SYSTEMS of ACIDS and BASES1 NORRIS F. HALL2 University of Wisconsin. Madison, Wisconsin
I
N QUITE early times chemical workers become familiar with certain volatile materials such as spirit of vinegar or oil of vitriol which in proper dilution changed the color of vegetable dyes, tasted sour, and acted on various metals, earths, and alkalies, often with effervescence. I n so acting their sour or acid principle was killed or neutralized and one could often show that the product was a "neutral salt," i . e., a soluble, crystalline substance with a characteristic taste, such as seasalt or sugar. Later the notion of the "base" (of a salt) was introduced, and this was often a non-volatile residue left when a salt yielded up its spirit or "acid" on distillation. Not only the sulfur acids hut also their anhydrides were recognized as.acidic as were also, for ex-
5
USANOVICH
ELECTROPHILE :ATCON,
(Cl,NR4Xj
CATtON-DONOR,
4 ELECTRON-PAIR
--
ELECTRON-PAIR ACCEPTORI OR OXIDANTI
LEWIS
ACCEPTOR
(SO,,
BCl,)
ample, fixed air (COz) and oxymuriatic acid (CIS while bases included other earths beside the alkalies and also spirit of hartshorn. The most important early theory of acidity is due to Lavoisier, who was encouraged by his studies of oxygen and of chemical compositionin general to postulate that all a d s were of what he called an acidifiable .~ ~ - com~ounds base (an element such as S or P) with the acidifying principle which was oxygen. Many of the best chemists became convinced adherents of this theory, and it required many years and repeated attacks before it was ~
-.
~~~~
~
.~
abandoned. Its opponents successfully showed that (1) many acids contain no oxygen (HCl, HzS, Clz), and (2) many binary oxygen compounds are not acids (KzO, CaO, and so forth). One reason for the fall of this theory was apparently that i t was unduly restrictiwe, in attempting to tie up acid character with the presence of a single element. (Also of course, Lavoisier's choice of oxygen was unfortunate.) A sturdy rival of Lavoisier's oxygen theory was the hydrogen theory of acids, which is still widely held today. This theory may be stated as follows: "An acid must be a hydrogen compound, but not all hydrogen compounds are acids." Both the Arrhenius or electrolytic dissociation theory formulation of the acid-base problem (which I will sometimes call the water-ion theory) and the Brginsted-Lowry point of view are forms of this hydrogen theory of acids. This view has, however, been rejected by many chemists. Even Humphry Davy whose work did more than any other to displace the oxygen theory by the hydrogen theory of acids, wrote in 1814 "that acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances." Among modem opponents of the hydrogen theory inay be listed A. F. 0. Germann,a M. U~anovich,~ and especially G. N. Lewis.= Berzelius, who had adopted a modified form of the oxygen theory, in that he regarded all acids as oxides of non-metals, of all bases as oxides of metals, was also the first proponent of the dualistic electrochemical view which was revived by Kosse16 and whose modem developments are also seen in the work of Germann, Usanovich, and Lewis. Berzelius could not properly appreciate the force of the hydrogen theory as advanced and supported by Liehig, and clung to the oxygen theory to the end of his life. I n spite of his authority, however, the hydrogen theory slowly gained ground, and, helped simultaneously by the growth of structural organic chemistry and by the increasing importance in inorganic chemistry of reactions in water solution, i t secured and still holds today an almost universal acceptance. WATER-ION SYSTEM Without retracing all the intermediate steps we may THE
~
Contrilwtion ra the Symposium on Theories and Teaching of Acids and 13asrs conducrcd by the Diesions of Physical Chcmistrv Education at the nmctv-sevcnth meetina- of - ~ - and Chemical the.^. C. S., Baltimore, Md., April 4, 1939: 2 I am greatly indebted to Mr. Fred Meyer for helpful discussion during the preparation of this paper. 1
k J. Am. ~ h c m . 3. Franklin 1nst1 226. 293-313 (193%. . . ~ e also Soc.. 61, 1886. 1894 (1939). %OSSEL,Ann. Physlb, 49 (4). 229 (1916); Z. E l e k t r o c h . ,
26.314 (1920).
