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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.
