Arrhenius Acid-Base Theory, Lewis Acid-Base Theory, conjugate pairs

  Arrhenius Acid-Base Theory 

      The Arrhenius acid-base concept classifies a substance as an acid if it produces hydrogen ions H(+) or hydronium ions in water. A substance is classified as a base if it produces hydroxide ions OH(-) in water.

Acids

According to the Arrhenius acid-base concept, a substance is classified as an acid if it ionizes to form hydrogen(+) ions in aqueous solution. For example, hydrochloric acid reacts with water to form hydronium ion.

Bases 

According to the Arrhenius acid-base concept, a substance is classified as a base if it ionizes to form hydroxide ions OH(-) in aqueous solution. For example 

   NaOH  -------  Na+  +  OH- 

Bronsted-Lowry Acid-Base Concept

  The Bronsted-Lowry theory classifies a substance as an acid if it acts as a proton donor, and as a base if it acts as a proton acceptor.

Example: When hydrogen chloride gas dissolves in water to produce hydrochloric acid,

the hydrogen chloride molecule gives a proton (a hydrogen ion) to a water molecule.  

A co-ordinate (dative covalent) bond is formed between one of the lone pairs on the oxygen and the hydrogen from the HCl. Hydroxonium ions, H3O+, are produced.

Lewis Acid-Base Concept

  The Lewis theory classifies a substance as an acid if it acts as an electron-pair acceptor and

as a base if it acts as an electron-pair donor. Other ways of classifying substances as acids or bases

are the Arrhenius concept and the Bronsted-Lowry concept.

Conjugate pairs

  When hydrogen chloride dissolves in water, almost 100% of it reacts with the water to produce hydroxonium ions and chloride ions. Hydrogen chloride is a strong acid, and we tend to write this as a one-way reaction:

H2O +HCL  -------. H30+   + cl -

Thinking about the forward reaction:

#The HA is an acid because it is donating a proton (hydrogen ion) to the water.

#The water is a base because it is accepting a proton from the HA.

But there is also a back reaction:

#The H3O+ is an acid because it is donating a proton (hydrogen ion) to the A-ion.

# The A-ion is a base because it is accepting a proton from the H3O+.