now state the water-ion form of the hydrogen theory which the success of Arrhenius' ionic ideas has brought into widest use today. Definitions An acid is a hydrogen compound which in water solution gives hydrogen ion, a base is a hydroxyl compound which in water solution gives hydroxyl ion. (Note: CHI, SOa, and SnCL are not acids; K20, NH3, and CzH50Hare not bases.) Acidity and alkalinity are restricted to water solutions and refer to an unbalance in the concentration of the solvent ions (i. e., HC1 dissolved in toluene is not acidic, nor is pure HzSOI; neither fused KOH nor a solution of sodium amide in liquid ammonia is alkaline). (Note: The operational meaning of "hydrogen-ion concentration," and so forth, may be readily stated in terms of indicator colors, E.M.F. and conductance measurements, catalytic activity, and so forth.) Corollary Certain ions such as HS04- or NH4+, MgOH+ or BaOH+ may be acids or bases on this view-nly hydroxyl compounds may be &photeric. These definitions are clear and self-consistent, are perhaps adequate to the needs of many beginning students of chemistry, and have permitted the formulation of the relative strengths of many water-soluble acids and bases in terms of their so-called ionization constants. They have, however, been attacked from various points of view. SOLVO-SYSTEM In the first place i t has been pointed out that water, while a most exceptional solvent, is by no means unique, either in its self-ionization or in its power to dissolve acids, bases, and salts, and to serve as avehiclefor their interactions. E. C. Franklm' in pariicular has stressed the profound similarity between reactions in water and in liquid ammonia and has pointed out &at parallel to the aquo-system of acids, bases, and salts related to water, there exists an ammono-system of substances similarly related to ammonia. Thus, while the strong acids dissolved in water exist principally as oxonium salts OH,+, X-, in ammonia they exist as ammonium salts NH4+, X-. Corresponding to KzO and KOH in the water system, we have K3N, KzNH, and KNHz as alkalies in the ammonia system. Aminomerruric chloride is an ammono-basic salt as MgOHCl is an aquobasic salt. This point of view has been applied with varying success to many of the hydrides as parent solvents. Thus hydrazine, hydrogen fluoride, hydrogen sulfide, and even methane bave been made the basis of similar systems. In each case, parallel to the selfdissociation of water THE GENERAL
2HxO
~2
HaOf
+ OH-
FRmpLm, I. Am. Chem. Soc., 46, 2137 (1924); 27, 820 (1905); also Am. Chem. J., 47, 285 (1912), and "The nltrogen system of compounds," Reinhold Publishing Corp., New York City, 1935.
'
there is a corresponding self-dissociation of the other solvent, such as 2NHa Ft NH4+
+ NHI-
and the conceptions of acidity and alkalinity may be logically extended to solutions in each solvent where the lyoniums and lyate8 ions are active. AU the solvents mentioned above are hydrides, but there seems no reason why the nomenclature should not be extended to others as well. Thus, Walden,#Jander,Io and others bave ascribed acid-base character to reactions in the solvent SOz, which is supposed to dissociate as follows.
