ACID AND BASE MODELS

Bronsted – Lowry and Lewis concept of acids and bases

The Bronsted-Lowry and Lewis concepts are two different ways of defining acids and bases.

The Bronsted-Lowry concept defines an acid as a substance that donates a proton (H+) and a base as a substance that accepts a proton. This definition emphasizes the role of protons in acid-base reactions.

For example, in the reaction HCl + H2O → H3O+ + Cl-, HCl is the acid because it donates a proton (H+) to water, which acts as the base by accepting the proton to form a hydronium ion (H3O+) and a chloride ion (Cl-).

 

The Lewis concept, on the other hand, defines an acid as a substance that accepts a pair of electrons and a base as a substance that donates a pair of electrons. This definition is broader than the Bronsted-Lowry concept, as it includes reactions that do not involve protons.

For example, in the reaction BF3 + NH3 → F3B-NH3, BF3 is the acid because it accepts a pair of electrons from the lone pair on the nitrogen atom in NH3, which acts as the base by donating a pair of electrons to form a coordinate covalent bond.

Overall, both the Bronsted-Lowry and Lewis concepts provide a useful framework for understanding acid-base reactions, and they are both widely used in chemistry.

 

Conjugate acid-base pair concept in terms of equilibrium

The concept of conjugate acid-base pairs is central to understanding equilibrium in acid-base reactions. In an acid-base reaction, an acid (HA) donates a proton (H+) to a base (B), forming its conjugate base (A-) and the conjugate acid (BH+).

For example, in the reaction between hydrochloric acid (HCl) and water (H2O):

HCl + H2O ⇌ Cl- + H3O+

HCl acts as an acid by donating a proton (H+) to water, which acts as a base by accepting the proton. The resulting products are the chloride ion (Cl-) and the hydronium ion (H3O+). In this reaction, HCl is the acid, H2O is the base, Cl- is the conjugate base, and H3O+ is the conjugate acid.

The equilibrium constant for this reaction is defined as:

K = [Cl-][H3O+] / [HCl][H2O]

where [ ] denotes the concentration of each species. At equilibrium, the concentrations of the reactants and products are related by this equilibrium constant.

The concept of conjugate acid-base pairs helps us to understand the direction of an acid-base reaction. If an acid is strong, its conjugate base will be weak, and vice versa. Similarly, if a base is strong, its conjugate acid will be weak, and vice versa. This is because strong acids and bases have a tendency to transfer protons completely to their conjugate bases and acids, respectively, whereas weak acids and bases only partially transfer protons.

In summary, the concept of conjugate acid-base pairs is essential to understanding equilibrium in acid-base reactions. It helps us to predict the direction of a reaction and to understand the strength of acids and bases.