ACID-BASE REACTIONS' Their Analogy to Oxidation-Reduction Reactions in Solution THOMAS H. HAZLEHURST Lehigh University, Bethlehem, Pennsylvania
ture of considerable energy -- to remove a proton from the EACHERS and textbooks of elementary them- acid. This work is done by the base by means of its are swinging more and more toward the strong proton affinity. Thus the terms "donor" and "acce~tor" might profitably be replaced by "loser" and Rrdnsted theorv of and to the .. r . . ..acids .~ .--. - ~ - - bases -~ ~ .. ~ ~ -.-... -.-~ . "winner." of the older view [(I). (2). /3)1. This offers the oossi,, ,, ,. bility of combining the teaching of acidity and oxidation DISCUSSION in a very attractive way. A comparison of the last two sections will show that For a long time chemical reactions have been classified are mere paraphrases of one another, the words into two large categories: oxidation-reduction (o/r), they " electron," "reducer," and "oxidizer" being the analogs and the rest of the reactions. The latter class included double decomposition reactions, of which ueutraliza- of the words "proton," "acid," and "base." Furthertions, or, more properly, acid-base (a/b) reactions were more, amphoterism exists among oxidizers and acids, one type. It is proposed to show in this paper that a/b alike: HS0,- is both acid and base; Sn+= is both reactions are not double' decomposition reactions, but oxidizer and reducer. The analogy between a/b and olr reactions remains are closely analogous to o/r reactions. valid even after closer consideration of the detailed OXIDATION-REDUCTION REACTIONS physical mechanism of the processes in question. DurBy definition, a reducer is an electron donor. An ing o/r reactions, an electron is extracted from the oxidizer is an electron acceptor. An o/r reaction is one valence shell of one atom and inserted into that of anin which a reducer loses an electron to an oxidizer. A other. Can it be said that during an a/b reaction a reducer cannot demonstrate its reducing power unless proton is extracted from the valence shell of one atom an oxidizer is present, and vice versa. The reaction is and inserted into that of another? Hydrogenous compounds aie unique among molea mutuul affair between oxidizer and reducer. The driving force of the reaction is really the electron cules because the core of the hydrogen atom, alone a5nity of the oxidizer, because the reducer does not among atomic cores, is a bare nucleus. In any other actually repel electrons. On the contrary, i t requires atom the core, consisting of the nucleus plus any comthe expenditure of considerable energy to remove an pleted shells of electrons below the valence shell, is electron from a reducer. This work is done by the immensely larger because of the presence of a t least one oxidizer by means of its strong'electron affinity. It is such completed shell. Atomic cwes have diameters of thus clear that the terms donor andvacceptor are in a 1 to 2 A. Bare nuclei (including the proton or core of certain sense misnomers. The reducer's "donation" is the hydrogen atom) have diameters of the order of A,, and so are almost incomparably smaller. The a forced one, and the oxidizer's "acceptance" is of a very active variety. It is rather like saying that a man "thickness'! of the valence shell of an atom is not, of "accepts" a stone which the ground "gives" him as he course, very definite, but will be a t least 0.1 A. Hence lifts it. More appropriate terms might be "electron it is easily possible for the proton to bury itself in the valence shell of the atom to which it is attached, whereas loser" and "electron winne~."~ it is impossible for the core of any other atom to do the ACID-BASE REACTIONS same thinp. semi-quantitative pictures of the arrangement of By definition, an acid is a proton donor. A base is a proton acceptor. An a / b reaction is one in which an electrons and protons in hydrogenous molecules have acid loses a proton to a base. An acid cannot demon- been published (4) showing how the proton embeds strate its acidity unless a base is present, and vice versa. itself in the valence shell of another atom. Hence it may 'be said truthfully that hydrogenous molecules The reaction is a mutual affair between acid and base. The driving force of the reaction is really the proton possess valence shells which are composites of protons affinity of the base, because the acid does not actually and electrons, and if a proton is lost by the molecule i t repel protons. On the contrary, i t requires the expendi- has come from the valence shell.s INTRODUCTION
T
-
.~
~
~
Presented before the Division of Chemical Education at the one hundredth meeting of the A. C. S., Detroit, Michigan, Sc~tember9-13. 1940. i "Electron snatcher" would be a mare descriptive and striking tam!
-
"t is probably superfluous to state that the proton which i s . transferred cannot possibly come from the nucleus of the atom concerned anv more than electrons can he extracted from completed shells bf the core by the energy available in a chemical : reaction.
