acids and Bases conducted by the Divisions of Physical Chemistry and Chemical Education at the ninety-seventh ineeting of the A. C. S., Baltimore, Md., April 4. 1939.
and not very useful. Other instructors either omit the classical concepts or pass over them very briefly. They prefer to accept the Br@nsted-Lowryconcept of protontransfer as the sole basis upon which the student is asked to understand and explain the behavior of acids and bases. A few instructors may adopt the more generalized classification of G. N. Lewis. The writer believes that there is still a third method of approach and procedure in teaching this subject. It is not claimed that this method is new. No doubt many instructors use it. It is based upon the principle that the student must iirst know the facts concerning the behavior of acids and bases, and that facts are of far greater importance than all the theories that have ever been formulated. With the facts before him, the student can then examine critically all the theories that have been offeredto explain the chemical characteristics and reactions of acids and bases. We must not lose sight of the origin of theories or of their proper place and use in the science. No theory is perfect. If it were, it would not be a theory. A theory serves its purpose only so long as, and to the extent that, i t is useful. Does it offer a reasonable and successful basis for the correlation and integration of
facts and for future work on the problem with which it is concerned? Is it, in short, a satisfactory working hypothesis which we can use to guide and shape our studies until we have more complete information and can build a better theory, or, perhaps, attain the 6nal truth? For the students of general chemistry, a theory must serve the same purpose that it offersthe graduate student or the independent scientist. It must serve as a reasonable basis for the understanding and correlation of the many facts and observations which they study in the text, and in the classroom and laboratory. Some of the difficulties that we find in our general chemistry classes may arise from the confusion of facts and theories. Students are inclined to accept theories on an equal basis with facts. It appears to the writer that this danger may be lessened if the student is made to understand that more than one theory has a t different times been offered to explain the same body of knowledge. Perhaps the real danger lies in teaching a single theory as the perfect solution of all the problems with which that theory is concerned. When this procedure is followed, the student may lose sight of the real meaning and the proper place of the theory in the science that he is studying. The writer believes, therefore, that in presenting a new subject, such as acids and bases, the student should study 6 r s t the properties and behavior of these substances, not theories concerning their chemical character, or theories concerning the manner in whicli they react. I n approaching the study of acids and bases, the student should be acquainted first with such information as can be provided on his level concerning the properties and constitution of electrolytes as distinguished from non-electrolytes. It is assumed, also, that he will be acquainted, a t least in an elementary fashion, with modern knowledge concerning the atom and interpretations of atomic structure that are based upon this knowledge. He will also h o w something about the diierences in properties of hetero olar and homopolar compounds; electrical conducti t y of solutions; electrolysis and Faraday's laws; the vapor tensions, boiling points, freezing points, and osmotic pressures of solutions; and the abnormal behavior of electrolytes. We have mentioned here some of the information that will have been provided before the student 6nally comes to the study of acid-base equilibria. The student is now ready to examine the theories that have been proposed to explain the behavior of electrolytes in general, and acids and bases in particular. The writer does not believe that the thorough presentation of Arrhenius' theory can be disregarded a t this point, or that to follow its presentation with the new concepts is inconsistent with good teaching. We can say to the student that here are the facts as Arrhenius knew them in 1887, and here is the theory that he based upon these facts. But we shouldn't stop a t this point. We should also ask the student to examine the facts and the theory. Does this theory explain all the facts? Is i t the only theory that Arrhenius might have offered to explain the facts? Furthermore, is the state of our
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knowledge on this subject no further advanced than i t was in 1887? Or have we discovered other information that makes possible a better, more satisfactory explanation? And here, i t may be said, is an opportunity for really effective teaching; an opportunity to develop in some small measure, a t least, a critical attitude. Was it necessary for Arrhenius to assume that all electrolytes, except a t infinite dilution, are only partially ionized? Was his assumption that the electrical conductance of a solution depends only upon the concentration of the ions and that the velocity of the ions is independent of the concentration either necessary or justified? Let us be even more critical. How does his theory explain the heats of reaction of strong acids and strong bases? Is it consistent with modem knowledge of the crystal structures of salts like sodium chloride? The student, himself, will find many other diiculties that stand in the way of his complete acceptance of the theory. He may ask why hydrogen chloride, which is a non-conductor in the pure state, should become a very excellent conductor when it is dissolved in water but not in benzene. He will not be satisfied by the answer that in the presence of water some of the molecules of hydrogen chloride dissociate into electrically charged atoms of hydrogen and chlorine, and other molecules do not. The ideas of equilibrium and reversibility do not help a great deal. Even with their help, he must still wonder why ions that once separate should combine again under the same conditions. If hydrogen chloride were a heteropolar compound, as sodium chloride is, the effect of the water dipoles might help somewhat. Even then, however, he might-still wonder how water dipoles could bring about the separation of ions and a t the same time allow the same ions to recombine. When he turns his attention to the solvent, water, in which the hydrogen chloride dissolves, his wonder and amazement may be still more increased. For here is a substance that, although a very feeble conductor, must produce a few ions. How is i