Explain the concept of Bronsted acids and Bronsted bases. Illustrate the answer with suitable examples.

Explanation

Bronsted-Lowry theory: Any substance that can lose a proton or protons is an acid.
Ex: `HCl, H_(2)SO_(4), H_(3)PO_(4), CH_(3)COOH`
An substance that can gain a proton or protons is a base.
Ex: `NH_(3), H_(2),OH^(-)` etc.
When an acid react with base neutralization takes place. As per Bronsted Lowry theory proton transfer from an acid to a base is called neutralization.
Ex: `HCl+H_(2)OhArrH_(3)O^(+)+Cl^(-)` ltbr The above reaction is reversible. In this reaction,
(i) HCl donates proton to water. So it is a Bronsted acid.
(ii) `H_(2)O` gains the proton. so it is a Bronsted base.
(iii) `H_(3)O^(+)` donates a proton to `Cl^(-)`. So it is Bronsted acid.
(iv) `Cl^(-)` can gain a proton. So it is a Bronsted base
The acid base pair which diller by a single proton is said to be conjugate acid-base pair. In the above reaction HCl & `Cl^(-)` and `H_(3)O^(+)` and `H_(2)O` are conjugate acid -base pairs.
Every Bronsted acid has its conjugate base and every Bronsted base has a conjugate protonic acid.
According to Bronsted -Lowry theory greater the tendency to donate proton stonger is the acid. HIgher the tendency of the base to accept protons, stronger is the base. In the above reaction HCl gives proton easily than `H_(3)O^(+)`. So HCl is stronger acid than `H_(2)O`. `H_(2)O` accepts the proton more easily than `Cl^(-)`.So `H_(2)O` is a stronger base than `NH_(3)`.
In all acid-base reactions, the reaction takes place in teh direction of formation of weaker acid and a weaker base.
A stronger acid has a weaker conjugate base and a stronger base has weaker conjugate acid.
Advantages of Bronsted-Lowry theory:
(i) This theroy explains the behaviour of acids and bases in both aqueous and non-aqueous solvents.
(ii) It explains the behaviour of `NH_(3), CaO`, etc. as bases and `CO_(2),SO_(2)` etc. as acids.
Drawbacks of Bronsted-Lowry theory:
(i) Proton donation or acceptance happens only in the presence of other substances.
(ii) It could not explain the behaviour of electron deficient molecules like `AlCl_(3), BCl_(3)` etc. as acids.