466
The analogy is thus complete, even in the intimate details of the transfer of proton and electron, respectively. Why are a / b reactions usually classified among the double decomposition reactions, whereas o/r reactions are not? Chiefly because the protons are represented explicitly in the equation by a symbol, H, whereas the electrons are represented only indirectly by the charges assigned to the various elements present. Also because reactions are customarily written in the molecular rather than in the ionic form. Some schemes for balancing o/r equations include as one step the formation of half-equations showing the action of oxidizer and reducer separately, thus: Compare these with the half-equations HCI = C19, OHz 9 = OHs'
+
+
On one side of the first two equations the electrons are represented only implicitly in the charges assigned to the elements, but in the last two equations the protons are explicit on both sides a s p and H, respectively. When the half-equations are combined, all trace of the electrons will have been lost: Mg V z O . = MgO On the contrary, the protons will still be explicit: .
+
HCI
+ OH2 = Cl- + OHa+
The usual type of double decomposition reaction is an ionic reaction in which one pair of ions is removed from the sphere of activity by precipitation, volatilization, or the formation of a very slightly ionized complex. The last is usually exemplified by the typical neutralization of a hydroxide by an acid:
2Hg0 = 2Hg HISO, = Ha0
++ Oz SO*
(0/1) (not o/r)
This pair of types can never be considered as a / b reactions, however, because the typical a / b reaction is acid
+ base = base + acid
which requires two species on each side of the equation with few exceptions. (3) Displacement can always be classified as oxidation or neutralization: Zn" SiO.
++ Cu+' = Zn12 + Cu" Na*COa = NanSiOa+ COX
(oh)
(not o/r)
When written ionically, the last reaction conforms to an extended definition of acidity, and is often referred to as "the displacement of a volatile acid by a non-volatile one" : Si01 (acid)
+ COa-' (bare) = Si08-* (base) + CO, (acid)
(4) Double decomposition reactions, as has been shown, should not include neutralizations as a subclass. Apart from a/b reactions, the processes commonly placed in this group are more properly included under the head of combination. For example, equation ( A ) above, written ionically : Baf2
+
=
B~SO,
The second objection to 'the usual classification is that it fails to emphasize the essential reversibility of all reactions. It is not wise to accustom the student to read an equation always and only from left to right, Every typical process has its inverse: for oxidation there is reduction; for acidifying,. "alkalizing"; for combination, decopposition. .. CONCLUSION
A rational classification of reactions would thus be: NaOH + HCI = NaCl + OH2 (un-ionized) (1) a/b, ( 2 ) 017, ( 3 ) c/d (combination-decomposition), Written in the true ionic form, however, it is clearly not in which the mutual relationship of the inverse processes a combination of H+ and OH- to form un-ionized OH*, is manifested and in which the number of classes is but rather a transfer of a proton from OHa" to OH-: reduced to three. Many reactions would still be placed in more than one category (for example, reaction (B) OHf + OH- = OH% OH% above), but even this ambiguity could be avoided by Any other type of double decomposition is more truly using the c/d class only if the process fails to fall into a combination of ions to form long-lived complexes: the class of 017. Reactions represented by complex equations may naturally fall into more than one class. For example, THE CLASSIFICATION OF CHEMICAL REACTIONS nearly all o / r reactions involving oxygenous ions are are definitely transIt is illuminating to review the usual classification of a / b reactions as well, since chemical reactions in the light oi the preceding discus- ferred and the acidity of the solution is altered: sion. Consider the types called: (1) combination, (2) decomposition, (3) displacement, (4) double decomposition, (5) neutralization, (6) oxidation. There are two objections to this scheme or to one of its variants. The ionic equation is the simplest possible for the overFirst, such a classification is never unique: all reaction, although i t is recognized that the actual (1) Combination may or may not involve oxidation: reaction undoubtedly proceeds in steps much simpler than that represented by this equation. Here definitely there is a / b as well as o/r, into which latter class (2) The same is true of decomposition, which, after the reaction is commonly placed. With the growing emphasis, even in elementary chemall, is only a combination read from right to left:
+
istry, upon ionic equations and modern theories of acidity, i t would seem feasible to reduce the number of descriptive terms applied to types of reaction to the three mentioned above, and to stress strongly the analogy between a/b and o/r reactions. To the beginner, chemistry is already sufficiently complex and bewildering.
LITERATURE CITED
(1) HALL, "Systems of acids and bases." J. CHEM.EDUC., 17,124 (1940). (2) Bnrscos, ,'Teaching the new concepts of acids and bases in general chemistry,'' ibid.. 17, 128 (1940). (3) JOHNSON, "The advantages of the older methods."ibid., 17, 132 (1940). (4) H~z~snunsr, of acid.~asereactions,s~ 17, 374 (1940).
iba.,
