91Ó°ÊÓ

Acids differ in their ability to donate H⁺. Stronger acids, such as HCl, react almost completely with water, whereas weaker acids, such as acetic acid (CH₃COOH), react only slightly. The exact strength of a given acid HA in water solution is described using the acidity constant (${\mathit{K}}_{\mathbf{a}}$)for the acid-dissociation equilibrium.

Example:

$$\begin{gathered} {\text{HA +H}}_2\text{O}⇄{\text{A}}^{-}{\text{+H}}_{\text{3}}{\text{O}}^{-} \\ {\text{K}}_a\text{=}\frac{{\text{[H}}_{\text{3}}{\text{O}}^{+}{\text{][A}}^{-}\text{]}}{\left[HA\right]} \end{gathered}$$

Acid strengths normally uses $\text{p}{K}_a$values rather than $K_a$values, where the $\text{p}{K}_a$is the negative common logarithm of $K_a$:

$\text{p}K\text{a = - log}{K}_a$

The stronger acid has a smaller $\text{p}{K}_a$and a weaker acid has a larger$\text{p}{K}_a$.

Stronger Acid - $\text{p}{K}_a=19$

weakker Acid - $\text{p}{K}_a=36$

A reaction takes place if the stronger acid is the reactant. The above reaction will take place to virtual completion due to large difference in a value.