The strength of an acid or base depends on what fraction of it dissociates, or ionizes, usually in water. A strong acid or base, such as HCl or NaOH, completely dissociates in water and has a pH close to one end of the spectrum. In a solution containing a weak acid or base, only a fraction of the molecules dissociates, resulting in a low concentration of H+ or OH– ions and a pH closer to the middle of the range.
Each acid and base has an ionization constant (Ka for acids, Kb for bases) that represents the ratio of concentration of products to reactants at equilibrium. It can be used to calculate the strength of an acid or a base.For strong acids, , where HA is the undissociated acid, A– is the acid's conjugate base, and H3O+ is the hydronium ion.
The ionization constant can be used to calculate the concentration of each ion at equilibrium by setting the change in concentration of the initial acid or base to x and creating an initial, change, equilibrium (ICE) concentration table.
Acid-Base Strength and Molecular Structure
The likelihood of an acid or base to ionize in water is determined by the strength of the bonds in the molecule and the arrangement of electronegativity among the atoms.The strength of an acid arises from its ability to lose a proton, in other words, to break the bond between a hydrogen atom and another atom. The more unequally the electron is shared in the bond, the more likely the bond is to break. When hydrogen is bonded to a highly electronegative atom such as chlorine, the electron density becomes much greater around the electronegative atom, weakening the bond and making it easier for the proton to escape. This phenomenon explains the strength of acids such as HCl and HBr. An oxygen bonded to a hydrogen will produce a strong acid, and the more oxygens present, the stronger the acid.