Bronsted - Lowry Theory

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What is 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.


FAQ (Frequently Asked Questions)

1. Give the comparison between Bronsted Lowry, Arrhenius, and Lewis acids and bases?

Arrhenius theory is a protonic concept. Any substance that gives H+ ions when dissolved in water is called acid. HCl is an example. And the substance that dissociates to produce OH- ions when dissolved in water is called the base. NaOH is an example.


Bronsted Lowry theory is the proton donor-acceptor concept. A substance that donates protons is acid. HCl is an example. And, proton acceptor is called the base, where Cl- is an example. 


Lewis theory is an electronic concept. The donor electron pair is called the base—for example, all the nucleophiles. And, the acceptor of an electron pair is acid—for example, all the electrophiles.

2. List three types of acid-base theories?

Let us look at the acid-base theories.

  • Arrhenius theory of acid and bases - Acids are the substance that produces hydrogen ion in the solution, and the bases are the substances that produce hydroxide ion in the solution

  • Bronsted Lowry theory of acid and bases - An acid is given as a proton donor, and a base is given as a proton acceptor.

  • Lewis theory of acid and bases - Acid is an electron pair acceptor, and the base is an electron-pair donor.

3. Explain how to identify an acid-base reaction?

In an acid-base reaction, there exists a change in the pH of the medium. Any means that would tell us, the change in the pH will help us to identify the reaction as an acid-base one. Examples are given as a pH meter and a pH indicator.

4. What are acid and base?

Acid is defined as a substance when mixed in water; it releases H+ ions. The base is defined as a substance when mixed in water; it releases OH- ions. We can find them by indicators: Litmus paper and Methyl orange.