Strong and Weak Acids

Author: John Hutchinson

From the definition of an acid given in the Foundation, a typical acid can be written as HA, representing the hydrogen ion which will be donated and the rest of the molecule which will remain as a negative ion after the donation. The typical reaction of an acid in aqueous solution reacting with water can be written as

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

[1]

In this reaction, HA(aq) represents an acid molecule dissolved in aqueous solution. H₃O⁺(aq) is a notation to indicate that the donated proton has been dissolved in solution. Observations indicate that the proton is associated with several water molecules in a cluster, rather than attached to a single molecule. H₃O⁺ is a simplified notation to represent this result. Similarly, the A^-(aq) ion is solvated by several water molecules. Equation 1 is referred to as acid ionization.

Equation 1 implies that a 0.1M solution of the acid HA in water should produce H₃O⁺ ions in solution with a concentration of 0.1M. In fact, the concentration of H₃O⁺ ions, [H₃O⁺], can be measured by a variety of techniques. Chemists commonly use a measure of the H₃O⁺ ion concentration called the pH, defined by:

pH=-(log[H₃O⁺])

We now observe the concentration [H₃O⁺] produced by dissolving a variety of acids in solution at a concentration of 0.1M, and the results are tabulated in table 1.

Table 1: H₃O⁺ pH for 0.1M Acid Solutions

Acid	[H3O+] (M)	pH
H2SO4	0.1	1
HNO3	0.1	1
HCl	0.1	1
HBr	0.1	1
HI	0.1	1
HClO4	0.1	1
HClO3	0.1	1
HNO2	6.210-3	2.2
HCN	710-6	5.1
HIO	110-6	5.8
HF	5.510-3	2.3
HOCN	5.510-3	2.3
HClO2	2.810-2	1.6
CH3COOH (acetic acid)	1.310-3	2.9
CH3CH2COOH (propionic acid)	1.110-3	2.9

Note that there are several acids listed for which [H₃O⁺]=0.1M, and pH=1. This shows that, for these acids, the acid ionization is complete: essentially every acid molecule is ionized in the solution according to equation 1. However, there are other acids listed for which [H₃O⁺] is considerably less than 0.1M and the pH is considerably greater than 1. For each of these acids, therefore, not all of the acid molecules ionize according to equation 1. In fact, it is clear in table 1 that in these acids the vast majority of the acid molecules do not ionize, and only a small percentage does ionize.

From these observations, we distinguish two classes of acids: strong acids and weak acids. Strong acids are those for which nearly 100% of the acid molecules ionize, whereas weak acids are those for which only a small percentage of molecules ionize. There are seven strong acids listed in table 1. From many observations, it is possible to determine that these seven acids are the only commonly observed strong acids. The vast majority of all substances with acidic properties are weak acids. We seek to characterize weak acid ionization quantitatively and to determine what the differences in molecular properties are between strong acids and weak acids.