THE LOWRY-BRØNSTED THEORY

1. Definition of acids and bases

An ACID is a substance that can donate a PROTON (H+ ION) to some other substance. It is therefore a PROTON DONOR.

A BASE is a substance that can accept a proton from other substances. It is therefore a PROTON ACCEPTOR.

It follows therefore that an acid will react with a base by the transfer of a proton from the acid to the base.

For example, let us suppose that we have an acid AH and a base B. They will react according to the reaction

The substance AH acts as an acid, since it donates a proton to B, which acts as a base by accepting the proton. Notice that the reaction is a reversible one. This implies that BH+ can donate a proton to A-. In other words, BH+ is an acid, and A-is a base! A- is said to be the CONJUGATE BASE of the acid AH, while BH is the CONJUGATE ACID of the base B. Let us look at a specific example:

The acid HCl reacts with the base H2O to produce Cl-, the conjugate base of HCl, and H3O+, the conjugate acid of H2O. The difference between an acid and its conjugate base or a base and its conjugate acid is one H+ ion.

When an acid reacts with water in this way, it is said to IONIZE, or DISSOCIATE, and the process is known as IONIZATION, or DISSOCIATION. (The term "ionization" as used here must not be confused with the process of ionization whereby atoms lose electrons to form cations.)

When an acid and a base react to form a salt and water, the process is known as NEUTRALIZATION.

2. Polyprotic acids

Some substances have more than one proton that can be given to a base. Such acids are known as POLYPROTIC ACIDS. Sulphuric acid is an example of a polyprotic acid, since it can lose two protons to the base water (in other words it is a DIPROTIC ACID), in two ionisation stages:

3. Amphiprotic substances

Some substances can act both as acids (by losing a H+ ion) and as bases (by gaining a H+ ion). Such substances are known as AMPHIPROTIC substances, or AMPHOLYTES.

Water is a good example of an amphiprotic substance:


4. Additional questions