大学初年級の化学教育における酸-塩基理論の取り扱い方について

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Although acid-base reaction play one of the most important roles in the main part of chemistry, the definitions of acid and base have not been well established. It is not too far off in saying that the history of the acid-base concept has been the changes in the definition of acids and bases. Arrhenius, towards the end of the nineteenth century, presented a definition which is only applied to aqueous solutions. Then, in 1923, two important ideas were published by J. N. Brφnsted and G. N. Lewis. The older concept of Arrhenius was extended by Brφnsted to include all the processes involving protons. Acrcording to Brφnsted, "an acid is a species tending to lose a proton and a base is a species tending to gain a proton". The definition of Lewis is even broader and more general, and can be applied to reactions which do not involve protons. Lewis acids are defined as electron acceptors and bases as electron donors. When we study the content of the present high-school level texts, we find that most of them does not go beyond Arrhenins definition and the important ideas of Lewis are barely mentioned. Even when these more generalized ideas by Brφnsted and Lewis are introduced, the examples that are listed as acid-base reactions are confind to those in aqueous media. This could give students a wrong impression that acid-base reactions are a special type of interactions that only occur in aqueous systems. On the other hand, reactions between acidic oxides and basic oxides, for example,, are included without being mentioned that they can also be classified as acid-base interaction. Moreover, both Arrhenius and Brφnsted definitions even have to exclude one of the most simple acid-base gas reactions, e.g., NH_3(gas)+HCl(gas)→NH_4Cl. The generalized acid-base concept of Lewis can easily accomodate the above reactions within its scope. It includes reactions in polar or non-aqueous solvents, gas reactions or those between molecules in inert organic solvents. The operational difinitions of Lewis acids and bases can easily be incorporated in high-school level chemistry, and examples of experiments which demonstrate the idea can easily be found, using both inorganic and organic reactants and solvents. On the foundation of this generalized approach of Lewis acids and bases taught in high school level, one can easily build up the vast knowledge of chemistry in an organized and unified way. It also has an advantage of making it easier and more straightforward to appreciate modern publications. It is because many of the recent theories and arguments in this field are based on Lewis concept of acids and bases. There are many problems, however, which cannot be dealt with by Lewis definitions. For example, concept of. acid and base strength is not included in the definition. In general terms, a stable complex would be formed between a strong acid and a strong base, and an unstable one between a weak acid-base combination. There have been, several attempts to define scales for strength or classify acid and base ions and molecules according to the stability of the resulting acid-base complexes. Two of the more recent approaches are introduced in some detail. R. G. Peason, in 1963, devided Lewis acids and bases into two classes each; "hard" acid and bases and"soft" acids and bases. Then he proposed a principle which states that "hard acids prefer to associate with hard bases, and soft acids prefer to associate with soft bases." His principle gained quite a popularity in the latter half of the decade because of its practical usefulness and descriptive simplicity. It is still one of the most useful approaches in organizing vast quantities of chemical information. Yet it has also been pointed out that the principle lacks theoretical background and quantitativeness. One of the most scathing criticisms came from R.S. Drago who, in 1965, proposed an empirical equation to calculate the enthalpy of formation of Lewis acid-base complexes. Drago also tried to rationalize his equation using quantum mechanical treatment. At present, we have to say that the concept of acid-base strength is still in the middle of controversy and no one theory satisfies all the aspects of acid and bases. Therefore, one must be careful about the way of intoroducing the subject to students. Yet it seems true that many of the modern reseaches are based on the generalized acid-base concept which, at the same time, can be the best way of organizing the greater part of chemistry. It is our proposal, therefore, to introduce and maintain Lewis concept all through high school and colledge chemistry courses in order to unify and simplify the teaching of chemistry, the remebering of chemical facts, and the interpretation of data.
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大学初年級の化学教育における酸-塩基理論の取り扱い方について

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