Weak Acids

A weak acid reacts incompletely with water to produce hydronium ions. Therefore, a weak acid is only partially deprotonated in water. In the animation, nine HA molecules produce only two hydronium ions (plus two A^- ions), while the remaining seven molecules remain in their molecular form, HA, and are the major species in the solution.
HA(aq) + H₂O(l) <--> H₃O⁺(aq) + A^-(aq)

The animation also shows the dynamic nature of the equilibrium (<-->); proton transfer is always taking place between HA, A^-, H₃O⁺ and water. The value of K_a for a weak acid is small because the ratio [H₃O⁺][A^-]/[HA] is small. Typically, weak acids have K_a values about 10^-3-10^-10.