Protonic Concept (Bronsted -Lowry Concept)

A great advancement was made in the area of Acids and Bases when Bronsted and Lowry proposed in 1923 a new concept which is independent of solvents.

According to them, an acid is defined as any hydrogen-containing materials (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:

Molecular:	HCl → H+ + Cl-
Cationic:	[Al(H2O)6]+3 ↓  H+ + [Al(H2O)5(OH)]+2
Anionic:	HCO3- → H+ + CO3-2

Bronsted Bases:

Molecular:	Pyridine + H+→ [Py(H)]+
Cationic:	[Al(H2O)5(OH)]+2 + H+ ↓  [Al(H2O)6]+3
Anionic:	H+ + CO3-2 → 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 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	+	H2O	→	H3O+	+	Cl-
Acid		Base		Acid		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 proton are called conjugate acid-base pairs.

Acid	Formula	Base	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+	Chloride Ion	H2O
Hydrogen Sulfate	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 Sulphide	H2S	Hydro Sulphide Ion	HS-

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.

Examples of Acid-Base Reactions:

NH4+	+	OH-	⇆	NH3	+	H2O
Acid1		Base2		Base2		Acid2

HCl	+	NH3	⇆	Cl-	+	NH4+
Acid1		Base2		Base1		Acid2

HNO3	+	H2O	⇆	NO3⁻	+	H3O+
Acid1		Base2		Base1		Acid2

H2SO4	+	H2O	⇆	HSO4-	+	H3O+
Acid1		Base2		Base1		Acid2

NH4+	+	H2O	⇆	NH3	+	H3O+
Acid1		Base2		Base1		Acid2

H2O	+	F⁻	⇆	OH⁻	+	HF
Acid1		Base2		Base1		Acid2

H2O	+	CH3NH2	⇆	OH⁻	+	CH3NH3+
Acid1		Base2		Base1		Acid2

NH4+	+	CH3COO⁻	⇆	NH3	+	CH3COOH
Acid1		Base2		Base1		Acid2

Problem

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 conjugate base of a species is one that is obtained on the release of a proton.

H2O	+	HX-	⇆	OH-	+	H2X
Acid1		Base2		Base1		Acid2

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

HX-	+	H2O	⇆	X-2	+	H3O+
Acid1		Base2		Base1		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.

Problem

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^-

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