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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 for learning chemistry or chemical science. According to this definition, an acid is any hydrogen containing material (molecule or ion) that can release a proton or hydrogen ion to any other substances, whereas a base is any substances (molecules or ion) that can accept a proton to any other substances to form the conjugate acid-base pair. In short, an acid is a proton donor and a base is a proton acceptor in chemistry.

Conjugate acid base pair list

The neutralization reaction going through donates or accepts protons by acids and bases according to strength. 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 vacant orbital and the orbital which contains lone pair of electron. This can be explained by the Lewis acid base theory.

Conjugate acid base pair examples

For example, nitric and sulfuric acid easily donates a proton to water to show acid character. But water easily accepts a proton from sulfuric or nitric acid to shows acid base properties. In the below picture, we provide some examples of conjugated acid-base pairs with their name and formula.

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

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 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 effective for the electric polarization of other ions or molecules according to Fajan’s rules. Water(H2O) accepts a proton from HCl, which is a base.
  • In the reverse reaction which at chemical equilibrium proceeds at the same rate as the forward reaction where H3O+ ion donates a proton to Cl ion and Cl ion accepts a proton from H3O+ ion.

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

Conjugate acid base pairs 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 bases from acids 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 form one pair, and acid2 and base2 form another conjugate acid base pair in chemistry.

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 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 acids molecules form conjugate bases and the basicity decreases from NH2 to F ion. 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 respectively. The acidity of CH3COOH is greater than HCN.

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 and hydroxyl ion. 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 series of chlorine oxoacids HOCl, HClO2, HClO3, and HClO4 is ClO, ClO2-1, 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. But 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.