A SIM-PLIFIED NOMENCLATURE for the PROTON TRANSFER CONCEPT of ACIDS' HUBERT N. ALYEA Princeton University, Princeton, New Terser
T
.~
HIS COMMITTEE was formed to propose a simplified nomenclature for an elementary presentation of the proton transfer concept of acids. Since this latter material is just beginning to appear in elementary textbooks, an attempt among teachers to agree to a common terminology seems timely. It is not a function of the committee to express itself for or against the use of this system in beginning chemistry; indeed, certain of its members personally oppose a general extension of the system. A statement of the committee's methods is appropriate. Some four hundred papers in the field were examined, and condensed into the representative bibliography of some seventy papers attached to this report. From this, a hundred-page record was compiled, showing every sense in which every term in the system has been used since its beginning. Examination of this record led to a committee agreement on certain terms suitable for an elementary presentation of the subTentative recommendations of the Committee on Nomenclature of Acids and Bases, Division of Chemical Education. Presented at the ninetv-seventh meetine of the American Chemical L i e t y , ~ a l t i m o r e , . ~ dApril . , 4, 19s9.
PART I.
ject, certain terms which were correct but superfluous, and certain terms which were incorrect. The committee realized only too well that some words listed as "avoid" are entirely proper when correctly defined; and that use rather than committee fiat is the proper criterion. A list, with definitions, of these words was sent to the authors in our bibliography. For their immediate and generous response, and for their valued suggestions the committee is indebted. These comments were re-worked into the final list, which is submitted tentatively herewith. In issuing this recommendation the members of the committee invite correspondence from the readers. We shall continue to receive these criticisms and recommend a final list. L. P. HAMMETT, Columbia University J. H. HUDEBRAND, University of California W. C. JOHNSON, The University of Chicago E. A. WUDMAN, Earlham College H. N. ALYEA(Chuirman), Princeton University The Committee on Nomenclature of Acids and Bases, Division of Chemical Education.
NOMENCLATURE ACCORDING TO THE BRPNSTED SYSTEM
SUGGESTED TERMINOLOGY SUBSTANCES
SUPERFLUOUS OR INCORRECTITERMINOLOGY
acid, base
acid anhydride, basic anhydride1, anhydro base, antiacid, basic oxide, protodose, protogene, proton donor, proton acceptor, protophile, pseudo-, simple or undissociated hase
proton, hydrogen ion, oxonium or hydroniurn ion, lyate, and lyonium ion
H20Hf ion, hydrion hydrate, hydroxonium ion, hydrated hydrogen ion, solvated H+ ion, solvated hydrion
cation acid or base, anion acid or hase, neutral molecule acid or base
uncharged acid or base, neutral acid or base
SUGGESTED TERMINOLOGY mono-, di-, poly-protic acid or base ,
SUPERFLUOUS OR INCORRECTLTERMINOLOGY monoacid acid, monobasic acid, monoacid base, and so forth
conjugate acid, base, or pair
corresponding acid, base, or pair
amphiprotic, aprotic substance
amphoteric, amphoprotic' substance
salt, strong electrolyte, weak electrolyte, non-electrolyte, ester, derivative, indicator, molecttle, substance
acidate, acid salt, basic salt', metal ester, neutral salt, non-salt; pseudo-acidi base, or salt.
