Summary

Acids and bases are important in domestic, environmental and industrial contexts. Acidity in aqueous solutions is caused by hydrogen ions and a logarithmic scale, pH, has been devised to measure acidity. Buffer solutions, which can be made from partially neutralised weak acids, resist changes in pH and find many important industrial and biological applications.

Specification

3.1.12.1 Brønsted–Lowry acid–base equilibria in aqueous solution

An acid is a proton donor.

A base is a proton acceptor.

Acid–base equilibria involve the transfer of protons.

3.1.12.2 Definition and determination of pH

The concentration of hydrogen ions in aqueous solution covers a very wide range. Therefore, a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration.

pH = –log10[H+]

Students should be able to:

• convert concentration of hydrogen ions into pH and vice versa

• calculate the pH of a solution of a strong acid from its concentration.

3.1.12.3 The ionic product of water, KW

Water is slightly dissociated.

KW is derived from the equilibrium constant for this dissociation.

KW = [H+][OH–]

The value of Kw varies with temperature.

Students should be able to use Kw to calculate the pH of a strong base from its concentration.

3.1.12.4 Weak acids and bases Ka for weak acids

Weak acids and weak bases dissociate only slightly in aqueous solution.

Ka is the dissociation constant for a weak acid.

pKa = –log10 Ka

Students should be able to:

• construct an expression for Ka

• perform calculations relating the pH of a weak acid to the concentration of the acid and the dissociation constant, Ka

• convert Ka into pKa and vice versa.

Notes