Acids and Bases: A Critical Reevaluation" ROBERT GLNELL Polytechnic Institute of Brooklyn, Brooklyn, New York
T
HERE exists a t present in the field of acids and bases considerable controversy between the proponents of the various theories. The controversy is not, however, over the experimental facts but rather over the definitions of the words, acid and base. I t does not appear necessary to reiterate here the experimental facts or the evidence in support of the various theories. This has been ably done in many recent publications (which incidentally also give extensive bibliographies) (I,.!?). We shall rather r e h m i n e some of the fundamentals. Let us go back to the definition of the term "acid as it was first used. Boyle (3) listed the properties of acids as follows: "They dissolve many substances, they precipitate sulfur from its solution in alkalies, they change blue plant dyes to red, they lose all these properties in contact with alkalies." Luder (I) comments on this defmition thus, "These were recognized as the properties of aqueous solutions of acids. If the solution of a substance in water had these and other typical acid properties, the substance itself was called an acid." This very accurate description is the starting point of acid-base theory. However, due to the state of science at that early date, this description was interpreted in a manner that led to certain misconceptions when considered in the light of present day science. These misconceptions have been the cause of the various definitions that plague acid-base theory today. I t seems therefore that it would be profitable to analyze these misconceptions. At the time of Boyle and for a long while after him, scientists thought that changes of matter could be sharply divided into two classes, changes of a chemical nature and changes of a physical nature. Among the various physical changes was listed solution. The reasoning was that since substances could be recovered unchanged from a solution, solution was equivalent to dividing a substance into many smaller parts-a "purely" physical change. As a consequence of this theory, the scientists of that day were certain that if hydrogen sulfate, dissolved in water, had certain properties, these properties must, of necessity, belong to the hydrogen sulfate itself. Hence they asked: "We know of a large number of substances which have similar properties in solution; what is it that they have in common which can be the cause of these peculiar properties?" Since they *Presented at the one hundred and fourth meeting of the American Chemical Society in Buffalo. New York. September 7 to 11.1942.
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knew that all compounds were composed of a limited number of elements, it seemed to them that one of these elements must be an "acidifying principle." This type of thinking was both reasonable and traditional. Thereafter, the search was on in full swing. We all know the subsequent history-the oxygen theory of acids, the hydrogen theory, the hydrogen-ion theory, etc. Let us now go back to the original definition and examine it in the light of modern chemical theory. We now think of solution as a chemical process. It differs from chemical change (in the older concept) in a quantitative sense rather than in a qualitative one. The processes occurring are of ihe same type but the energy changes are of a dierent order of magnitude. The chemical changes that occur can be illustrated by examining the solution of hydrogen acetate ("acetic acid") in water. In pure hydrogen acetate the strongest forces are probably the dipole forces which make the molecules dimerize. (The molecular weight of gaseous hydrogen acetate at temperatures just above its boiling point leads us to this conclusion.) In addition, there must be weaker van der Waals forces which hold these dimers together as a liquid. Hence we can write pure liquid hydrogen acetate as [(CH&!OOH),],. When this substance is mixed with water, the first reaction probably is to break the weaker forces and form hydrated dimers
+ mHzO
[(CH,COOH)*l,
-
n[(CH3COOH)9.xH201 (1)
(We know that a similar reaction occurs in benzene, as shown by molecular weight determinations.) The next reaction is the reaction of water with the hydrated dimers to form hydrated monomers. (CH,COOH)a.xHnO
+ (2y - x ) H ~ O - ~ ( C H I C O O H ~ ~ H(2)I ~ )
The structure of the monohydrate of hydrogen acetate can be shown symbolically by some picture of this sort. H
I //O H\ 0-H I \o-H.,' H
H-C-C
\
H-C-C
0-H
I
H
Lt
d - C
&H
1
This monohydrate undergoes further reaction with water giving the acetate ion and hydronium ion (CHCOOH.H.0)
+ H.0 a (CHzCO0.H20)- + H80+
(4)
The resonance structure of the acetate ion can be represented symbolically by
Now if solution is not a "physical" change but a chemical change then the original theory that the properties of a solution of hydrogen sulfate in water are the properties of hydrogen sulfate is no longer necessarily true. If we examine the facts, we soon discover that certain properties are common to both the pure substance and to its solution while others are peculiar to the solution. Let us now examine critically one of the characteristic properties of acidic solutions. We know that acidic solutions will react with basic solutions. We also know that the "acid," the substance whose solution is acidic, will react with the "base," the substance whose solution is basic. If these two reactions are truly the same then we can say that this property of the solution is due solely to the solute. If, however, the reactions are dierent then we must say that a solution can have properties which neither the pure solute nor the pure solvent possesses. Let us take as our examples the reaction between hydrogen chloride and ammonia and that between their water solutions. When hydrogen chloride reacts with animonia, we get ammonium chloride. This can be shown symbolically thus
Finally, we have the third reaction, traditionally called neutralization
I t seems self evident that reaction (8) is not the same as reaction (5). In reaction (8) the important thing that has happened is that the hydronium ion has combined with the hydroxyl ion. Let us take another examplethe reaction between carbon dioxide and calcium oxide and the reaction between their solutions. In the pure state the reaction is Ca++O=
+ C02 e Ca++COrm
(9)
Here the bond between the calcium and oxygen in the calcium oxide is broken and the oxygen ion attaches itself to the carbon in the carbon dioxide molecule. In the case of the water solutions the whole picture changes. F i t , when the calcium oxide dissolves in water, the following reaction occurs Caf+O=
+ (x + 1)HsO
-
Ca(H1O),++
+ 2 OH-
(10)
And similarly, the carbon dioxide reacts with the water (the predominant species) when it dissolves
When the two solutions are then mixed, two reactions occur.
