Equilibrium of Acids and Bases
Arrhenius Model of Acids and Bases
The classical, or Arrhenius, model was developed by Svante Arrhenius in the nineteenth century. He defined an acid as any substance that liberates or yields hydrogen ions (H+) or protons in water. An example would be hydrogen chloride, HCl, gas, which when put in water ionizes to yield hydrogen ions, H+, and chloride ions. The resulting water solution of ionized H+ and Cl- is known as hydrochloric acid.
          HCl(g) + H2O  <------------->   H3O+(aq) + Cl-(aq)
This process involving the breakdown of a substance into ions is known as ionization.
An Arrhenius base is a substance that dissociates in water to produce hydroxide ions, OH-. Two examples of strong, or almost completely dissociated bases are potassium hydroxide, KOH, and sodium hydroxide, NaOH or lye.
Brønsted-Lowry Acid-Base Model
The Arrhenius theory applies only when water is used as the solvent. It restricts the term acid to substances yielding hydronium ions and the term base to those yielding hydroxide ions.
In 1923 J.N. Brønsted and T.M. Lowry independently proposed a much broader and more useful concept of acids and bases. According to their model, a Brønsted-Lowry acid is any substance capable of donating a hydrogen ion or proton to another substance, and a Brønsted-Lowry base is any substance capable of accepting a proton or hydrogen from another substance. In other words, acids are proton donors, and bases are proton acceptors.
According to this concept, any reaction involving the transfer of a proton or H+ from one substance to another is an acid-base reaction.
A base is a proton acceptor and an acid is a proton donor.
For ex.            NH3 + H2O ---->      NH4+    +     OH-
                       base     acid           conjugate        conjugate
                                                        acid                base
An aqueous solution of ammonia is sometimes called ammonium hydroxide, NH4OH. This usage is a matter of convenience since such a compound has never been isolated. A better notation for this is NH3(aq).
 ex.                 2 HCl + H2O ------>      H3O+    +        Cl-
                          acid    base              conjugate        conjugate
                                                             acid                base
Notice that water can act as either an acid or as a base. For this reason it is called amphoteric.
Hydronium ions
Consider HCl, a gas composed of polar covalent molecules. When HCl gas is passed through water we achieve the classic substance, hydrochloric acid, HCl(aq). The original gas does not have any of the properties of the resulting solution. It is reasonable to assume that molecules of HCl react with the water to produce ions. It is these ions that ultimately give the water and HCl solution it's acidic properties.
                    H2O + HCl ----> H3O+(aq) + Cl-(aq)
The reaction above consists of a breaking away of a proton, H+, from the HCl molecules. A stable co-ordinate bond is formed when a proton, H+, shares a pair of electrons with an oxygen atom of the highly polar water molecule. A hydrated proton, called the hydronium ion, H3O+, is formed.
Also, to some extent are the very exotic ions, H5O2+, H7O3+, and H9O4+. It is these ions along with the hydronium ion that impart the acidic properties to a solution.
Most solutions formed by the reaction of polar molecular compounds with water are observed to have either acidic or basic properties.