Conjugate acid base pair

Conjugate acid-base pair definition chemistry

A great advancement was made in the area of acids and bases 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.

Bronsted Lowery concept

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

What are the Bronsted acids?

HCl(molecular) → H⁺ + Cl⁻

[Al(H₂O)₆]⁺³(cationic) → H⁺ + [Al(H₂O)₅(OH)]⁺²

HCO₃⁻(anionic) → H⁺ + CO₃⁻²

What are the Bronsted bases?

Pyridine(molecular) + H⁺→ [Py(H)]⁺

[Al(H₂O)₅(OH)]⁺²(cationic) + H⁺ → [Al(H₂O)₆]⁺³

H⁺ + CO₃⁻²(anionic) → HCO₃

What are conjugate acid-base pairs give 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, thus forming conjugated acid-base pairs.

Thus, 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(acid) + H₂O(base) → H₃O⁺(acid) + Cl⁻(base)

In the above reaction, HCl donates a proton to H₂O and is, therefore, an acid. H₂O, on the other hand, accepts a proton from HCl and is, therefore, a base.

In the reverse reaction which at equilibrium proceeds at the same rate as the forward reaction, H₃O⁺ ion donates a proton to Cl⁻ ion, hence H₃O⁺ ion is an acid. Cl⁻ ion is a base because it accepts a proton from H₃O⁺ ion.

HCl + H₂O ⇆ H₃O⁺ + Cl⁻

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

Conjugate acid-base pair

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

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.

Acid-base reaction equations

Acid-base reaction

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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.

H₂O(acid1) + HX⁻(base2) ⇆ OH⁻(base1) + H₂X(acid2)

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

HX⁻(acid1)+ H₂O(base2) ⇆ X⁻²(base1) + H₃O⁺(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.

Acidic character in the periodic table

These hydrides become increasingly acidic order

CH₄ㄑNH₃ㄑH₂OㄑHF.

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 in the periodic table the stability of their conjugate bases increases in the order,

CH₃⁻ㄑNH₂⁻ㄑOH⁻ㄑF⁻.

Conjugate acid base pair