Theories of acids and bases

Theories of Acids and Bases

Arrhenius Concept

  • Swedish chemist Svante Arrhenius proposed the Arrhenius concept of acids and bases.
  • According to this theory, an acid is a substance that, when dissolved in water, increases the concentration of H+ ions (protons).
  • A base is a substance that, when dissolved in water, increases the concentration of OH- ions.
  • This theory is an effective explanation for many acid-base reactions. However, it only applies to reactions in aqueous solutions and cannot explain the behaviour of acids and bases outside a watery environment, which is a substantial limitation of the theory.

Bronsted-Lowry Theory

  • The Bronsted-Lowry theory defines acids and bases in terms of proton donors and acceptors, which widens the scope beyond just the aqueous solutions.
  • According to this theory, an acid is a substance that can donate a proton (H+) to another substance, and a base is a substance that can accept a proton from another substance.
  • In addition, this concept introduces the idea of conjugate acid-base pairs. This refers to an acid and a base that differ by the presence or absence of a proton. For example, H2O and OH- are a conjugate acid-base pair.
  • While more inclusive and explanatory than the Arrhenius concept, the Bronsted-Lowry definition of acids and bases is still inadequate for explaining certain reactions that do not involve proton exchange, such as reactions involving metal oxides and hydroxides.

Lewis Theory

  • To overcome the restrictions of the Bronsted-Lowry theory, the Lewis theory was proposed.
  • According to this theory, an acid is a substance that can accept an electron pair from another substance, and a base is a substance that can donate an electron pair to another substance.
  • This concept encompasses nearly all acid-base reactions, because nearly all involve an exchange of electrons. This makes the Lewis theory the most comprehensive definition of acids and bases. However, it is also the most abstract and complex, and is only necessary for understanding some of the more complex and specialised areas of chemistry.