s o 1e SO++ + soa-
Germann3 bas pointed out that phosgene (COClp)may be regarded as the parent of an acid-base system. Here, however, the dissociation presumably takes the form
*
++2c1c1-
COCI, COC+ COCll Ft COCl+
Acids in these solvents do notcontain or liberate protons, but other cations instead. The latter author has explicitly given the most general formulation to this point of view. We have thus been led a long distance, both from the traditional, pre-Arrhenius view and also from the highly restricted water-ion system, and many are ready to cry, "Halt!" a t this point. Opposition to the general solvosystem may be summarized under these heads. (1) , , Manv feel that while it may have formal merit, the practical importance of non-aqueous systems is too slight to warrant giving up the more specific and familiar water-ion system. (2) Others feel that when we abandon the hydrogen theory of acids, the system becomes too general to be useful-that there is something infrinsically and exclusively acidic about hydrogen compounds, not shared by non-hydrogenic lyonium salts. (3) A third group would hold that the theory is not general enough. In tying up acid-base behavior to particular individual solvents it ignores the widespread feelmg that certain substances such as SO3 and NH3 are acidic or basic in their own right, irrespective of the presence of any solvent and so is no better in this respect than the water-ion theory. Such critics would feel that the solvo-system is too much an empty formalism which fails to emphasize the essential underlying mechanism of acid-base action. Whether for these or for other reasons, the generalized solvo-system has had more lip senice than actual use. Elementary texts have stuck for the most part to the water-ion system, and while some research workers have made use of it, much more attention has been paid to
-
BJERRUM. Chem. Revinus, 16, 287 (1935). WALDEN."Salts, acids, and bases." McGraw-Hill Book Co., New York City, 1929: lo JANDER, Z . physzk. Chem., 178, 57 (1936); ibid.; 179, 43 (1937); Ber., 70, 251-7 (1937); Z. enorg. Chem., 230, 105; ibid., 232, 229 (1937). %
the Brglnsted-Lowry system described below in Lewis' phrase.
In the year 1923, T. M. Lowry in England and J. N. Brglnsted in Denmark independently proposed a novel set of definitions which have aroused wide attention, not only among textbook writers, but among many students of reaction kinetics. As Br#nsted has done more than Lowry to develop and use the system, it is now generally known by his name. Br#nsted regards acids and bases as fundamentally differentin behavior from salts, i. e., electrolytes. Salts (which may include acidic and basic ions) are assemblages of ions, and their behavior is essentially described in physical terms as by the theories of Debye, Onsager, and others. Acids and bases, on the other hand, are uniquely characterized by a tendency to exchange protons, and are defined by the fundamental reaction equation acid
s base + proton
Here an acid mnst obviously be a hydrogen compound, but a base may in principle be any substance whatever. Of course in practice only compounds of labile hydrogen are acids, and many molecules show no measurable basic behavior. Since the reaction suggested above can occur to a measurable extent only if another proton acceptor is available to compete with the base, tlie real acid-base reactions are always a t least as complicated in type as:
ing substances like HC1 "acids." However, solvated acids such as HCl (HzO) and NHC1 are not themselves acids but salts wbich contain the acids HaO+ and NH4+, and KOH and KNHa are not bases, but contain the bases OH- and NH2-. (6) Other things being equal, acid strength increases with positive charge and basic strength with negative, but some anions are hardly bases a t all (CIOa-, RSOa-), while many cations (Fe+++, NRa+) are definitely not acids, though their hydrogen-containing solvates are in general acidic. (7) Some substances such as SOa, BC13, and CO*, which the early chemists would have called acids, are excluded for lack of hydrogen, as are also such things as Germann's "phosgenoaluminic acid" (COA12Cls). (8) The system is peculiarly adapted to discussing catalytic phenomena of prototropic systems in a variety of solvents," and in principle permits arranging all acids and bases in a single monotonic strength series under all conditions and in all solvents. (9) The system facilitates a number of instructive comparisons with the system of oxidizing and reducing agents.12 The fundamental defming reaction of this system is Reductant = Oxidant
4-n
(electrons),
and thermodynamic treatment in terms of proton activity, acidity potentials, and equilibrium constants can be applied to Br#nsted acids and bases in exactly the same way as to redox systems, except that here electron activities mnst be used. acid' + base' s acid' + base1 (1) The fa$ that a snbstanc6 is a strong acid by no means prevents its being also, under other circumSuch reactions a t equivalent concentrations always run stances, an active base. Thus the cations of fluoronium spontaneously in the direction of the formation of the and nitroninm perchlorates HzF+ and HzNOa+ are weaker acid and base. hydrogen ions formed from the bases H F and HNOs. This formulation differs from the preceding ones in For further elaborations and explanations of the many ways. system the reader should consult Brglnsted'sr3 many (1) Acid-base behavior is formally independent of anv solvent and not esoeciallv related to anv.. but in- journal articles in recent years, his textbook of physical chemistry, and also papers by Klages14and Schwarzeneludes in an unforced way the action of all protolytic bach,ls and the bibliography recently given by Alyea in solvents. EDUCATION.^^ (2) Ions, instead of appearing as a highly exceptional the JOURNAL OF CHEMICAL It might be better in some ways for me to retire from sub-class, are accorded a status as acids and bases enthe discussion a t this point imfavor of the papers wbich tirely equivalent to that of uncharged molecules. follow and which are to present in more detail the ad(3) An acid and a base react, not to produce a salt and a solvent, but to form a new acid and a new base. vantages and disadvantages of the Brfinsted and Arrhenius systems. I feel, however, that this introducThe conception of the "neutrality" of a solvent loses its tory survey would be very incomplete if I did not now meaning. call my readers' attention to still other important pro(4) Acid dissociation and acid solvolysis are seen to posals which no student of this field can afford to be entirely similar phenomena: neglect.
