Bronsted - Lowry Theory

Bronsted–Lowry theory is also known as the proton theory of acids and bases. In 1923, this theory was introduced independently by the Danish chemist named Johannes Nicolaus Bronsted and an English chemist named Thomas Martin Lowry by stating that any compound that can transfer a proton to any other compound is defined as an acid, and the compound that accepts the proton is defined as a base. A proton is considered as a radioactive particle having a positive electrical charge unit, where the symbol H+ can be represented as it constitutes the nucleus of a hydrogen atom.

About the Theory

A material only acts as an acid in the presence of a base, according to the Bronsted Lowry scheme; similarly, a substance only functions as a base in the presence of an acid. Moreover, when an acidic substance lacks a proton, it creates a base called the acid conjugate base, and when a proton is acquired by a basic substance, it produces an acid called the base conjugate acid. Therefore, the reaction between an acidic substance, like hydrochloric acid, and a basic substance, like ammonia, can be represented by the equation given below:

HCl +NH3⇌NH4++Cl-

In the equation given above, the ammonium ion (NH+4 ) is given as the acid conjugate to the base ammonia, and the chloride ion (Cl-) is given as the base conjugate to hydrochloric acid.

The Bronsted–Lowry theory also enlarges the number of compounds, which are considered to be the acids and bases to include not only the neutral molecules (for example, the alkali metal hydroxides and nitric, sulfuric, and acetic acids) but also some molecules and atoms having positive and negative electrical charges (called as cations and anions). The hydronium ion, the ammonium ion, and some hydrated metal cations are considered to be acids. The phosphate, acetate, sulfide, carbonate, and halogen ions are considered as bases.

Definitions of Acids and Bases

According to the Arrhenius theory, acids are described as the substances that dissociate in an aqueous solution to form H⁺ (hydrogen ions), while bases are described as substances that dissociate in an aqueous solution to form OH⁻ (hydroxide ions).

Physical chemists Thomas Martin Lowry in England and Johannes Nicolaus Bronsted in Denmark both independently proposed the theory in 1923 that carries their names. The acids and bases are represented in this Bronsted Lowry theory in the way they react with each other, which makes for greater generality. This definition is expressed in terms of an equilibrium expression.

Acid + base ⇌ conjugate acid + conjugate base.

With an HA acid, this equation can be written symbolically as follows:

HA+B ⇌ A- + HB+

The equilibrium sign (⇌) can be used because the reaction can take place in both forward and backward directions. The HA acid can lose a proton to turn into its conjugate base, A−. The base, B, can accept a proton to turn into its conjugate acid, HB+. Most of the acid-base reactions are fast so that the reaction's components are usually in dynamic equilibrium with each other.

Aqueous Solutions

Consider the below acid-base reaction:

CH3COOH+H2O⇌CH3COO-+H3O+

Acetic acid, which is CH3COOH, is acid due to the reason it donates a proton to water (H2O) and becomes its conjugate base, which is the acetate ion (CH3COO−). H₂O is given as a base because of the reason it accepts a proton from CH3COOH and turns into its conjugate acid, which is the hydronium ion (H3O+).

The acid-base reverse reaction is also an acid-base reaction between the conjugate base of the acid and the conjugate acid of the base in the first reaction. In the above-given example, acetate is given as the base of the reverse reaction, and the hydronium ion is given as an acid.

H3O++CH3COO-⇌CH3COOH+H2O

The power of the theory of Bronsted-Lowry is that it does not require acid to dissociate, unlike the theory of Arrhenius.

Comparison with Lewis Acid-Base Theory

In the year when Bronsted and Lowry published their theory, G. N. Lewis proposed an alternative/substitutional theory of acid-base reactions. This Lewis theory is based on the electronic structure. A Lewis base can be defined as a compound, which can donate an electron pair to a Lewis acid, which is a compound that can accept an electron pair. Lewis's proposal explains the Bronsted–Lowry classification in terms of the electronic structure.

HA+B⇌A-+BH+

Both the conjugate basis, A-, the nucleus, B, are seen in the above representation holding a lone pair of electrons, namely the proton, which is a Lewis acid that can be passed between them.