CONTEMPORARY ACID-BASE THEORY W. F. LUDER Northeastern University, Boston, Massachusetts
THE SUBJECT of acids and bases has been one of the
most controversial in chemistry. Since the time when distinctive properties of acids and bases were first recognized, no less than seven different theories of acids and bases have been proposed. However, it is nom- possible to be certain of the trend toward greater generalization. One test bf any theory is: How wide an area of facts does the same explanation cover? The history of our ideas about acids and bases is an excellent illustration of the gradual broadening of an originally narrow concept. An equally important test of a new theory is: Does it give a more fundamental explanation of the facts npon which the old theory was based? Let us consider the characteristic experimental behavior of acids and bases. In the seventeenth century Robert Boyle listed the properties of acids in water solution as follows: (1) (2) (3) (4) (5)
They have a sour taste. They dissolve many substances. They precipitate sulfur from its solution in bases. They change blue plant dyes to red. They lose all these properties on contact with bases.
Of the five properties, the fourth and fifth are still definitive. The "plant dyes" which change color are now called indicators, and many are known that do not come from plants and that show other color changes. When the acid "loses its properties on contact with bases" the change is called neutralization., I n Boyle's description of acids, bases Lave been implicitly defined. The two properties of bases that are to be emphasized in this discussion are those which are opposite to the corresponding properties of acids. Bases in water solution change the red color of the plant dye (litmus) to blue, and they neutralize acids. So whether we are thinking of acids or bases the two properties to keep in mind are: (I), the effect npon indicators; and (2), neutralization. In some way acids and bases are opposites, and one definition must depend upon the other. This is one aspect of their behavior that must be explained by any theory of acids and bases. In Boyle's time the composition of few chemical compounds was k n o m . In fact the distinction between an element and a compound was first clearly stated by Boyle himself. Nevertheless, by their properties hydrogen chloride, sulfur dioxide, carbon dioxide, and vinegar were recognized as acids, and lye, soda ash, and quicklime mere called bases. Their properties were
always referred to water as the solvent since at that time very few other solvents were known. After the discovery of oxygen, Lavoisier proposed the first theory of acids. According to Lavoisier one element, oxygen, was the essential constituent of all acids. I n some mysterious way oxygen was supposed to confer acidic properties upon all substances containing it. In fact its name, given it by Lavoisier, means ',acid-former.'' A few years later Davy showed that hydrogen chloride is a compound of hydrogen and chlorine with no oxygen present. He also demonstrated that many oxides are not acids, but instead are bases. These discoveries led to the adoption of hydrogen instead of oxygen as the one element supposed to be necessary for acidic properties. With the advent of the Arrhenius theory of ionization, acid and base were defined as follows: Acid: a substance containing ionizable hydrogen atoms. Rase: a substance containing ionizable hydroxyl radicals. . . The word ionizable meant ionizable in water solution. These definitions are called the water definitions because, in making them depend upon the two ions that unite to form water, chemists limited acid-base phenomena to aqueous solutions.
.
THE PROTON DEFINITIONS
Proton Acceptors. When sodium carbonate, sodium acetate, sodium phosphate, ammonia, and piperidine are dissolved in water they each affect indicators in the same way that metallic hydroxides do, and they all neutralize solutions of hydrogen acids. Hundreds of other compounds which do not contain hydroxyl radicals behave in the same way. Furthermore, it is possible to observe neutralization and the color changes of indicators in a number of nonaqueons solvents such as benzene. These facts led Rronsted, Lowry, and G. N. Le~ris, in the same year, 1923, but independently, to propose that any substance that could accept protons (hydrogen ions) from an acid should be called a base. This definition makes acid-base phenomena independent of any solvent and enlarges the number of bases enormously. In the proton definitions of acids and bases: An acid is a snbstance that can donate protons to a base. A base is a substance that can accept protons from an acid.
555
JOURNAL OF CHEMICAL EDUCATION
556
Displacement. According to the proton definitions, then, all reactions between acids and bases are displacement reactions. In all of them one base displaces another from combination with the proton. Examples of this behavior are given in Table 1. In the first equation of Table 1, the base HOH displaces the other base, chloride ion, from its combination with the proton. In the second. equation the base piperidine displaces hydroxyl ion from its combination with the proton.
