There is perhaps no other class of equilibria as important as that involving acids and bases. As wc continue the study of chcmistry, wc shall find that the classification "acid-base reaction" includes a vast number
of chcmical changcs, so that the principles and practical points arc ofvery general use. Now wc must devote some time to a discussion of nomenclature and classification of acids and bases.
The classification of substanccs as acids was at first suggested by their sour taste, and alkalis were taken as those substanccs that could reverse or neutralize the action of acids. It was thought also that an acid must have, as ncccssary constituent, the clement oxygen, but in 1810 Davy demonstrated that hydrochloric acid contained only hydrogen and chlorinc. Shortly thereafter the view was taken that all acids had hydrogen as an essential constituent.
An explanation of why acids had differing strengths was one of the important results of the Arrhcnius ionic dissociation theory, developed between 1880 and 1890. The chcmical activity and electrical conductivity of solutions of acids were taken to be consequences of their reversible dissociation into ions, one of which was H*:
HC1 = H* + ci-.
The fact that different acids had different strengths was explained as a result of a variation of the degree of dissociation. A similar scheme applied to the behaviour of bases, which were all thought to producc the hydroxyl ion in solution:
NaOH(s) = Na*(aq)+ OH" (aq).
Thus, the proton was responsible for acidic properties, and the hydroxyl ion was responsible for basic properties.
While this point of view was a considerable advance in chcmical theory, it led to certain difficulties. The first of these concerned the nature of the proton in aqueous solution, and the second had to do with the fact that substanccs which did not contain hydroxyl ion were capable of acting as bases.
Упр. 3. Передайте основное содержанке текста в нескольких предложениях.
Упр. 4. Какое слово в тексте означает «главный, основной»?
Упр. 5. Найдите в тексте ответы на следующие вопросы:
1. Почему было трудно классифицировать вещества на кислоты и основания? 2. Что сначала считалось характерным признаком кислот? 3. Какой элемент счкпгался необходимым компонентом кислот до открытия Дэви? 4. Какое важное обстоятельство, касающееся свойств кислот, объяснила теория Аррсниуса? 5. Что являстся определяющим в поведении кислот и оснований?
Упр. 6. Закончите следующие предложения:
1. The subject of this text is... 2. In 1810 it was showed... 3. Hydrogen was considered to be ... 4. Different strengths of different acids were due to ... 5. Bases were thought to producc ...
Section III
up short dialogues according to the example.
— What is the subject of the next Iccture?
— Why should I know? (I've no idea.)
— Haven't you been at the previous lecture?
Ex. 2. Respond to the following statements:
1. The classification of substanccs into acids and bases has greatly changed. 2. The theory of ionic dissociation has played a great role in the development of chcmistry. 3. The problem of nomenclature is still very important in chcmistry.
Ex. 3. Give detailed answers to the questions.
1. What arc the characteristic properties of acids? 2. What is the modem definition of a base? 3. What was the role of the theory by Arrhcnius in the development of acid-base classification? 4. What is dissociation?
Ex. 4. Discuss the following topics:
1. The Problems of Classification in Chemistry.
2. The Evolution of the Theory of Acids and Bases.
3. The Present Conccpt of Acids and Bases.