ACIDS, BASES, and SALTS* MARTIN KILPATRICK The University of Pennsylvania, Philadelphia, Pennsylvania
T
HE purpose of this paper is to review the teaching of the ideas of acids, bases, and salts; to note that the common definitions are often illogical and hazy; and to see what part of the definitions should be retained and what discarded so that our teaching will provide the necessary background for a logical approach to the modem ideas of acids and bases (1). Our purpose is to achieve definitions such that the student will not have to unlearn and discard, later on, the ideas which have been well taught and firmly fixed in his mind in his first course in chemistry. Let us review the common definitions. First, what is an acid? (1) A substance which tastes sour and in aqueous solution colors litmus red. (2) A hydrogen compound which can split off hydrogen ions.
In the teaching of the Arrhenius theory the emphasis has been on aqueous solution, and the question arises as to whether the substance is an acid before it separates into ions. For example, would one consider the ion HP04= an acid except in so far as it gives hydrogen ions to the solution? Taking a case in non-aqueous solution, would one consider acetic acid an acid when dissolved in heptane? Another requirement for an acid is that it should combine with metal hydroxides to form a salt and water. As Bronsted (2) points out, until now we have been more concerned about what substances are acids than we have with the question of what is an acid. What is a base? (1) A substance which in aqueous solution turns litmus blue. (2) A substance which gives hydro71 ions in aqueous solution.
In organic chemistry, however, one learns that the amines are bases, and again we encounter the requirement that a base should form a salt with an acid; aniline, for example, forms aniline hydrochloride. But in general the hydroxyl ion has been thought of as the carrier of basic properties. For example, ammonia itself has not customarily been considered a base, but the ammonium hydroxide formed by its reaction with water has been considered a base. Yet ammonia reacts directly with hydrogen chloride to form the salt ammonium chloride. What is a salt? (1) Salts are substances famed upon neutralization of acids ' Presented helare the Division of Chemical Education symposium on ".\lodemizing the Coursein Gencrnl Chemistry."at the Clcvcland meeting of the American Chemicd Society. September 12. 1934.
with bases. Their formulas may be derived by replacing the hydrogen of the acid by a metal. (2) To be regarded as a salt, the product of neutralization of an acid with a base must be ionized.
This second specification appeared after the coming of the theory of ionization. It serves to distinguish between esters and compounds like Hg(CN)2, HgC12, etc., and those whose solutions are good conductors of the electric current. The acid or basic reaction of salts of weak acids or bases has been explained on the basis of hydrolysis. We see that the nature of acidity (or basicity) is not always clear, and consequently we must attempt to analyze the situation and discard what is illogical and ambiguous. The real stumbling block is the requirement that an acid should combine with a metal hydroxide to form a salt and water. Let us discard this. Let us also examine critically the equation commonly written to express the dissociation of an acid in water, for hydrogen chloride, for example, The same equation is written for the dissociation of hydrogen chloride in alcohol. Yet neither in water nor in ethyl alcohol do we have any protons to speak of, but rather HsO+ and C2HsOH2+(3). We should there: fore write ..
.
HCI
+ H1O e H1O+ + CI-
HCI 4- GHsOH S CsH,OH$+
+ C1-.
Now let us go over our definitions and see if we can logically put together what we have taught in the past. Our second definition of an acid was a hydrogen cornpound that can sfilit off hydrogen ions. But remembering that the hydrogen ion as we used to write it is really a proton, let us make the formal definition a hydrogen compound that can sfilit off protons. Let us make a list of such substances. This is done in the following equations. Acid =Base
+ Proton
What about the ion H2POn-? It also can give off a proton, as can HPOp=. The following equations involve these negatively charged acids. Many more examples will immediately occur to the reader. Anion Acid.
HsPOI- S HP0,HP04- S PO4-
+ K+ + H+
Can we have positively charged acids? Let us ex. amine a few more equations. Acid + Base Cation Acids
+ Proton
+
NHICe NHa H* C6HJiHa+ =GH&NHI H+ Hf [ C U ( H ~ O[Cu(&O)r(OH)IC ) ~ ~ [Fe(HzO)sl e IFe(HzOMOH)I++ H + HIOt e H20 H +
+
+
++
The hydrolysis of ammonium chloride is thus explained as a double acid-base equilibrium. For ferric perchlorate in water we have: At [Fe(H1O)ol+++