.
HCI NH,+
+ H.0 = HaO+ + CI+ H1O = HaOf + NHI
as are basic dissociation and basic solvolysis: NH, GHsOa-
+ H20 = NHlt + OH+ H1O = HC2H10n+ OH-
( 5 ) Brglnsted's system agrees with the others in call-
" Cf., especially HAMMBTT, J. Am. Chem. Soc., passim; Chem. Revievs, 13,61 (1933). l a Cf,, LATIMER. "Oxidation potentials," Prentice-Hall, Inc., New York City, 1939, and GURNEY,"Ions in solution." The Macmillan Co., New York City, 1936. 1 J Cf., J . CHEM. EDUC., 16, 535 (1939). 1' KLAGES, Z. Elektmchem., 39, 683 (1933). IS SCHWARZBACH, Helf. Chim. Ada, 13,870, 897 (1930). I6 ALYEA, J. CHEM.EDUC., 16, 535 (1939).
..
ELECTRON-PAIR SYSTEM
I t is quite beyond the scope of this naner to develoo the interesting- and important consequences of this The first or electron-pair system of acids and bases is nomenclature, for which Lewis' own papers must be by no means new, as it was put forward explicitly by G. consulted. One apparent disadvantage of Lewis' N. Lewis in 1923 (i. e., in the same year as the proposals system is that acids here have different relative of Br'nsted and Lowry). However' as this point of strengths depending on the base with which they react, view was not further elaborated by Lewis until 1938, and the underlying ideas have been given a very differ- so that no unique order of acid strengths can be formuent treatment by Sidgwick,l7this point of view has re- lated. Chemists must decide whether the restriction ceived little attention and now appears almost as novel which in Br'nsted's theory makes this possible is a desirable one. as if it had iust been pronosed for the first time. Lewis definesan acid moleche;radical, or ion as one in which THE POSIT^-NEGAT~" SYSTEM the normal electron grouping (often an octet) surroundThe Iast set of ideas in this field to which I wish to ing some atom is incomplete, and can accept for its direct attention was outlined only last year by a Russian completion an (non-bonding) electron pair from some of the Central Asiatic Uniother atom. Conversely, a base is a structure which has chemist, M. Usano~ich,~ such an electron pair which it can share with an acid. versity a t Tashkent. Usanovich is, like Lewis, disTypical acids are: Monobasic--0, HC1, SO3, turbed by the "illogicality" of Br6nsted in restricting acid properties to hydrogen compounds alone, but he B(OR)3; dibasic-Hi, Agf, SnC14; polybasic-NH4+, goes further than Lewis in identifying general electroFe+++, Cu++, H4Fe(CN)6. Typical bases are: Monoacidic--NRa, CN-, OH-, positive or electrophilicls character with acidity, and nu~leophilic'~ behavior with basicity. For him, acids 4sC; dibasic-(GH&O, 0-. As Lewis points out, these acids and bases are exactly (1) form salts with bases and effect neutralization; what Sidgwick has called "acceptor" and "donor" (2) give up (H+ or other) cations; and molecules, respectively, but Lewis prefers to apply to (3) add to themselves anions and free electrons. them the more ancient names. It thus appears that oxidation and reduction become in In this system an acid does not metastasize with a this system a special case of acid-base behavior. base, but combines with it. Later, rearrangement or Examples are decomposition may occur. Thus the acid oxygen atom, Nan0 SO3 2Na+ SO4combines with the base triethylamine to give a stable amine oxide, but the weaker acid COa gives a product (Na20 is a base because it gives up the anion 0- and with ammonia which must rearranpe to carbamic acid SO* is an acid because i t takes i t un.), in order to be stable. Readers w$ observe that this 3(NH&S f SblS 6NH.t i'2sbS' system includes all the acids and bases of the Br$nsted system and no other bases, while i t points out a host ((NH&S is a base because it gives up S-, and so forth.) of new acids (including most cations) which the Br$nsted system does not recognize as such. In one sense, by including such things as SOaand SnClr, it constitutes (Na is a base and C1 an acid because they give and take a return to