Bronsted-Lowry Acids and Their Conjugated Bases

In 1924, chemists Johannes Nicolaus Bronsted and Thomas Martin Lowry autonomously characterised acids and bases based on their ability to donate or accept protons (H+ ions). The theory describes acids as proton donors, while bases are proton acceptors. Amphoteric compounds can function as both a Bronsted-Lowry acid and a base. 

According to the theory, the base creates its conjugate acid whenever a base and an acid reacts. Likewise, the acid forms its conjugate base. This phenomenon happens by exchanging a proton. 

Bronsted-Lowry acid-base theory is a stereotype of the Arrhenius theory of acids and bases.

Definitions of Acids and Bases

Arrhenius’s theory defines acids as substances that dissociate in a given aqueous solution to give H+ (hydrogen ions). In contrast, bases are substances that dissociate in a given aqueous solution to provide OH- with (hydroxide ions).

The following equation expresses the definition:

Acid + base ⇌ conjugate base + conjugate acid.

Because the reaction can occur in both forward and reverse directions, the above expression uses an equilibrium sign. The acid, HA, can lose a proton to make its conjugate base, A. B, the base, can accept a proton to become HB+, its conjugate acid. 

Since most acid-base reactions are quick, the reaction components are generally in dynamic equilibrium.

The Bronsted-Lowry Theory

A proton (H+) donor represents an acid, and a proton acceptor indicates a base, according to the Bronsted–Lowry theory of acids and bases. When a Bronsted–Lowry acid loses one of its protons, it forms a conjugate base. Similarly, gaining a proton by a Bronsted–Lowry base leads to the formation of conjugate acid.

Bronsted-Lowry defines the proton transfer from one molecule or ion to another as an acid-base reaction.

Take a look at the acid-base reaction shown below:

CH3COOH + H2O⇌CH3COO- + H3O+

Acetic acid (CH3COOH) is acidic because it donates a proton to water (H2O) and forms its conjugate base, the acetate ion (CH3COO). H2O is designated as a base because it accepts a proton from CH3COOH and transforms into its conjugate acid, the hydronium ion (H3O+).

In the first reaction, the conjugate base of the acid and the base’s conjugate acid undergo an acid-base reaction. In the preceding example, acetate is the reverse reaction’s base, and the hydronium ion is the acid.

Points to Remember

• Bronsted-Lowry Acid: A chemical group that loses a proton (H+).
• Bronsted-Lowry Base: A chemical group with a single electron pair that can gain a cation/proton. 
• Conjugate Base: Accepts protons from the Bronsted-Lowry Acid. 
• Conjugate Acid: Loses proton to a Bronsted-Lowry Base 
• The molecular formula of the conjugate acid-base pair is the same as that of the original acid-base pair, except that the acid has one more H+ than the conjugate base.
• Strong Acids/Bases: Compounds that completely ionises in water or aqueous solution 
• Weak Acids/Bases: Compounds that dissociate only partially in aqueous solutions or water.
• Amphoteric Compounds: Acts both as a Bronsted-Lowry acid and a Bronsted-Lowry base. E.g., water.

Comparison with Lewis Acid-Base Theory

G.N. Lewis gave an alternative theory of acid-base reactions the same year Bronsted and Lowry published their theory. 

The electronic structure underpins this Lewis’s theory. A Lewis base is a compound capable of giving an electron pair to a Lewis acid, which can gain a pair of electrons.

Lewis’ demonstration describes the Bronsted–Lowry classification using electronic structure.

HA + B ⇌ A- + BH+

In the above representation, all of the two  Conjugate bases, A- and B, hold a solitary pair of electrons. Therefore, the proton, a Lewis acid, may pass between them.

Conclusion

Any species that can donate a proton H+ is called Bronsted-Lowry acid. Any species which has the capability of accepting a proton and requires a lone pair of electrons for making a bond with H+ is a Bronsted-Lowry base.

The species established after an acid donates its proton is the conjugate base of a Bronsted-Lowry acid. The species formed after a base accepts a proton is the conjugate acid of a Bronsted-Lowry base. A conjugate acid-base pair has the same molecular formula as the conjugate base, except the acid has an extra H+.

An example of an amphoteric substance is water, which means that it can function as a Bronsted-Lowry acid and a Bronsted-Lowry base.