Question

Discuss in extensive detail the Brønsted-Lowry model of acids and bases including equations for the - a. The definition of Brønsted-Lowry acids and bases b. Application of this model to non-aqueous solvents and gas phase reactions c. Relation between acid-strength and solvents d. Factors that influence the strengths of acids and bases

Answer 1

(a) In this theory, acids are defined as proton donors; whereas bases are defined as proton acceptors. A compound that acts as both a Bronsted-Lowry acid and base together is called amphoteric.

The Bronsted-Lowry theory of acid and bases took the Arrhenius definition one step further, as a substance no longer needed to be composed of hydrogen (H+) or hydroxide (OH-) ions in order to be classified as an acid or base. For exmaple, consider the following chemical equation:

HCl(aq)+NH3(aq)→NH+4(aq)+Cl−(aq)(1.1)

Here, hydrochloric acid (HCl) "donates" a proton (H+) to ammonia (NH3) which "accepts" it , forming a positively charged ammonium ion (NH4+) and a negatively charged chloride ion (Cl-). Therefore, HCl is a Bronsted-Lowry acid (donates a proton) while the ammonia is a Bronsted-Lowry base (accepts a proton). Also, Cl- is called the conjugate base of the acid HCl and NH4+ is called the conjugate acid of the base NH3.

A Bronsted-Lowry acid is a proton (hydrogen ion) donor.
A Bronsted-Lowry base is a proton (hydrogen ion) acceptor.

(b) In non aqueous solutions, this model can be applied as below:

NaNH2 + NH4Cl 2NH3 + NaCl

Here, NaNH2 is a base as it accepts an H+ ion while NH4Cl is an acid as it gives up a proton to form two NH3 molecules and produce a salt.

F9r Gaseous phase consider the reaction between Hydrogen Chloride gas and Ammonia gas.

NH3 + HCl NH4Cl.

Here, HCl donates it's proton while NH3 accepts H+ ion. Hence, NH3 is a Bronsted-Lowry base while HCl is an acid.

(c) If an acid completely ionizes in the solvent, then it's acid strength is higher. But, if it does not ionize completely in a solvent then it's strength is low in that solvent. This can be seen from ionization of Acid in water as an example.

K_a (strength of an acid) = [H₃O⁺]*[Conjugate base]/[Acid]. In this formula, we don't include concentration of water as it is the solvent and is in liquid form. Higher dissociation leads to higher H+ ion concentration and higher Ka value.

(d) Factors influencing acid strength:

1. Bond strength - The weaker the bond, more is the acidity. This is because as the bond strength is less, the dissociation of H+ ion becomes more easier.

2. Bond polarity - A high polarity bond makes it easier for the H+ ion to leave the molecule.

3. In case of Oxoacids, the number of Oxygen atoms on the atom X decide the strength of the acid. As the number of Oxygen atoms increase, the strength of the acid also increases.