the "good old days" before the hydrogen an electron.) NRaH+is an acid for Br$nited, while NR4+ theory, when a thing that acted like an aciQwas called is not. For Usanovich both are acids because they one without askmg of what i t consisted. On this point are cations, and because they will give up cations to Lewis says,'S "To restrict the group of acids to those bases. substances which contain hydrogen interferes as seriNRsHf + OHNRs + HIO ously with the systematic understanding of chemistry NRI + ROH NRIC + OHas would the restriction of the term oxidizing agent to substances containing oxygen." In this system a In the inclusion of oxidation-reducfion phenomena and primary acid such as SO3may neutralize itself by virtue reactions.between odd molecules in the acid-base system of its basic properties, forming double molecules or poly- Usanovich has apparently pushed these ideas to their mers. If, then, it is dissolved in a basic solvent such as logical limits. In accounting for the origin of acidwater, the stronger base displaces the weaker from base behavior Usanovich essentially returns to the point combination and we have the new product, H2S04. In of view of Kosse16 which explains the properties of the presence of more and more water this molecule may molecules primarily in terms of ionic charges. Also, add a t least two more molecules of the base, but the however, he stresses the importance of "coordination resulting electrical strain is so great that the system unsaturationU-the ability of an atom to increase its splits off one or two oxonium ions, leaving a t last the covalence. For him the acid function depends on the S o b ion which is purely basic, and may perhaps com- presence of a coordinatively unsaturated positive particle and the basic function on a similarly unsaturated bine with one or more molecules of the acid solvent. negative atom. Other things being equal the acid or basic property "The electronic theory of valemy," Oxford UniSIDGWICK, versity Press, New York City, 1927. LEWIS, I.Franklin Insl., 226, 297 (1938). THE
+
-
--
+
.
of a molecule depends on the charge or polar valence of the atoms present. Usually the effect of the atom of highest valence will be dominant. Hence oxides of low-valent atoms (alkalis and alkaline earths) are predominantly basic, while those of the third and higher groups are predominantly acidic. The corresponding halides are less basic than the oxides for the same reason and so, also, we find AsCls, AIF3, and so forth, much more acidic than the corresponding oxides. Of course most compounds may be expected to show some amphoteric behavior, and other influencesz0than ionic charge are involved or Thoz would be as acidic as C02. This point of view, unlike Lewis', does not require of the student a detailed familiarity with atomic structure theory, and is therefore apparently simpler to grasp and use. In its formal aspects, i t seems more closely related to the ideas previously current, and in Cf,,discussions of ion sizes.
its extreme generality i t satisfies the logical demand for a thoroughgoing application of electrostatic theory. The original paper must be consulted for explanations, amplifications, and detail. In summary, a diagram (see page 124) may serve to emphasize certain of the relationships discussed above. The whole group of substances which have been called acids are divided into five regions. Number 1 includes those substances which dissociate in water to give hydrogen-ions. This is the Arrhenius region. Number 2 includes Number 1 and represents substances which give the lyonium ion in any particular solvent. Number 3 also includes Number 1, but only a part of Number 2. It is the Br#nsted region and includes all proton donors. Number 4 (the Lewis region) includes the others and in addition all other substances that are electron-pair acceptors. Number 5 (the Usanovich region) includes all the others and, in addition, all other cations, catiouoid reagents, and oxidants.
