A weak acid is an acid that does not completely donate all of its hydrogens when dissolved in water. These acids have higher pKa compared to strong acids, which release all of their hydrogens when dissolved in water.

While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane superacid (H(CHB₁₁Cl₁₁), which is one million times stronger than sulfuric acid, is entirely non-corrosive, whereas the weak acid hydrofluoric acid (HF) is extremely corrosive and can dissolve, among other things, glass and all metals except iridium.

Explanation

Weak acids do not ionize in a solution to a significant extent; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissociated HA still remains. Weak acids in water dissociate as

\mathrm{ HA_{(aq)} \, \leftrightarrow \, H^+\,_{(aq)} +\, A^-\,_{(aq)} }.

The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (K_a):

\mathrm{ K_a\, =\, \frac {[H^+\,][A^-\,]}{[HA]} }

The greater the value of K_a, the more the formation of H⁺ is favored, and the lower the pH of the solution. The K_a of weak acids varies between 1.8×10^-16 and 55.5. Acids with a K_a less than 1.8×10^-16 are weaker acids than water. Acids with a K_a of greater than 55.5 are strong acids and almost totally dissociate when dissolved in water.

Examples

The vast majority of acids are weak acids. Organic acids are a large subset of weak acids. However, there are some mineral acids in this field.

• acetic acid
• citric acid
• boric acid
• phosphoric acid
• hydrofluoric acid

See also

• Strong acid
• Weak base

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