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 is therefore involved a release of proton by acid and the acceptance of proton by 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 base is the species formed on the addition of 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 foreword 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 conjugated 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	Di hydrogen 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 1:

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 conjugated 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 conjugate acid of X^-2 is HX^- but X^-2 cannot have any conjugated base because there is no proton that can release.

• Problem 2:

Arrange the following compounds in order of increasing acid strength: 

(i) NH₃ (ii) CH₄ (iii) HF (iv) H₂O

• Answer:

These hydrides become become increasingly acidic order as, 

CH₄ㄑNH₃ㄑH₂OㄑHF. 

Thus CH₄ has negligible acidic properties, but NH₃ donates a proton more to strong base to from NH₂^-, H₂O loses a proton even more readily and HF is a fairly strongly 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^-