Sunday 2 September 2007

WHAT'S ACID? AND WHAT'S BASE?

Acids were first recognized as a class of substances that taste sour and Bases sometimes called alkalis, are characterized by their bitter taste and slippery feel. Scientists have postulated some theories to defined acids and bases as follow :

Arrhenius Theory
An acid is any substance that ionizes in water to give hydrogen ions, and a base ionizes in water to give hydroxyl ions. This theory is obviously restricted to water as the solvent.





Theory of The Solvent Systems
Franklin introduced the solvent systems theory in 1905. This theory is similar with the Arhenius theory, but applicable also to other ionizable solvents. The theory said :An acid is defined as a solute that yields the cation of of the solvent while the base is a solute that yields the anion of the solvent.


See at the picture, when we dilute sodium ethoxide into ethanol, sodium ethoxide ionize to give C2H5O- ion. Because of diluting sodium ethoxide yields anoin of the solvent so we can say that sodium ethoxide is strong base. Meanwhile diluting ammonium chloride in liquid ammonia produced NH4+ ion, that is cation of the solvent. So we say that ammonium chloride is a strong acid.

Bronsted-Lowry Theory

This theory states that an acid is any substance that can donate a proton and a base is any substance that can accept a proton. For the example see this reaction below :




Acid1 and base1 or acid2 and base2 are called acid-base conjugated.


Lewis Theory
G. N. Lewis introduced electronic theory of acids and bases in 1923. This theori said that an acid is a substance that can accept an electron pair and the a base is a substance that can donate an electron pair.




In the first reaction the aluminium chloride is an acid because it accept electron pair from the ether. In the second reaction the water is a base because water gives its electron pair into proton.














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