Bronsted-Lowry acid-base theory and explanation
Brønsted-Lowry Theory (Protonic Concept)
To overcome the limitations of the Arrhenius theory, Danish chemist J. N. Brønsted and British chemist T. M. Lowry independently proposed a modern definition of acids and bases in 1923. This theory is widely known as the Protonic Concept.
Acid
Chemical species that can donate a proton (H+) to another substance are called acids. In other words, an acid is a proton donor.
• H2SO4 → H+ + HSO4–
Classification:
1. Neutral Molecules: HCl, H2SO4, HNO3, CH3COOH, H2O, H2S, HCN, etc.
2. Cations (Positive Ions): H3O+, NH4+, [Fe(H2O)6]3+, etc.
3. Anions (Negative Ions): HSO4–, HCO3–, HS–, H2PO4–, etc.
Base
Chemical species that can accept a proton (H+) from another substance are called bases. In other words, a base is a proton acceptor.
• H2O + H+ → H3O+
Classification:
1. Neutral Molecules: NH3, H2O, R-NH2, etc.
2. Cations (Positive Ions): Cationic bases are generally not observed.
3. Anions (Negative Ions): OH–, Cl–, CO32-, SO42-, HCO3–, CH3COO–, CN–, etc.
Conjugate Acid and Conjugate Base
Conjugate Acid: The acidic species formed after a base accepts a proton is called the conjugate acid of that base.
Conjugate Base: The basic species left behind after an acid loses or donates a proton is called the conjugate base of that acid.
Figure: Representation of conjugate pairs in the reaction between ammonia and water.
Figure: Representation of conjugate pairs in the reaction between hydrochloric acid and water.
- Inability to Explain Aprotic Acids: This concept fails to justify the acidic behavior of various non-metallic acidic oxides (such as CO2, SO2, SO3, N2O5) and Lewis acids (such as BF3, BCl3, AlCl3, FeCl3), because they do not contain any replaceable protons (H+).
- Inability to Explain Protonless Bases: Similarly, several metallic oxides (such as Na2O, CaO, BaO) exhibit distinctly basic properties without accepting any proton, which remains unexplained by this theory.
- Reactions Involving No Proton Transfer: According to this framework, acid-base interactions are restricted solely to the donation and acceptance of protons. However, in reality, there are many acid-base reactions where no proton transfer occurs. For example:
• BF3 + NH3 → H3N·BF3 (Coordinate bond formation)
• Ag+ + 2NH3 → [Ag(NH3)2]+
• SiF4 + 2F– → [SiF6]2-
• PCl3 + Cl2 → PCl5 - Complexity of Amphoteric Oxides: Certain oxides exhibit both acidic and basic behaviors in aqueous or non-aqueous media (amphoteric oxides), which is difficult to explain comprehensively through simple proton transfer equations alone.
