Acid-Base Theories: Arrhenius, Brønsted-Lowry, and Lewis - SS2 Chemistry Lesson Note

Acid-base theories are fundamental concepts in chemistry that describe the behaviour and properties of acids and bases. Three prominent acid-base theories are the Arrhenius theory, Brønsted-Lowry theory, and Lewis theory.

Arrhenius Theory:

Proposed by Svante Arrhenius in the late 19th century. According to this theory, acids are substances that release hydrogen ions (H+) in aqueous solution, while bases are substances that release hydroxide ions (OH-) in aqueous solution. Acids and bases react to form water and a salt in neutralisation reactions. This theory is limited to aqueous solutions and does not account for acid-base reactions in other solvents.

Brønsted-Lowry Theory:

Proposed independently by Johannes Brønsted and Thomas Lowry in the early 20th century. According to this theory, acids are substances that donate protons (H+) to other substances, while bases are substances that accept protons. In this theory, acid-base reactions involve proton transfer from the acid to the base. This theory is more general than the Arrhenius theory and can explain acid-base reactions in various solvents, not just aqueous solutions.

Lewis Theory:

Proposed by Gilbert N. Lewis in the early 20th century. According to this theory, acids are substances that accept pairs of electrons, while bases are substances that donate pairs of electrons. Lewis acids can be molecules or ions that have an electron-deficient site, such as a positively charged ion or a molecule with an empty orbital. Lewis bases are molecules or ions that have a lone pair of electrons available for donation. The Lewis theory is the broadest of the three theories and includes both the Arrhenius and Brønsted-Lowry definitions.

Acid-Base Equilibrium in the Brønsted-Lowry Theory:

In the Brønsted-Lowry theory, an acid-base equilibrium involves the transfer of protons from the acid to the base. The acid that donates a proton becomes its conjugate base, while the base that accepts a proton becomes its conjugate acid. The strength of an acid is determined by its tendency to donate a proton, while the strength of a base is determined by its tendency to accept a proton.

Acid-Base Equilibrium in the Lewis Theory:

In the Lewis theory, an acid-base equilibrium involves the formation of a coordinate covalent bond between the Lewis acid and Lewis base, with the Lewis acid accepting a pair of electrons from the Lewis base. Lewis acids and bases can form complexes and coordinate compounds through the sharing of electron pairs.

Understanding these acid-base theories provides a comprehensive understanding of how acids and bases behave and interact in different chemical systems. The Arrhenius theory focuses on aqueous solutions, the Brønsted-Lowry theory emphasises proton transfer, and the Lewis theory encompasses a broader concept of electron pair transfer. These theories are fundamental in many areas of chemistry, including chemical reactions, solution chemistry, and coordination compounds.