BI = HsOf A, + 3C10'- + H20 + B, [Fe(OH) (H9O)slf+ +
3 C10,-.
H30t' The dissociatiou constantof the acid [Fe(H20)8]+++
In the last equation there appears the acid which is the hydrogen ion in aqueous solution. I t is , only one of many acids. m a t about bases? 1s there anything unique about the hydroxyl ion? Let us examine the right-hand members of the preceding equations. Now let us define a base as a substance which can take up protons: if we do so, we see that these substances forme2 by the abstraction of a proton from an acid can all be called bases. In other words an acid is defined formally by the equation, A$B+H,+
where A is an acid irrespective of charge, B is a base irrespective of charge (the charge of B is of necessity equal to that of A minus one), and H + is a proton. This equation furnishes, as Bronsted (4) has shown, a general basis for the consideration of acids and bases in aqueous and nou-aqueous solution. Now let us see what liappens when hydrogen chloride is dissolved in water. The hydrogen chloride gives up its proton to the water to form hydroxonium ion and chloride ion. Since h ~ d r o g echloride ~ has a great tendency to give UP its Proton, thereaction proceeds far to the right, as indicated below.
has been measured (5) and found to be about the same as that of monochloraceticacid. F, the case of sodium acetate in waterwe have: Na+
BI A2 BS AI + CHCOO-4 H 2 0 SOH- + CHLOOH + Naf.
These three equations illustrate the amphoteric character of water, It is a base with acid HJO+, and an acid with conjugate base OH-, As is evident from the new definitions of an acid and a base, the salt-buildinl: property is no l o n ~ e ran important one in defining acid o; a base. -A salt can be defined as a system built from oppositely charged ions which do not neutralize each other. In aqueous solution the salt would be a good conductor and in the solid state it would form a crystal lattice.* Bjerrum (6) in a review of the subject of acids, bases, and salts, introduces the new term acidate which includes salts as defined above. so that a neutral acid and a base react to form an acidaie and water, The introduction of the new idea of acids makes no change in the quantitative treatment of the titration of acids and bases in aquequs solution, but it has led to advances in titration in non-aqueous solution. It is now realized that the proton exchanze is the imoortant step in acid-base titration and that titrations can he made quantitative even in inert solvents like benzene. In the field of homogeneous catalysis (7) the new ideas have led to new and significantresults. They have also led to new results in the pioblem of the reaction of metals with acids (8). Just as the phenomenon of oxidation and reduction involves a study of the exchange of an electron, the study of acidity involves another fundamental building stone-the proton. The two are related by the analogous equations:
-
Conversely, the base C1- does not have any appreciable tendency in aqueous solution to acquire a proton. So the chloride ion is a very weak base, while hydrogen chloride is a very strong acid. In the case of strong acids like hydrogen chloride in aqueous solution the reaction goes so far to the right that the molecular acid practically disappears. Now take the case of acetic acid. Here we are dealing with a weak acid and mnsequently we have many acetic acid molecules left a t equilibrium. The third equation represents the double acid-base reaction of hydrogen chloride in ethyl alcohol, and the fourth equation that of hydrogen chloride in liquid ammonia. Going back to aqueous solution, let us see what happens when ammonium chloride dissolves in water. We have: .
+ Proton Reduetant e Oxidant + Electron, Acid= Base
and by the equation: Hydrogen Atom
Proton
+ Electron.
In accordance with this definition hydrogen chloride in aqueous solution would he considered a salt, as would sodium hydroxide. In the reaction between sodium hydroxide and hydrogen chloride in aqueous solution we have
so that the process of neutralization in this sense is not saltforming but salt-destroying (2).
REFERENCES
(1) For an historical review of acids, bases,andsalts.seeWERo, L.,"S~ra.Bas och Salt." Uppsala Universitets Arbkrift. P.,"Salts. acids, and bases," McGraw1925,and WALDEN. Hill Book Co., New York, 1929. For a review of the modern conceptions see HALL, N. F., J. CHEM.EDUC.,7, 782 (1930). (2) BRONSTED, J. N.,Ber., 69,2049 (1928).
(3) V O L ~ RA., , .Ann.. 440, 200 (1924); KOLTHOP~, I. M., Rec.tmu. chm., 49,401 (1930). (4) BRON~TED,J, N.,&id., 42,718 (1923). ( 5 ) BRON~TED, J. N.AND K., 2.*hySik, Chcm., Is4, 97 (1928). (6) BJERRm, N., Fysisk Taskrift, Nr, 1931. (7) LIVXNG~~ON. R.,J. CBX. EDUC.,7,2887(1930). (8) KUPATRICK. M.. ibid ,8,1566 (1931).