. with
the indicator and will neutralize hydrochloric acid or sulfuric acid in water. The color of the indicator in acid solution is yellow. In another solvent such as toluene .or chlorobenzene, a drop each of the two bases gives the blue color with crystal violet as an indicator. Now if a drop of stannic chloride or a small amount of aluminum bromide is added to the blue basic solutions the color changes to yellow, shelling that stannic chloride and aluminum bromide are acids. Table 2 shows the experiments.
TABLE 1 B r k s t e d Displacements (One Base Displaces Another) HCI HOH HaOt HCl
+ HOH
+ CIHI,N + OH+ CsHllN
--
--
TABLE 2 Experimental Definitions
+
Color of Crystal Violet Solution i n
HJO+ CIt CsHuNH+ OHHOH HOH CsHllNIIf C1-
++
Even the neutralization represented by the third equation is a displacement: the hydroxyl ion displaces the water molecule from the hydronium ion. Thus the proton theory includes the water theory as a special case. This is one of the essentials of any new theory, that it include the familiar properties as well as those that compel the adoption of the new theory. The reaction between hydrogen chloride and piperidine (fourth equation in Table 1) may take place in a number of ways. For example, if each substance is dissolved separately in two portions of chlorobenzene, the two solutions have the same effect, respectively, upon a properly chosen indicator (e. g., crystal violet) as do their water solutions. And there is no evidence of any reaction between chlorobenzene and either of these two solutes. Yet when the two solutions are mixed in equivalent amounts neutralization takes place, as indicated by the color change of the indicator, and piperidinium chloride is obtained. According to the proton definitions piperidine has displaced the chloride ion from combination with the proton. The same product is obtained when the two substances are permitted to react directly in the gaseous state. It is plain that no solvent i s necessary for neutraC ization reactions.
.
Bases
CrHnN CHaCOOH (CH,)&O
C5HuN CH,COOH iCHd.CO
Blue
To those who have not seen it before, the demonstration that acetic acid may act as a base is almost as striking as that aluminum bromide may behave as an acid. In performing the experiment, for example the neutralization of aqueous hydrogen chloride by acetic acid, care must be taken not to add too much of the stronger acid. Only just enough dilute hydrochloric acid to change the indicator color from blue t o a yellowish green should be used. Then a large amount of glacial acetic acid must be added to bring back the blue color of the solution. Other similar experiments s h o ~that there are hundreds of substances which do not contain hydrogen and yet will behave as acids towardjndicators and in neutralization. Such experiments were first performed by Germann, Wicked, G. B. L. Smith, and G. N. Lewis. A few of these nonhydrogen acids are listed in Table 3. This work indicated the need for a return to an experimental basis for the definition of acids and bases.
THE ELECTRONIC THEORY OF ACIDS AND BASES TABLE 3
Experimental Definitions. of Acids and Bases. AlSome Acids That Do Not Contain Hydrogen though the proton definitions included all the subZnCb SO3 rn " ,, stances that show basic wro~erties.thev limited acids to those substances whkh' contain hidrogen. Yet there are many substances that behave exactly the same way toward indicators, and will neutralize bases just as well as any hydrogen acid. Consequently, a broadening of the definition of acids was needed which The experimental d e f i n i t i ~ sof G. N. Lewis are: would correspond to the enlarging of our ideas of bases. (1) An acid is any substance that will neutralize a base A few simple experiments, which anyone can perform such as sodium hydroxide. (2) A base is any substance in an elementary laboratory with common chemicals, that will neutralize an acid such as hydrogen chloride. will demonstrate that acids do not have t o contain The Theoretical Ezplanation. The explanation of hydrogen. Using crystal violet as the indicator, bases acid-base phenomena was suggested by Lewis in 1923, such as piperidine and acetone will give a blue color but was largely ignored until the appearance of Lewis' 7,
OCTOBER. 1948
551
second paper on the subject in 1938. I t is as follows: (1) An acid can accept a share in an electron-pair. (2) A base can donate a share in an electron-pair. (3) Neutralization is the formation of a coordinate covalent bond between the acid and the base. An example would be: F F-A
I
F Arid
H
+
I :X-H I
-
F
H : A--H
F-13
I
P'
13 Base
TABLE 5 Displacement of One Base by Another
1
H
In the coordinate covalent bond one atom has supplied both electrons of the pair that holds the nit,rogen and boron atoms together. The formation of this hond is neutralization. The electronic theory of acids and bases inrlwles both the water and the proton theories. When the equation for the reaction between hydronium and hydroxyl ions is written in simplified form (neglecting to shov the water molecule attached to the proton in the hydronium ion), it is plain that the explanation of this reaction is
