Question.1: Explain Bronsted -Lowry concept (protonic concept) of acid and base.
Ans.: J.N. Bronsted and T.M. Lowry (1923) proposed a more general theory known as the Bronsted Lowry proton transfer theory. According to this concept
Acid: Any hydrogen containing species (a molecule, a cation or a anion), which is capable of donating one or more protons to any other substance, is called an acid.
Basc: Any species (molecule, cation or anion), which is capable of accepting one or more protons from an acid is called an base.
Thus, according to be Bronsted Low concept, an acid is a proton donor, and a base is a proton acceptor. An important feature of this concept is that the acids and bases are interdependent, i.e., for a substance to act as an acid, there should be another substance to act as a base i.e., an acid and a base should be present simultaneously in any system. The reaction of an acid with a base involves transfer of a proton from the acid to the base.
Example: NH4+ and Cl– ions are produced in the solution when HCl acid and NH3 react.
In this reaction HCl giving off a proton becomes Cl– ion. So as a proton donor HCl is an acid here.
HCl → H+ + Cl–
Again NH3 accepts that protons which is donated by HCl, NH3 become NH3 ion. So, as NH3 is capable in accepting proton, it is a base.
NH3 + H+ → NH4+
Question.2. What is conjugate acid and conjugate base? Explain with examples.
Ans.: Conjugate base: According to the protonic concept when an acid losses a proton in solution it becomes an ion (anion); again that ion will have tendency to gain the proton back and thus it acts like a base. This anion is so, called conjugate base of that acid.
Conjugate acid: According to the protonic concept when base accepts a proton in solution it becomes an ion (cation); again that ion will have the tendency to donate the proton back and thus it acts like a proton donor that is, it acts like an acid. This cation is so, called the conjugate acid of that base.
From this behavior, it is clear that at equilibrium there cxist two pairs of acids and bases. These are called conjugate acids and bases.
Example: The reaction between HCl and NH3 is given below:
Question.3 : Descrive Conjugate pair Explanation:
In this reaction, HCl donales a proton to ammonia and becomes Cl– ion. This Cl– ion can accept proton. So Cl– ion is the conjugate base of HCl. Again NH3 gains proton from HCl and becomes NH4+ ion and this ion has the tendency to donate proton. So, NH4+ ion is the conjugate acid of NH3. So, in this reaction HCl, Cl– and NH3, NH4+ are two conjugate acid & base pairs.
The conjugate base of strong acid is weak base and that of weak acid is strong is strong base. The conjugate acid of strong base is weak acid that of weak base is strong acid.