STRENGTH OF ACIDS AND BASES

Contents for this page	Related topics
Ionization of acids The ionization constant Ionization of bases Additional questions	The common ion effect The pH scale The hydrolysis of ions	Data Glossary
Learning Outcomes
After studying this section, you will be familiar with (a) the concept of the strength of acids and bases and (b), understand by what is meant by the "ionisation constant" of an acid and a base.

Ionization of acids:

An acid-base reaction is a reversible reaction:

More specifically, when the acid AH reacts with the base H₂O, we talk about the IONIZATION, or DISSOCIATION of the acid AH.

Such dissociation produces an anion, A^-, and the HYDRONIUM ION, H₃O⁺, which is frequently abbreviated to H⁺, the hydrogen ion or proton.

With some acids, such as hydrochloric acid (a solution of hydrogen chloride, HCl, in water), the equilibrium lies on the right. In other words, the acid is almost completely ionized:

while with others, such as hydrocyanic acid, HCN, the equilibrium may lie on the left. In other words, it is dissociated only to a small extent:

Acids like HCl are said to be strongly ionized in water, and are known as STRONG ACIDS, while acids like HCN are known as WEAK ACIDS, since they are only weakly ionized in water.

Note that whether an acid is "strong" or "weak" has nothing to do with its concentration!

The ionization constant:

Since the ionisation of an acid in water is a reversible reaction, there will be an EQUILIBRIUM CONSTANT for the above reaction :

In dilute aqueous solution, only a small fraction of the [H₂O] is used up, so we can say that for all intents and purposes, [H₂O] is also constant. Grouping the constants together

K_a is known as the IONIZATION CONSTANT or DISSOCIATION CONSTANT of the acid HA. Its value varies widely, depending on the acid. The higher the value of K_a, the stronger the acid. For strong acids it is so high that the values of K_a are not normally measurable.

For convenience, dissociation constants are frequently expressed as the logarithmic function pK_a, where

We must bear in mind that Ka (and therefore also pK_a values) are temperature dependent, like is normally the case with all equilibrium constants.

Some values for weak acids.

Ionization of bases:

Bases also ionise in water. Taking the case of ammonia

we can formulate the IONIZATION CONSTANT, K_b:

The value of K_b for ammonia is 1.8 x 10^-5 at 25 ºC ( pK_b = 4.74), which means that it is quite a weak base. Strong bases will produce high concentrations of the HYDROXIDE, OH^-, ion in aqueous solution.

Additional questions

Some values of the dissociation constants for weak acids: