Weak acid

A weak acid is an acid chemical which does not dissociate (split into ions) completely in water solution. This means it does not give all its hydrogen ions into the water. Weak acids typically have a pH between 3 and 6.

Acetic acid (CH₃COOH) and oxalic acid (H₂C₂O₄) are examples of weak acids. If an acid is represented by HA, then there is a balance between molecules which are split (ionised), and molecules which are not:

[math]\displaystyle{ \mathrm{ HA_{(aq)} \, \leftrightarrow \, H^+\,_{(aq)} +\, A^-\,_{(aq)} } }[/math]

The relative strength of an acid can be expressed using K_a, the acid ionization constant. In general, the larger a K_a, the stronger the acid, while a smaller K_a indicates a relatively weaker acid. Weak acids will have a K_a value of less than 1, while strong acids will have a K_a value greater than 1.

Weak Acid Media

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  Image of a strong acid mostly dissociating. The small red circles represent H+ ions.

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  Image of a weak acid partly dissociating

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  2-chlorobutanoic acid, used in acid

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  3-chlorobutanoic acid, used in acid

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  4-chlorobutanoic acid, used in acid

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  en:butyric acid aka butanoic acid