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Concepts of Acids and Bases

There are three concepts of acids and bases. The Arrhenius definition states that an acid produces H+ in aqueous solution and a base produces OH.  After that, two more sophisticated and general concepts of Bronsted-Lowry and the Lewis were described.

The Arrhenius Concept of Acids and Bases

In 1884, the Swedish chemist Svante Arrhenius proposed this concept of acids and bases.

Arrhenius Acid

Acid is a substance which dissociate in aqueous solution to give hydrogen ion (H+).

Arrhenius Base

Base is a substance which dissociate in aqueous solution to give hydroxide ion (OH).

Limitations of Arrhenius Concept

1. Arrhenius concept can explain only aqueous solution. It can not explain non-aqueous solution.

2. It says acid must give H+ ions, whereas there are acids which do not contain hydrogen.

Example: AlCl3, BF3, CO2

In Base, similarly Arrhenius concept says that Base must have OH which it releases in aqueous solution but there are bases which give OH even though they do not contain OH.

Example: NH3

1. Arrhenius concept can explain only aqueous solution but it cannot explain non-aqueous solution.

2. It says acid must give H+ ions, whereas there are acids which do not contain hydrogen.

Example: AlCl3, BF3, CO2

In Base, similarly Arrhenius concept says that Base must have OH which it releases in aqueous solution but there are bases which give OH even though they do not contain OH.

Example: NH3




Bronsted-Lowry Concept of Acids and Bases

In 1923, chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently developed definitions of acids and bases based on (H+) proton transfer.

Bronsted Acid

Any substance during chemical reaction transfer or donate proton to other substance is called Acid.

Bronsted Base

Any substance during chemical reaction accepts proton is called Base. HCl is donating H+ to NH3, so HCl is Acid, where as NH3 is accepting proton so it is Base.

Conjugate Acid

Species formed by the accepting a proton by base.

Base accepts proton from acid so it forms a relation with acid.

Conjugate Base

Species formed by donating a proton by acid.

After donating proton to base, it forms a relation with base so Cl is conjugate base.

Amphoteric Substances

Any substance which behaves as both acid and base.

Example: H2O is an amphoteric substance.

Limitations of Bronsted Lowry Concept

This concept is based on proton transfer because proton transfer is compulsory in this concept. According to this concept, we will be able to know that substance is acid or base, when there is proton transfer in the reaction but there are substances, in which there is no proton but still they are acid and base.

Examples:

SO3 Acid

CaO Base




Lewis Concept of Acids and Bases

In 1923, G.N. Lewis from UC Berkeley proposed more broad and general concept which was based on electron pair transfer.

Lewis Acid

A substance which can accept electron pair.

Lewis Base

A substance which can donate electron pair.

Lewis Acid: Acid can be +ve charge species or neutral species. Neutral species in which central atom has incomplete outermost shell.

Simple cations are Lewis acid which accepts es pair.

Example: H+ , Ag+ , Cu2+ , H3O+

Exception: Na+, K+, Ca2+, they cannot be Lewis acids because they always donate electrons. They do not have capacity to accept electrons.

Lewis Base: Neutral species having at least one lone pair or negatively charged species.

Lewis Acid-Base Reaction

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