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 Ka for a weak acid is small because the ratio [H3O+][A-]/[HA] is small. Typically, weak acids have Ka values about 10-3-10-10.