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.
What is the Bronsted-Lowery Concept of Acids and Bases?
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 + H2 ⇆  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 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 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 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 2:
    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, 
    CH4NH3H2OHF. 
    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-NH2-OH-F-

Protonic Concept (Bronsted -Lowry Concept), Bronsted Acids, Bronsted Bases and Conjugate Acid - Base Pairs with related problems and solutions

Inorganic Chemistry

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