Define conjugate acid-base pair

A great advancement was made in the area of the conjugate acid-base pair when Bronsted and Lowry proposed in 1923 a new concept that is independent of solvents.

According to them, an acid is defined as any hydrogen-containing material (a molecule or a cation or an anion) that can release a proton (H⁺) to any other substance.

Whereas a base is a substance (a molecule or a cation or an anion) that can accept a proton from any other substance.

Conjugate acid base pair
Conjugate acid-base

In short, an acid is a proton donor and a base is a proton acceptor.

A neutralization process has therefore involved a release of a proton by acid and the acceptance of proton by the base.

Neutral compounds or even ions could be designated as acids or bases according to this concept.

Examples of Bronsted acids

Acid Acid Base
HCl H+ + Cl
[Al(H2O)6]+3 H+ + [Al(H2O)5(OH)]+2
HCO3 H+ + CO3-2

Examples of Bronsted Bases

Base Acid Acid
Pyridine + H+ [Py(H)]+
[Al(H2O)5(OH)]+2 + H+ [Al(H2O)6]+3
CO3-2 + H+ HCO3

What is conjugate acid-base pair with example?

When an acid release a proton (H+), the residue must be a base and this base can take up a proton (H+) to form original acid.

Thus acid-base neutralization involved two acid or two bases forming conjugated acid-base pairs.

Hence a conjugate base of an acid is that part left after the proton lost. Similarly, the conjugate acid of the base is the species formed on the addition of a proton to the base.

HCl + H2OH3O+ + Cl

In the above reaction, HCl donates a proton to H2O, which is an acid. But H2O accepts a proton from HCl, which is a base.

In the reverse reaction which at equilibrium proceeds at the same rate as the forward reaction

H3O+ + ClHCl + H2O

H3O+ ion donates a proton to Cl ion, hence H3O+ ion is an acid. Cl ion accepts a proton from H3O+ ion is a base.

Acid and conjugate base examples

The members of which can be formed from each other mutually by the gain or loss of protons are called conjugate acid-base pairs.

Thus a conjugate base of an acid is the part left after the proton (H+) lost. Similarly, the conjugate acid of a base is the species formed on the addition of a proton to a base.

Conjugate Acid Conjugate Base
Perchloric acid
Perchlorate ion
H+ + ClO4
Sulfuric acid
Hydrogen sulfate
H+ + HSO4
Hydrochloric acid
Chloride ion
H+ + Cl
Nitric acid
Nitrate ion
H+ + NO3
Hydronium ion
H+ + H2O

Conjugate acid-base pair reactions

An acid exhibits its acid properties only when it is allowed to react with a base. Similarly, a base displays its basic properties only when it exposed to an acid.

acid1 base2 base1 acid2
HCl + NH3 Cl + NH4+
HNO3 + H2O NO3 + H3O+
H2O + F- OH + HF
H2SO4 + H2O HSO4 + H3O+
NH4+ + H2O NH3 + H3O+

How do you identify acids and bases from an equation?

Conjugate acid of a species is the one that is obtained on the addition of a proton and the conjugate base of a species is one that is obtained on the release of a proton.

acid1 base2 base1 acid2

In the above reaction, HX acts as a base and its conjugate acid is H2X.

HX H2O X-2 + H3O
acid1 base2 base1 acid2

In this reaction HX acts as an acid thus its conjugated base is X-2.

In the same way, the conjugate acid of X-2 is HX but X-2 cannot have any conjugated base because there is no proton that can release.

Conjugate base stability and acid strength

These hydrides become increasingly acidic order


Methane has negligible acidic properties, but ammonia donates a proton more to the strong base to form NH2, water loses a proton even more readily and hydrogen fluoride is a fairly strong acid.

The increase in the acidic properties of these hydrides is due to the fact that as we move from CH4 to HF, the stability of their conjugate bases increases in the order,