Strong and Weak Acids and Bases A contribution to the difficulty encountered by students in a consideration of acid-base theory is found in the definitions of strong and weak acids and bases. The words "strong" and "weak" are introduced to assist the student in the prediction of the position of equilibrium in an acid-base reaction. However, "strong" and "weak9'are used in various ways, and often with respect to the same compound in different situations. In Arrhenius theory, the strength of an acid is defined in terms of its percent dissociation in water. However, some strong electrolytes, such as NHaN03 and AlCls, which increase the hydrogen ion concentration of aqueous solutions, are not strong acids. In BrCsted theory, conjugate bases of weak acids are often said to be strong. This generalization is correct if attention is focussed on the reaction of a proton with the conjugate I,j,r. Ilwvrwr, t h e w n , j ~ g . ~llnieut tc 3 w r ~ .+rid k i. w t twces-lr~lynstruns h7:r in $11 n,arlwnr in a h n h n p n m n tsacicpterl. F m Itlitnnre, r h nretatc ~ enion i\ a i t n m y ha,? in the rw,tit,n i.HICiltW 4 H 0 CII ('i)011 1 1110 t)ut 3 ~ e i i khiire In i h r rem tlon CH ,CUW T H,O . C H ,COilH T OH-.'Ihe d~ffirult\Ilr< ~n the d r i ~ n ~ t mu nt r t n , n i :md n.edk acid; and bases in an ahsolute way. As is most apparent in the Lewis theory of .&ids and bases, the strength depends as much on the properties of the solvent as on the intrinsic properties of the acid or base. Statements concerning the strength or weakness of an acid or base must be made in comparison with some reference. The most suitable reference is thesolvent and the ions produced by its autoinnizatiun. Water is the common solvent of choice. The Br#nsted model is considered most extensively in the development of general acid-base theory a t the introductory level. When a Brvinsted acid or base is added to a solution, a proton transfer reaction takes place and equilibrium is established when a minimum in the total free energy of the system has been obtained. A table of acids and their conjugate bases, arranged in order of decreasing dissociation constant, KHA,of the acid in water should be used for a qualitative discussion of acid strength. (A comparable table of bases and their conjugate acids could just as well be used.) An acid, HA, will transfer a proton to a base, B, which is below it in the table HA B * HBt A-. This can he viewed as the sum of two acid-base half reactions:
..
.
+
+
HAsH++AB-+ Ht s BHt The value of K for a particular reaction gives a quantitative aswith an equilibrium constant, K, given by K = KHA/KHB+. sessment of the extent of the reaction. The pedagogical use of acid-base half reactions has been noted recently1 and an analogy with standard reduction potentials has been made pre~iously.~ Acid-haw strengths are measured relative to the couples HaO+/HsO with KH.o+ = l, and H2010H- with KHIO= 1 X 10-14, This development presents the principles of the position of equilihrium as applied to all aqueous acid and base reactions. The ambiguous terms "strong" and "weak" can he avoided. These terms can be introduced subsequently, but placed in proper perspective, since coupled reactions need to he specified. The format lends itself to a meanineful discussion of acids and bases and to an understanding of the statement that the nosition of equilibrium
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Atkinson, G. F., J. CHEM. EDUC., 56,238 (1979). %Pacer,R. A,, J. CHEM. EDUC., 50,178 (1973). University of Louisville Louisville, KY 40208
I14 1 Journal of Chemical Education
Joseph C . Deck,