Displacement Reactions of Acids and Bases. (1) One Ease displaces another .from combination with a n acid. Tahle 5 shows some further reactions of the three neutralization products of Table 4. In each example another base displaces the original base from its combination with the acid. In equation (4) of Tahle 5, the type of displacement emphasized by the proton definitions is illustrated. In HCL + :o:H-I II:O.TI .. this reaction one base, water, displaces another, cyanide Acid Base ion, from combination with the proton. The base, the same as that for other neutralization reactions. The water, forms a new coordinate bond with the proton hydrogen ion is an electron-pair acceptor; the hydroxyl from the hydrogen cyanide. Hydrogen cyanide is ion is an electron-pair donor. called a secondalyl acid because its acidic properties deThe proton, boron chloride, stannic chloride, and pend upon the presence of a primary acid, the proton, many other acids have electronic formulas in which the which is in combination with a primary base as the possibility of accepting pairs of electrons to form co- result of the reaction represented by equation (1) in ordinate covalent bonds is obvious. Such acids are Tahle 4. . . called primary acids. In a corresponding nay, bases BrBnsted rea!ictions are all df this type. But the same that can readily donate a share in one or more pairs of kind of reaction may be observed involving primary electrons are called primary bases. Three more ex- acids other than the proton. For example, the base amples of neutralization reactions between primary pyridine xi11 react with the neutralization product of acids and bases are listed in Table 4. In each example e-luation (2) to displace the base acetone from comhinathe product contains both an acid and a base held to- tion uvith the primary acid, boron chloride, as s h o ~ nin gether by a coordinate bond. This p r d u c t is the result equation (5). A new coordinate hond is formed h e t w ~ n of neutralization; yet it can still undergo further re- the pyridine and the boron chloride. action. When another acid or base is added displacement reactions oft,en occur.
-
TABLE 6 Disdacernent of One Acid bv Another
TABLE 4
-
H+l + :C; iN: -1 Acid Base
Acid
Acid
H : C ; ; X : (1) Formation of Coordinate Bond
C1 Forination of Coordinate Bond
Bsse
Bsse
1
+ 4H+ --cu+~ + S H 4 + + 3H' hlC3 + 3HOH + Cut& CuOH+ + H +
Cu(:NHn),+2
1
Formation of Coordinate Bonds
Al(OII), HOH
-
171 (8)
I!))
Equation ( G ) represents one. of the displacement r e actions that can. he observed with the neutralizat,ion product of equation (3). If concentrated sodium hydroxide is added to a solution of the compl? ion containing only a slight excess of ammonia, cupric hydroxide is readily precipitated. Hydroxyl ion has displaced ammonia from combination vith the acidic cupric ion. I t is clear that the Bronsted definitions represent a special case of the displacement of one base by another.
558
JOURNAL OF CHEMICAL EDUCATION
Furthermore they represent only one of the two types CONCLUSION of displacement included in the electronic theory of The development of the Lewis theory has been the acids and bases. The other type is the displacement of result of a gradual broadening of the experimental one acid by a second acid. foundation. With the discovery of more facts it has (2) One a d displaces another from combination with a become obvious, first, that the water dehitions mere base. Table 6 includes three examples of the second inadequate, and, later, that the proton definition mas kind of displacement. When aqueous hydrogen chlo- still limited. Finally it has become evident that one ride is added to the neutralization product of thereaction theory can include not only neutralization and the between the primary acid and base shown in equation action of indicators, for acids other than hydrogen (3) of Table 4 the result may be represented as shown in acids, but also the great body of phenomena represented equation (7) of Table 6. One primary acid, the proton, by the terms hydrolysis, amphoteric behavior, and displaces another primary acid, the cupric ion, from its complex ions. Furthermore, as discussed elsewhere,' combination with ammonia. the electronic theory of acids and bases can correlate a Another example in which one acid displaces a second large amount of organic chemistry, and, through its is provided by the reactions of amphoteric behavior as clarification of the relationship between acids and illustrated by equation (a), in which hydrogen ions dis- bases and oxidizing and reducing agents, can lead place aluminum ions from combination with hydroxyl to a greater amount of systematization in all chemions. istry. Reactions of hydrolysis also can he included in the electronic theory of acids and bases. An example is ' LUDER.W. F., and S. ZUFFANTI, '(The Electronic Theory of equation (9),in which copper ions displace hydrogenions. Acids and Bases," John Wiley & Sans, New York, 1946.
