Definition of Conjugate Acid Base Pair
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 atom 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. 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.
The neutralization reaction going through donates or accepts of 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. But this theory can not explain the conjugate acid base phenomena in terms of electronic energy levels for the formation of the coordinate covalent bond between vacant orbital and the orbital which contain lone pair of electron particles. This can be explained by the Lewis acid base theory.
Donor Acceptor Properties
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 conjugate acid base solution.
HCl + H2O → H3O+ + Cl–, in the 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.
Bronsted Lowry Acids Bases
The members of which can be formed from each other mutually by the gain or loss of protons are called conjugate acids and bases pairs. 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 of conjugate acid base pairs 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.
Periodic Variation of Acids and Bases
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 acids produce strong conjugate bases and strong acids produce weak conjugate bases. If we consider hydracids of the 2nd periods in the periodic table like methane (greenhouse gas), ammonia, water, and hydrogen fluoride. Polarity of bonds 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.
Strength of Conjugate Acid-Base Pair
According to Bronsted theory, a strong acid has a strong electron 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 acid base pair 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 greater than HCN. The second method is the comparative pyrolysis or thermal decomposition by specific heat. In this method, we determined the equilibrium concentration of the conjugate acid base pair. 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.
Oxidation Number and Conjugate Acid Base
The acidic character of the conjugate acid base pair of oxyacids of the same chemical element which in different oxidation numbers increases with the increasing oxidation number. For example, the oxyanions of the acids HClO, HClO2, HClO3, and HClO4 are, ClO–, ClO2-1, ClO3-1, and HClO4– respectively. Hence 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.