Re: why was Arrhenius theory rejected and what was the faults in it?

Arrhenius was very limited in his concept of acid and bases. His basic theory was that acids must contain H+ protons and bases must contain OH- hydroxyls. The important part of his theory was the idea of electrolytic dissociation which leads to the very familiar (although very simplistic) acid-base reactions:

This would be for an acid:
HA ---> H+ + A¯

This would be a generic base:
XOH ---> X+ + OH¯

When acids and bases react according to this theory, they neutralize each other, forming water and a salt:
HA + XOH ---> H2O + XA

The Arrhenius theory of acids and bases was fully supplanted by the theory proposed independently by Johannes Brønsted and Thomas Lowry in 1923; the so-called Brønsted-Lowey theory. The Arrhenius theory was not incorrect but it just didn't work in all situations. It was too limited in scope. Here are some of the reasons why.

1) The solvent has no role to play in Arrhenius' theory. An acid is expected to be an acid in any solvent. This was found to not be the case. For example, HCl is an acid in water, behaving in the manner Arrhenius expected. However, if HCl is dissolved in benzene, there is no dissociation, the HCl remaining as undissociated molecules. The nature of the solvent plays a critical role in acid-base properties of substances.

2) All salts in Arrhenius' theory should produce solutions that are neither acidic or basic. This is not the case. If equal amounts of HCl and ammonia react, the solution is slightly acidic. If equal amounts of acetic acid and sodium hydroxide are reacted, the resulting solution is basic. Arrhenius' theory cannot explanation this.

3) The need for hydroxide as the base led Arrhenius to propose the formula NH4OH as the formula for ammonia in water. This led to the misconception that NH4OH is the actual base, not NH3. Arrhenis does not explain why ammonia is a base.

4) H+, a bare proton, does not exist for very long in water. It reats with water nearly completely to form hydronium ions H3O+. Arrhenius theory cannot explain this either.

5) Odd acid and base types, like ternary acids and polyprotic acids can not be explained or analyzed according to Arrhenius Theory.

It should also be noted that the Brønsted-Lowey theory was also too narrow in it's description of acids and bases and was itself supplanted by the Lewis theory which was the last great theory of acid-base chemistry. According to Lewis theory, an acid is any molecule that accepts a pair of electrons. On the other hand, a base is any molecule that donates a pair of electrons. Lewis theory gets at the very heart of the acid/base effect, the sharing of electrons. It does not require that an electron pair must transfer from one atom to another, as Bronsted-Lowry specifies, rather it is enough that an electron pair originally on one atom must be shared.

A good source for acid-base information and some of the above came from the following web site:
http://dbhs.wvusd.k12.ca.us/webdocs/AcidBase/AcidBase.html