SOLVENTS acid, basic, amphiprotic, aprotic, and mined solvents
amphoteric, amphoprotic', indifferent, or inert solvents; ampholyte
REACTIONS protolysis, protolytic reaction
acid-base equilibrium, dissociation', double acid-base equilibrium, double decomposition, metathical reaction, proton exchange reaction, protonic transfe~,ionization
ammonolysis, autoprotolysis, hydrolysis, lyolysis, or solvolysis
amphoteric reaction, medium protolysis, self-dimxiation'. self-ionization, self-protolysis, solvent ionization
neutralization or quantitative protolysis, salt formation, displacement, addition reaction, endpoint of a titration, cquivalence point, salt effect
titration to the neutral point', half-neutralized solution
CATALYSIS acid catalyst, basic catalyst, protolytic catalysis, general acid and base catalysis; specific acid and ba.x catalysis
general proton catalysis, specific proton catalysis, proton catalysis
STRENGTH pK, acidity, basicity, strengths of acids and bases
acid property, dissociation exponent1, normal acidity potential, proton activity; rational, conventional, or strength constant; strength exponent, true acidity
PART 11. DEFINITIONS. INCLUDING NOTES EXTENDING THE DEFINITIONS, OR STATING WHY CERTAIN COMMONLY USED WORDS HAVE BEEN CLASSED AS SUPERFLUOUS OR INCORRECT ACID-in the Brglnsted sense is a substance having the chemical property of losing a proton t o another substance. Examples: HaOf, HCI, HS0.C. Note 1: Deuton, omitted for simplicity, can of course he substituted for Woton i n all of these definitions. Note 2: An acid may shore a proton with a base rather than transfer it completely. ACID ANHYDRIDE-Avoid i n the Br$nsted system. I t can of course be retained for organic compounds like acetic anhydride. BASIC ANHYDRIDE, on the other hand, has no justification. ACIDATE-a compound formed by substituting metal for hydrogen in an acid, irrespective of the character of the substance formed. Avoid t o simplify an elementary presentation. ACID CATALYSTcatalyst which acts in virtue of its being an acid. Example: HAc, H80+, NHd+. Note: The mechanism of the catalysis is a losing of a proton t o reactants. A C I D I T Y 4 e e STRENGTRS OP Acms which is different. ACID SALT-Avoid in the Br#nsted system. See BASICSALT. ACID SOLVENT-a solvent which acts as an acid by losing a proton to the solute. Example: CH8COOH. AMMONOLYSISSee LYOLYSIS. AMPHIPROTIC SOLVENT-a solvent which may either lose a proton t o the solute or gain a proton from the solute, depending upon the acid-hare nature of the solute. Example: H,O.
AMPHIPROTIC S U B S T A N C G a substance which can act either as an acid or as a base. Example: HCOs-. ANION ACID-n acid the molecules of which are anions. Example: HSOci. ANION BASE-See ANIONACID. Example: CI-. APROTIC SOLVENT-a solvent which will neither lose a proton to the solute, nor gain a proton from the solute. Exampk: C&. APROTIC SUBSTANCEsubstance which can act neither as an acid nor a base. Example: CaHa. AUTOPROTOLYSIS-a protolytic reaction between two molecules of the same substance, which results in the formation of ions. Example: 2 H ~ 0 + HsO+ OH-. BASE-in the Br#nsted sense is a substance having the chemical property of gaining (taking up) a proton from another substance. Example: OH-; C1-. BASICCATALYST-Example: Co(NH&OHt+; NH*; OH-. See Acm CATALYST. B A S I C I T Y 4 e e STRENGTH OP BASESfrom which this differs. BASIC SALT-Avoid in Br#nsted system. B j e m m emphasizes the incorrectness for example of calling Cr(0H)Cl basic chromous chloride: i t is correctly named hydroxy-chromochloride, or chromous-hydroxy-chloride. BASIC SOLVENT-a solvent which acts as a base by gaining (taking up) a proton from the solute. Example: NHz. CATION A C I D a n acid the molecules of which are cations. Example: [AI(HpO)al+++.
+
CATION B A S E S e e CATIONACID. Example: OHIf+.
[Co(NH&
CONJUGATE ACID, CONJUGATE BASE, CONJUGATE PAIR-If the reactions A H C B can occur a t all, A may be called the conjugate acid, whatever its charge, and B may he called the conjugate base. Together they form a conjugate pair. Examples: NH4+ and NH8; HCOs- and
= +
coi.
DIPROTIC ACID-Example:
H.S04.
DIPROTIC BASE-Examples: PROTIC ACm.
See MONOPROTIC Acm.
HP0,-;
SO,-.
