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The Bronsted-Lowry theory of Acids and Bases
We generally define acids as being ones which give H+ ions in water and bases as those which form OH- ions in water . So far this definition is limited to reactions taking place in water only. Therefore The Bronsted-Lowry theory is used to describe acids and bases in a more general way.
This theory was given by Danish chemist J.Bronsted and English chemist T.Lowry in 1923.
The Bronsted-Lowry based on an idea that in an acid base reaction, a proton is transferred from an acid to a base (this proton is an H+ ion).
A Bronsted-Lowry acid is a proton doner.
A Bronsted-Lowry base is a proton acceptor.
When hydrochloric acid is formed, hydrogen chloride gas dissolves in water to form hydroxonium ions H3O+ and chloride ions.In the reaction illustrated below hydrochloric acid is acting as an acid because it donates a proton to water. Water is accepting a proton so it is acting as a bronsted lowry base.
Its is not necessary for the acids and bases to involve in aqueous solutions.
substances which can act either as a base or an acid are called amphoteric substances e.g water.
In the reaction I explained above water acted as a bronsted-lowry base. Now have a look at the following reaction.
When ammonia reacts with water, water donates a proton ammonia acting as an acid. This is illustrated in diagram above.
Conjugate acids and Conjugate bases
When an acid or base reacts with water an equilibrium mixture is formed meaning that reactants are being converted to products at the same rate as products are being converted to reactants. This reverse reaction can also be explained using the Bronsted-Lowry theory.
If a reactant is linked to a product by the transfer of a proton we call it a conjugate pair. look at the following reaction:
Looking at the forward reaction:
- Cl- is the conjugate base of acid HCL
- H3O+ is the conjugate acid of base H2O
Looking at the backward reaction:
- HCL is the conjugate acid of Cl-
- H2O is the conjugate base of H3O+
In a conjugate pair, acid has one more proton.
Hope this was helpful please comment if you have any queries.