Home Chemistry Acids Conjugate Acid Base pair

Conjugate Acid Base pair

Conjugate acid base pair definition

Conjugate acid base pair or protonic definition of acids and bases independently proposed by Bronsted and Lowery in 1923. According to this definition, an acid is a proton donor and a base is a proton acceptor in chemistry.

Conjugate acid base pair chart in order of decreasing strength of an acid or increasing strength of base

For learning chemistry, the conjugated acid base theory may be defined broadly as,

  • A conjugate acid is any hydrogen containing material (molecule or ion) that can release a proton or hydrogen ion to any other substances.
  • A conjugate base is any substance (molecules or ions) that can accept a proton to any other substances to form the conjugate acid base pair.

Conjugate acid base pair list

The neutralization reaction going through donates or accepts protons by acids and bases. Hence acids base pairs are the members of the balanced chemical equation that can be formed by mutual loss or gain of protons.

Examples of conjugate acid base pair or protonic definition of acids and bases proposed by Bronsted Lowery

Conjugate acid-base theory can not explain the conjugate acid base phenomena in terms of electronic structure by the formation of the coordinate covalent bond between the vacant orbital and the orbital which contains lone pair of electrons. This can be explained by the Lewis acid base theory.

Conjugate acid base pair examples

For example, nitric and sulfuric acid easily donate a proton to water to show acid character. Water easily accepts a proton from sulfuric or nitric acid to show base properties.

In the below list, we provide some examples of conjugated acid-base pairs with their name and formula.

Conjugate acid Conjugate Base
Name (in order of increasing strength) Formula Name (in order of decreasing strength) Formula
Perchloric acid HClO4 Perchlorate ion ClO4
Sulfuric acid H2SO4 Hydrogen sulfate ion HSO4
Hydrogen chloride HCl Chloride ion Cl
Nitric acid HNO3 Nitrate ion NO3
Hydronium ion H3O+ Water H2O
Hydrogen sulfate ion HSO4 Sulfate ion SO4−2
Phosphoric acid H3PO4 Dihydrogen phosphate ion H2PO4
Acetic acid CH3COOH Acetate ion CH3COO
Carbonic acid H2CO3 Hydrogen carbonate ion HCO3
Hydrogen sulfide H2S Hydro sulfide ion HS
Ammonium ion NH4+ Ammonia NH3
Hydrogen cyanide HCN Cyanide ion CN
Hydrogen carbonate ion HCO3 Carbonate ion CO3
Phenol C6H5OH Phenoxide ion C6H5O
Water H2O Hydroxide ion OH
Ethyl alcohol C2H5OH Ethoxide ion C2H5O
Ammonia NH3 Amide ion NH2
Methyl amine CH3NH2 Methyl amide ion CH3NH
Hydrogen H2 Hydride ion H
Methane CH4 Methide ion CH3

How do you find the conjugate acid base pair?

When an acid release a proton, the residue must be a base and this can take up a proton to form the original one. The neutralization reaction involved two acids or two bases forming conjugated acids and base pairs.

It defines acids and bases in terms of the substances themselves and not in terms of the pH scale of the acid base solution.

HCl + H2O → H3O+ + Cl

in this reaction, proton plays a key role in acid base function. Hydrogen ion has a very high charge density and is effective for the electric polarization of other ions or molecules according to Fajan’s rules. Water accepts a proton from hydrochloric acid. Therefore, water is a conjugate base of HCl.

In the reverse reaction which at chemical equilibrium proceeds at the same rate as the forward reaction. Here H3O+ ion donates a proton to Cl ion or Cl ion accepts a proton from H3O+ ion. Therefore, HCl is the conjugate acid of water.

Which of the following is a conjugate acid-base pair?

Conjugate acid base pairs are formed from each other mutually by the gain or loss of protons. The equilibrium reaction involves two acids and two bases.

The stronger acid and weaker base form one conjugate pair and the stronger base and weaker acid form another pair.

Some common examples for formation conjugate acids and bases are,

HClO4 ⇆ H+ + ClO4
H2SO4 ⇆ H+ + HSO4
HCl ⇆ H+ + Cl
HNO3 ⇆ H+ + NO3
H3O+ ⇆ H+ + H2O

On the basis of Bronsted Lewary theory or protonic definition, the neutralization reaction involves acid1 and base1 forming one pair, and acid2 and base2 forming another conjugate acid base pair.

Conjugate acid base pair examples in chemistry

Conjugate base of weak acid

Methane, hydrogen peroxide, and hydrogen molecule are the weakest acids, and their conjugate bases are CH3, H2O, and H. These are consequently the strongest bases.

Weak acid produces a strong conjugate base and strong acid produces a weak conjugate base. If we consider hydracids of the 2nd period in the periodic table like methane (greenhouse gas), ammonia, water, and hydrogen fluoride.

Polarity and acidity increase from methane to hydrogen fluoride. But after donating protons these acid molecules form conjugate bases. The basicity decreases from NH2 to F ion. Therefore, the fluorine ion (F) is a weak base, and NH2 ion is a strong base.

Conjugate base strength

According to Bronsted theory, a strong acid has a strong affinity to donate the proton but a strong base has a strong affinity to accept the proton.

When we compare the strength of the conjugate base of hydrogen cyanide and acetic acid, the experimentally observed ionization or acidity constant at 25°C,
KCH3COOH = 1.8 × 10−5 and
KHCN = 4.0 × 10−10

Therefore, the acidity of acetic acid is greater than that of hydrogen cyanide.

The second method is comparative pyrolysis or thermal decomposition by specific heat. In this method, we determined the equilibrium concentration of the conjugate acid base.

For example, ethoxide ions fairly chemical bonding with water to form ethyl alcohol than hydroxyl ions. Therefore, ethoxide ion is a stronger base than hydroxyl ion, and water is a stronger acid than ethyl alcohol.

Acid base and oxidation number

The acidic character of the conjugate acid base pair of oxyacids of the same chemical element increases with the increasing oxidation number.

For example, the oxyanions of a series of chlorine oxoacids HOCl, HClO2, HClO3, and HClO4 are ClO, ClO21, ClO3−1, and ClO4 respectively. All these four acids have one protonated oxygen atom.

  • The acidic character of the oxoacids increases from HClO to HClO4 because the oxidation number of the central atom or chlorine increases from HClO to HClO4.
  • The basic character of the conjugate base or oxyanions decreases from ClO to ClO4.

If we consider the conjugate acid base pair of the oxoacids of phosphorus this rule is not applied due to the structure of these oxoacids.