See MONO-
ENDPOINT OF A TITRATION-in the presence of an indicator, where CA (indicator) = Ca (indicator); that is, the point at which the indicator changes color. I n principle, the endpoint either in the presence of indicator or in electrometric or potentiometric titrations is the EQUIVALENCE POINT, that is, the point a t which the amount of acid added is equivslent to the amount of base started with, or vice versa. HALF-NEUTRALIZED treatment.
SOLUTION-Avoid
in
a simple
HYDROLYSISSee LYOLYSIS. HYDRONIUM ION-See
OXONIUM ION.
I N D I C A T O R a n y conjugate pair in which the conjugate acid differs in color from its conjugate base. Example: litmus.
-
IONIZATION-may he omitted in the Brylnsted system-an interaction between a molecule and its solvent resulting in the formation of ions. Example: HCI HnO HsO+ C1-. Note 1: Ionization is not dissociation. For example, an ion pair such as the solute forms in solvents of low dielectric constant may be said t o he ionized hut not dissociated. Note 2: Although certain oxidation-reduction reactions can formally he included in this definition, it is prohahly better t o exclude them in practice.
+
+
LEVELLING EFFECT OF WATER-a phenomenon suitable for elementary presentationprotolytic reaction between strong acids and water to form the oxanium ion, the presence of which gives an appearance of equal strengths t o acids whose strengths differ in a non-aqueous solvent. Examples: HNOl HCI
--
+ H,O + HzO
H80+ HsOf
+ NO,-;
+ C1-
A quantitative yield of oxanium ion is obtained in both instances, although the strengths of these acids differ. LYATE IONsolvent molecule minus a proton. OH- in water; CHaCOO- in glacial acetic acid.
Examples:
LYOLYSIS--or SOLVOLYSIS (also HYDROLYSIS, AMMONOLYSIS and so forth)protolytic reaction between a cation acid or an anion hase and the solvent (water, ammonia, and so forth). Example: NH,+
+ HnO=NHI + H,O+;
CN-
+ H1O e HCN + OH-
LYONIUM ION-a solvent molecule plus a proton. HsOCin water; NIX4+in liquid ammonia.
Examples:
MOLECULE-Common usage today loosely extends the definition to include ions. Example: H 2 0 ; HS04-; Na+.
,
Note: The term QUANTITATIVE PROTOLYSIS has been suggested. NEUTRAL MOLECULE ACID-an acid, the molecules of which are electrically neutral. Example: HCI; H d 0 4 .
NEUTRAL MOLECULE B A S E S e e N E W T ~ AMOLECULE L ACID. Example: NHs. NEUTRAL SALT-See
Basrc SALT.
NON-ELECTROLYTE-a substance whose solution is not ionized. Example: sugar. OXONIl~hlION or HYDROSIC>I ION --the ion 1130' Kutc: OXONIUhl IUN was much favored over HYDHONI1.'.\1 IOT. Both, huwever, ha\.c staunch supportrrs. POLYPROTIC ACID--See MONOPROTIC ACID. POLYPROTIC B A S E S e e MoNoPRorlc Acm. PROTOLYSIS, PROTOLYTIC REACTION-a reaction in which a proton is transferred from an acid t o a hase. Example: BI A2 BI; as in HCI CHCOO CHCOOH 4 A, c1-.
+
-
+
+
-
PROTOLYTIC CATALYSIScatalysis in which acids, bases, arhoth, act as catalysts by virtue of their undergoing protolytic reactions. Example: mutarotation of glucose (acid and basic catalysis); decomposition of nitramide (basic catalysis). PROTON-the ion H+. This is more specific than the term HYDROGEN ION, which is applied loosely not only to H + hut t o all lyonium ions. PSEUDO-ACID, PSEUDO-BASE-Avoid in a simple treatment. These words have been applied t o substances which exhibit slow neutralization. SALT-an
ionic compound. Example:
NaCI.
-
SELF-IONIZATION-Avoid, since it would apply not only t o protolytic reactions but t o a reaction such as LII-. See AuroPnoToLYsIs.
