Conjugate acid base pair

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.

Bronsted acids

HCl(molecular) → H⁺ + Cl⁻

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

HCO₃⁻(anionic) → H⁺ + CO₃⁻² 

Bronsted bases

Pyridine(molecular) + H⁺→ [Py(H)]⁺

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

H⁺ + CO₃⁻²(anionic) → HCO₃

Conjugate acid-base pairs

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 reactions

Acid-base reaction

Question

Name the conjugate acids and the conjugate bases of HX⁻ and X.

Answer

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.

Question

Arrange the following compounds in order of increasing acid strength: (i) NH₃ (ii) CH₄ (iii) HF (iv) H₂O

Answer

These hydrides become increasingly acidic order as,

CH₄ㄑNH₃ㄑH₂OㄑHF

Thus CH₄ has negligible acidic properties, but NH₃ donates a proton more to the strong base to form NH₂⁻, H₂O loses a proton even more readily and HF 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,

CH₃⁻ㄑNH₂⁻ㄑOH⁻ㄑF⁻