|Equilibrium of Acids and Bases|
|Arrhenius Model of Acids and Bases|
|The classical, or Arrhenius, model was developed by Svante
in the nineteenth century. He defined an acid as any substance
liberates or yields hydrogen ions (H+) or protons in water.
An example would be hydrogen chloride, HCl, gas, which when put in
ionizes to yield hydrogen ions, H+, and chloride ions. The
water solution of ionized H+ and Cl- is known
H2O <-------------> H3O+(aq)
|This process involving the breakdown of a substance into
known as ionization.
|An Arrhenius base is a substance that dissociates
to produce hydroxide ions, OH-. Two examples of strong, or
completely dissociated bases are potassium hydroxide, KOH, and sodium
NaOH or lye.
|Brønsted-Lowry Acid-Base Model|
|The Arrhenius theory applies only when water is used as
It restricts the term acid to substances yielding hydronium
and the term base to those yielding hydroxide ions.
|In 1923 J.N. Brønsted and T.M. Lowry independently
a much broader and more useful concept of acids and bases. According to
their model, a Brønsted-Lowry acid is any substance
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
another substance. In other words, acids are proton donors, and
are proton acceptors.
|According to this concept, any reaction involving the
a proton or H+ from one substance to another is an acid-base
|A base is a proton acceptor and an acid is a proton donor.
NH3 + H2O ---->
base acid conjugate conjugate
|An aqueous solution of ammonia is sometimes called
NH4OH. This usage is a matter of convenience since such a
has never been isolated. A better notation for this is NH3(aq).
2 HCl + H2O ------> H3O+
acid base conjugate conjugate
|Notice that water can act as either an acid or as a base.
reason it is called amphoteric.
|Consider HCl, a gas composed of polar covalent molecules.
gas is passed through water we achieve the classic substance,
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
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
from the HCl molecules. A stable co-ordinate bond is formed when a
H+, shares a pair of electrons with an oxygen atom of the
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
to a solution.
|Most solutions formed by the reaction of polar molecular compounds with water are observed to have either acidic or basic properties.|