+
SOLVOLYSISSee LYOLYSIS. STRENGTH EXPONENT-Use instead.
pK (pronounced Pee Kay)
STRENGTHS OF A C I D S t h e relative extent of reaction of the acids with a given base under identical conditions; the stronger acid reacts more completely. For most purposes a sathfactory numerical measure of strengths is given by the equilibrium constant for the reaction of the acid with water in dilute aqueous solutions; that is t o say, by the ionization constant (protolysis constant) in water solution. Note: The ACIDITY and the STRENGTH OF AN ACID are twodifferent things; and hear the same relation t o each other as pH and pK. We say 1NHCl in water has a certain ACIDITY (pH) hut theSTRENGTH OF THE ACID HCI, relative t o HdO* HNOa and so forth, is measured by pK. BASICITY and the STRENGTH OF A BASE differ in the same way. STRENGTH OF B A S E S i t is simpler t o refer only to the strength of the conjugate acid, just as pH is sufficient without introducing the concept pOH. See note under STRENGTHS OF AWS.
MONOPROTIC A C I D a n acid which has only one proton t o lose to a base. Examples: HCI; HS04-. Also DIPROTIC STRONG ELECTROLYTESee SALT. ACID, and so forth, POLYPROTIC ACID. A majority of referees preferred this to MONOACID ACIDor MONOBASIC ACID, SUBSTANCGany particular kind of matter, whether element, compound, or mixture; and including molecules, atoms, atomic and so forth. fragments, and ions. MONOPROTIC BASCExamples: HIPOc-; NIL. See UNCHARGED ACID. UNCHARGED BASE-Avoid, since MoNoPnoTIc Acw. it does not indicate internal charges, as d m NEUTRAL NEUTRALIZATIONprotolytic reaction which occurs when MOLECULE ACID. two substances, one of them acid with respect t o the solvent. substance whose solution is not the other basic, are mixed in equivalent quantities. Example: WEAK ELECTROLYTEcompletely ionized. Example: HgCI*. HIOt OH2H20.
+
-
BIBLIOGRAPHY ON THE PROTON T V N S P E R SYSTEM OF ACIDS AND BASES
The followine # a # m were selected from some four hundred because of their historical im$ortonce, and their readability by HENRICH,F.. "Theorien der Organischen Chemie," 4th ed.. Vieweg & Sohn, Braunrchweig, 1921, Chapter XVIII. KnPAmrcK, M., I. Phys. Chem., 34, 2180 (1930); J. C ~ M . EDUC.,8, 1566 (1931); ibid., 9, 1010, 1226 (1932); ibid., 12, 109 (1935). KOLTR&F,I. M., Rec. treu. chim.. 49,401 (1930). LAMER,V. K. AND H. C. DOWNES, Chem. Reuz'e~~s, 13,47 (1933); LA MER,V. K. AND S. H. MARON, I. A m . Chem. Sac., 60,2588 IlO?IL) ,A""v,. LEWIS.G. N., "Valence and the structure of atoms and molecules," Chem. Catalog Co., New York City, pp. 83, 142, 150, 1923; 1. of Franklin Institute. 226,293 (1938). LIVINGSTON, R.,J. CHEM.EDUC.,7,2887 (1930). LOWRY,T. M., Chem. and Ind. (Reuim), 42, 43 (1923); J. Chem. Sac., 123,822 (1923) ; Trans. Faraday Soc., 20, 13 (1924); J. Chem. Soc., 127, 1371 (1925); (-AND G. F. SMITH),ibid., 2539 (1927); ibid., LOWRY, T. M., 2554 (1927); Trans. Fewday Soc., 24, 545 (1928). MEERWEIN, H., ET AL., 1. prakt. Chem., 147,257 (1937). NOYES,W. A., J. A m . Chem. Sac., 55,4889 (1933). \."