+
--
Here two neutral molecules, each consisting of atoms HsOC. OH2H,O (I2) Ca(H90).++ + COs3 Ca++CO,- + xHnO connected by bonds which are essentially homopolar in character, react. The proton from the HC1 moleAresemblanceexists between equation (9) and equations cule moves over to the NH3 molecule, thus giving each (10) and (ll), but equation (9) bears no resemblance of the two new particles a charge. These charged to equation (12). particles (ions) are then held together by electrostatic To come back now to our original postulation, we forces. The reaction between water solutions of these have shown that the reaction between the pure solutes two substances is, however, more complicated. This is different from the reaction between the two solutions. reaction really consists of three reactions: the h s t two, Since we have traditionally labeled the reaction of an the reactions of each pure substance. with water, and acidic solution with a basic solution a neutralization, the third, the reaction between their solutions. The it seems hardly logical to give the same name to the first reaction is reaction between the pure solutes because it is so unlike a traditional neutralization. In this light we can again state that acidic solutions neutralize basic solutions, and further, that acids are substances which give acidic solutions and bases are substances which give basic solutions. However, the This reaction is very similar to equation (5). The latter part of this statement is open to grave objections. We have thus far only considered water as a solvent. next reaction is of a similar type.
}
What would happen if we chose another solvent? An extension in this direction has been discussed a t some length by the proponents of the solvent system theory. In any system of this sort, we only need one basic postulate. We must define neutralization. Once this is defined, the definitions of acidic and basic solutions follow naturally. Neutralization i s the union of soluo-positive ions with soluo-negatiue ions. In the water system the hydronium ion is the solvo-positive ion while the hydroxyl ion is the solvo-negative ion; in the ammonia system, the ammonium ion is the solvopositive ion while the amido ion, NH,-, is the solvonegative ion and so on. From this definition it follows logically that a n acidic solution is one which contains as one of the predominant species present a solvo-positive ion, and a basic solution i s one which contains as one of the predominant species present a solvo-negative ion. This definition removes many difficulties from our current acid-base theories, for according to it, we can readily understand how and why hydrogen chloride and zinc chloride both give acidic solutions in water, and we need not set any distinctions in our minds hetween them; both compounds are of the same type. Furthermore, without doing any mental gymnastics, we can understand how ammonium chloride can give a faintly acidic solution in water, a strongly acidic one in liquid ammonia and a basic one in selenium oxychloride. A theory of this sort places the emphasis where it belongs, on the solution. Acidity and basicity appear as properties of solutions and not of pure substances. Since solutions consist of two parts, a solute and a solvent, both must play a role in determining whether a solution will have acidic, basic, or neutral properties. As a consequence of this we cannot say that a substance is an acid or a base, per se,'but only that it is an acid or a base in the water system or the ammonia system, etc. The question now arises: what shall we call reactions like equation (5)? Must we invent a new terminology or does an appropriate one already exist? The answer is that a new terminology is unnecessary. We can use the bond nomenclature. In equation (5), two neutral molecules, each of which possesses a dipole, have reacted. A hydrogen atom stripped of an electron has moved from the HC1 molecule to the NHa molecule. The resulting two particles are now ions and are held together by a bond which is predominantly heteropolar in character. The reaction between calcium oxide
and carbon dioxide can be described in a similar way. The heteropolar bond between the calcium and the oxygen is broken and the oxygen ion attaches itself by means of a homopolar bond to the carbon in the carbon dioxide molecule. The resulting fragments are ions and are held together by a heteropolar link. This type of nomenclature, although it may seem at first a bit cumbersome, nevertheless says a great deal in an unambiguous manner. There is in this terfninology a varied assortment of bonds which can be used to classify reactions. First, the primary valence bonds or forces: the metallic bond, the heteropolar bond, the homopolar bond, and the semipolar bond; and second, the secondary valence bonds or forces: rigid dipoles, induced dipoles, and the dispersion effect or the quantum mechanical interaction of internal electron motions. Reactions can be classified according to the types and numbers of bonds broken and formed. One last word remains to be said. We have in our chemical nomenclature a number of usages which have grown up around the old theories, the principal one being the habit of naming those pure substances which contain hydrogen and which furnish hydronium ions upon solution in water, "acids." We also fail very often to make a distiuction between certain mixtures and pure substances. For instance, we often carefully point out the distinction between hydrogen chloride and its water solution, hydrochloric acid, but we usually ignore the distinction between hydrogen sulfate and its water solution, sulfuric acid. Of course, it is impossible to eradicate immediately these improper usages from scientific chemistry and chemical industry, but the distinction between acidic and basic solutions, which are mixtures, and pure substances should be carefully pointed out, and terms such as sulfuric acid for hydrogen sulfate should he avoided. ACKNOWLEDGMENTS
The author wishes to acknowledge the help given by many discussions with Dr. Raymond E. Kirk, Dr. Gilbert B. L. Smith, and Dr. Paul E. Spoerri. LITERATURE CITED
(1) L ~ D E R"The , electronic theory of acids and bases," Chem. Reu.. 27. 547-83 (1940). (2) "Acids a i d bases," a collection of papers from J. CHEM. Eouc., Mack Printing Co., Easton, Pennsylvania, 1941. 13) WALDEN. "Salts. acids. and bases." McGraw-Hill Book Co..
New ~ o r k 1629. .