t different from hydrogen chloride which apparently produces no ions? Certainly here, as elsewhere, Arrhenius' theory offers a very unsatisfactory explanation of the production of ions in the presence of water molecules. If the classical theories of electrolytes are thus critically examined, how can wejustify the elimination of the.new concepts? Surely it is not inconsistent with good teaching to have the students know that other explanations have been offered to modify and to replace the ideas that came into general use in 1887. Why should we acquaint the student with modern concepts of atomic and molecular structure, if it is not to offer a reasonable and satisfying basis for the understanding of such subjects as valence, the periodic classification of the elements, atomic weights, and the differences in properties of heteropolar and homopolar substances? Having done all this, why stop short of using this same background of understanding as a basis for explaining the ionization of electrolytes and the reactions of ions? Should the student of general chemistry become acquainted, therefore, with the Debye-Huckel theory and
the concept that substances which crystallize in ionic lattices are completely ionized in solution? Should he study the Br#nsted-Lowry concept that bases remove protons from acids and that a substance is an acid and may act as an acid regardless of whether or not it is in aqueous solution? Should he he offered the suggestion that the acid does not fnst ionize independently to liberate free protons? These questions must be answered from the student's point of view. If they provide a more reasonable and satisfying explanation for the facts that be studies and the observations that he makes, then they should bemade available to him. The writer believes that, when the problem is viewed in this light, there can be no doubt as to the answer. The presentation of more than one theory to explain the same group of facts may be argued by some to result in confusion in the minds of students. The writer wishes to state his opinion that there can he no confusion if we teach facts rather than theories, and if theories in genera1 chemistry, as well as elsewhere, are made to serve only the purpose for which they are built. Different theories can become confusing when they are studied and taught as worth-while and important ends in themselves, and quite apart from the facts that they attempt to correlate and explain. The beginning student in science should develop a critical attitude. His 6structor must make sure that he does not accept the first explanation that is offered as an ultimate truth, always to be revered, never to be questioned. The writer knows of no better way to develop the questioning attitude of science in the minds of students than by leading them to examine freely and openly all the suggested explanations and solutions of a given problem, especially when we admit that the problem has not yet been completely solved. If one of the functions of the teacher of science is to develop in his students this critical attitude, then surely this teacher must welcome the opportunity that comes his bay in teaching the subject of ionization and especially the chemistry involving acids and bases. Surely our students, even with their limited experiences, can he expected to study, compare, and criticize two theories as simple as the ArrheniusOstwald and Br#nsted-Lowry concepts. Much has been said and written on the dangers of unlearning. It might be said, for example, that confusion must result from the student's learning the classical concepts of acids and hases only to be told that they are now regarded as unsatisfactory and that newer concepts must be learned to take their place. There may be real danger here, but only when the theory is made all-important and comes to he regarded as fact. There is no danger if each theory is examined in the light of the facts that i t attempts to explain, and if the usefulness
of each theory as an intelligent basis for correlating and understanding these facts is tested sufficiently. There have been other occasions in the history of science when it has been necessary for teachers and students to consider more than one possible theory, as they have studied and taught certain subjects. As in the treatment of acids and bases, the writer believes that there is a great deal of value in having the student study critically a t least some of the older theories as well as the new. The atomic theory of John Dalton has been changed so radically during the twentieth century that some instructors regard its original form as worth only brief mention. The writer believes that it is very much in keeping with the objective of the general chemistry course to have students understand the factual evidence upon which the original theory was based. They should also understand the reasons that lay behind the inclusion of each concept in the theory. Then they should examine the adequacy of the theory as a basis for explaining valence, the reactions of atoms, and the structure of compounds. The need for new concepts will not be difficult to establish. It will then be time to present the relatively new knowledge of radioactivity, of electrons, of isotopes, and of the many other elements of twentieth-century chemistry and physics. Finally, the modern theory of the atom can be presented. Needless to say, this theory must be compared with Dalton's, and it, too, must be subjected to the tests used previously in the examination of the older concepts. The treatment here outlined is the same treatment that is recommended for the study of acids and bases and for the properties of electrolytes in general. " Where shall we use the new concepts of acids and bases in the general chemistry course? Wherever the student finds that they provide a useful and reasonable basis for his interpretation of facts and of observations made in the laboratory. Certainly use will be found for them in the study of the ionization of acids and bases, of the reactions occurring between these two classes of substances, of the theory of indicators, of titration, and of hydrolysis. Perhaps it may be necessary to extend and modify somewhat the work that the student does in the laboratory. We might add, for example, an experiment such as that described by Flowe i ~ in , ~which a solution of sodium acetate in glacial acetic acid is titrated against a solution of perchloric acid in the same solvent with basic fuchsin as the indicator. At least such experiments might show the student that there is need for the new classification of acids and bases and might emphasize the fact that chemical changes are not restricted to aqueous solutions. T m w s n s , J. CHBM. ED"=.,13,219 (1936).