--,. PAULING, L., ibid., 55, 1895 (1933). GERMANN. A. F. 0.. ibid.. 47.2461 (1925.5) PEDERSEN, K. J., ibid., 53, 18 (1931). GROVE,c;, ibid., 52, 1404 GO).‘ ~' SCHWARTZENBACH, G., ~ e i v C . h . A&, 13, 870, 897 (1930). GUGGENHEIN, E. A,, I. Phys. Chem., 34, 1758 (1930). SEDGWICK, "Electron theory of valency," Oxford Press, New HALL,N. F., J. CAEM.EDUC.,7, 782 (1930); Chem. RNieus, 8, York City, 1928. 191 11931). SHERK,K. W., J. CHEM.EDUC..13,358 (1936). HAM~~ETT, i.P., J. A m . Cham. sac., SO, 2666 (1928); (-AND A. VOLMER. . M... Ann.. 440. 200 (1924). J. DEYRUP), ibid., 54, 27?1 (1932); HAMMETT, L. P., ibid., 55, P., " ~ a l t iacihs and bas&," McGraw-Hill Book Co.. 1900 (1933); Chem. R m m s , 13, 61 (1933); "Solutions of elec- WALDEN. New York City, 1929. trolytes," 2nd ed., McGraw-Hill Book Co., New York City, WILDMAN, E. A., J. CHEM.E D U C . . ~ . 92 (1932); Proc. Indium 1936. Acod. Sci., 41, 259-61 (1932); J. CAEM.EDUC.,12,14 (1935); HANTzscH, A,, Z. Eledrochem., 24, 201 (1918); ibid., 29, 221 Sch. Sci. Math., 35, 141 (Feb., 1935). (1923); ibid., 30, 194 (1924); Ber., 60B, 1933 (1927); (-AND W. F. K., J. Chcm. Soc.. 1064. 1930. A. WEISSBERGER), Z.physik. Chem., 125,251 (1927); HANTSCH, WYNNE-JONES, ZINTL. E. AND S. NEUMAYR, Ber.. 63B, 237 (1930). A.. ibid., 134, 406 (1928); Ber.. 63B, 1789 (1930). ~
A TEACHER'S BIBLIOGRAPHY ON THE BRBNSTED SYSTEM OF ACIDS AND BASES
of divisions i n the field. These articles have b e a selected for their general treatment and of Hantzsch's work. "Solutions of electrolytes," 2nd ed., B J E R R ~N., , "Inorganic chemistry," trans. by R. P. BELL, McGraw-Hill Book Co., New York City, 1936. The Chemical Catalog Co., New York City, 1936; Chcm. M., 1. Phys. Chem., 34,2180 (1930). See especially KILPATRICK, R ~ e u s 16, , 287 (1935), trans. by MARYK~PATRICK. J. N., Chem. Rdms, 5, 231 pp. 2-1) Pages 407-14. J. CHEM. EDUC.,12, 109 (1935); ibid., 9, BR@NSTED, 1010, 1226 (1932), especially pp. 1013-6 for buffers. (1923). K o ~ ~ n o a rI. r , M., Rec. trau. chim., 49, 401 (1930). Excellent Co~anrr,J. B. AND N. F. HALL, J. A m . Chem. Soc., 49, 3062 review on pages 407-14, (1927). Especially the table of acid and basic strengths in L, V. K. AND H. C. DOWNES, them. xeuicvs, 13,47 (1933),
different solvents, page 3069. L. C., J. C ~ E MEnuc.. . 13,219 (1936). Titrations in aqueous and non-aqueous solutions as freshman laboratory experiments. HALL,N. F.. ibid., 7, 782 (1930); C h . R e u h s , 8, 191 (1931). HAMMETT,L. P., I. A m . Chem. Soc.. 50, 2666 (1928), summary FLOWERS.
for reactions in benzene. LIVINGSTON, R., J. CHEM.EDUC.,7,288742 (1930), forcatalysis. Lownu, T. M., Trans. Faradey Soc., 24,545 (1928), for catalysis in the mutarotation of sugar and so forth. SCAWARTZENBACH, G., Helu. Chim. A d a , 13, 870, 897 (1930). for mathematical treatment.
