Define an acid and a base according to Bronsted-Lowry concept and justify with examples that water is an amphoteric compound.
Answer: Bronsted-Lowry Concept : In 1923, the Danish chemist Bronsted and the English chemist Lowry independently presented their theories of acids and bases on the basis of proton-transfer.
According to this concept: An acid is a substance (molecule or ion) that can donate a proton (H+) to another substance.
A base is a substance that can accept a proton (H+)from another substance.
For example, HCl acts as an acid while NH3 acts as a base:
HCl + NH3 ⇔ NH4+ + Cl–
It is a reversible reaction. In the forward reaction, HCl is an acid as it donates a proton, whereas H2O is a base as it accepts a proton. In the reverse reaction, Cl– ion is a base as it accepts a proton from acid H3O+ ion. CI ion is called a conjugate base of acid HCl and H3O+ ion is called a conjugate acid of base H2O. It means every acid produces a conjugate base and every base produces a conjugate acid such that there is conjugate acid-base pair. Conjugate means joined together as a pair.
A conjugate acid is a specie formed by accepting a proton by a base.
A conjugate base is a specie formed by donating a proton by an acid.
Thus, conjugate acid-base pair differs from one another only by a single proton.
Explanation: According to Bronsted-Lowry concept, an acid and a base always work together to transfer a proton. That means, a substance can act as an acid (proton donor) only when another substance simultaneously behaves as a base (proton acceptor). Hence, a substance can act as an acid as well as a base, depending upon the nature of the other substance. For example, H2O acts as a base when it reacts with HCl as stated above and as an acid when it reacts with ammonia.
Such a substance that can behave as an acid, as well as, a base is called amphoteric.
Limitation of Bronsted Lowry Concept: It has been observed that there are certain substances which behave as acids though they do not have the ability to donate a proton, e.g.,SO3 . Similarly, CaO behaves as a base but it cannot accept a proton. These observations prove the limitations of Bronsted-Lowry concept of acids and bases.
|Do you know: All Arrhenius acids are Bronsted-Lowry acids, but except OH other Bronsted-Lowry bases are not Arrhenius bases.|
Table : Conjugate Acids and Bases of common species
|Do You Know?||All Arrhenius acids are Bronsted-Lowry acids, but except OH other Bronsted-Lowry bases are not Arrhenius bases|