The Bronsted-Lowry Theory of Acids and Bases
In 1923 J. N. Brønsted and T. M. Lowry independently proposed a broader concept of acids
and bases. According to them, a Bronsted-Lowry acid is any species capable of donating a
proton; a Bronsted-Lowry base is any species capable of accepting a proton. Hence
• An acid is a proton (hydrogen ion) donor.
• A base is a proton (hydrogen ion) acceptor.
A wide range of compounds can be classified in the Brønsted-Lowry framework: mineral acids
and derivatives such as sulfonates, carboxylic acids, amines, carbon acids, and many more.
Brønsted-Lowry Acid/Base Reaction
An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to
the base. The removal of a hydrogen ion from an acid produces its conjugate base, which is
the acid with a hydrogen ion removed. The reception of a proton by a base produces
its conjugate acid, which is the base with a hydrogen ion added.
Generally acid–base reactions according to the Brønsted–Lowry definition is:
acid + base ⇌ conjugate base + conjugate acid
Here, a conjugate base is the species that is formed after the Brønsted acid donates its proton.
The conjugate acid is the species that is formed when the Brønsted base accepts a proton from
the Brønsted acid. Therefore, according to the Brønsted-Lowry definition, an acid-base
reaction is one in which a conjugate acid and a conjugate base are formed.
Examples
Dissociation of hydrochloric acid (HCl) in aqueous solution
HCl + H2O ⇌ H3O+ + Cl−
The removal of H+ from the HCl produces the chloride ion, Cl−, the conjugate base of the acid.
The addition of H+ to the H2O (acting as a base) forms the hydronium ion, H3O+, the conjugate
acid of the base.
Reaction between acetic acid and water
CH3COOH + H2O ⇌ CH3COO− + H3O+
Here, acetic acid acts as a Brønsted-Lowry acid, donating a proton to water, which acts as the
Brønsted-Lowry base. The products include the acetate ion, which is the conjugate base formed
in the reaction, as well as hydronium ion, which is the conjugate acid.
Conjugate acid-base pairs
Consider a reversible reaction occurring during dissociation of acid HA
In forward reaction:
The HA is an acid because it is donating a proton (hydrogen ion) to the water.
The water is a base because it is accepting a proton from the HA.
In the backward reaction:
The H3O+ is an acid because it is donating a proton (hydrogen ion) to the A- ion.
The A- ion is a base because it is accepting a proton from the H3O+.
The reversible reaction contains two acids and two bases. These are in pairs, called conjugate
pairs.
When the acid, HA, loses a proton it forms a base, A-. When the base, A-, accepts a proton back
again, it reforms the acid, HA. These two are a conjugate pair.
Members of a conjugate pair differ from each other by the presence or absence of the
transferable hydrogen ion.
If HA is the acid, then A- is its conjugate base.
If A- is the base, then HA is its conjugate acid.
The water and the hydroxonium ion are also a conjugate pair. Water is a base; the hydroxonium
ion is its conjugate acid because it has the extra hydrogen ion which it can give away again.
The hydroxonium ion is an acid, then water is its conjugate base. The water can accept a
hydrogen ion back again to reform the hydroxonium ion.
For example.
Reaction between ammonia and water:
In the forward reaction, Ammonia is a base because it is accepting hydrogen ions from the
water. The ammonium ion is its conjugate acid - it can release that hydrogen ion again to reform
the ammonia. The water is an acid, and its conjugate base is the hydroxide ion. The hydroxide
ion can accept a hydrogen ion to reform the water.
In backward reaction, the ammonium ion is an acid, and ammonia is its conjugate base. The
hydroxide ion is a base and water its conjugate acid.
Relationship between the Bronsted-Lowry and the Arrhenius theory
Bronsted-Lowry concept is superior to Arrhenius concept.
1. Much wider scope:
Arrhenius concept is restricted to the study of substances which can release H+ and OH-
ions in water. Bronsted-Lowry concept includes all molecules and ions that can donate
a proton (acids) and those which can accept a proton (bases).
2. Not limited to aqueous solutions:
Bronsted-Lowry concept is not limited to aqueous solutions as in the case of Arrhenius
concept. It can be extended even to the gas phase. For example, gaseous ammonia (a
Bronsted base) can react with hydrogen chloride gas (a Bronsted acid) to give
ammonium chloride.
𝑵𝑯𝟑 (𝒈) + 𝑯𝑪𝒍𝒈 ⇋ 𝑵𝑯+
𝟒 + 𝑪𝒍−
3. Release of OH is not required for base:
Arrhenius acid is a substance that releases OH- ions in water. On the other hand, Bronsted
base is a substance that accepts a proton. Thus, liquid ammonia does not produce OH- ions
in water but is a base according to Bronsted Lowry concept as it can accept a proton to
form NH4+ (an acid).
𝑁𝐻3 + 𝐻 + ⇋ 𝑁𝐻4+
𝐵𝑎𝑠𝑒 𝐴𝑐𝑖𝑑
