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8.4: Acid/Base Classifications: Lewis Acids and Bases

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  • Learning Objectives
    • Define Lewis acid.
    • Define Lewis base.
    • Compare and contrast the definitions of Arrhenius, Brønsted-Lowry, and Lewis acids.
    • Compare and contrast the definitions of Arrhenius, Brønsted-Lowry, and Lewis bases.

    As stated in the previous section of this chapter, a Brønsted-Lowry acid/base reaction involves the transfer of a hydrogen ion, H+1, from a solute particle, which is classified as a Brønsted-Lowry acid, to a solvent particle, which is categorized as a Brønsted-Lowry base.  At the molecular level, this process involves the ionization of an electrolytic solute, the generation of an H+1 ion, which is also known as a proton, and the immediate absorption of that ion by a solvent molecule.  Furthermore, because the composition of a chemical is defined by the types of atoms that it contains, as well as on the ratio in which those atoms are present, the incorporation of a proton into a solvent molecule alters the identities of both substances.  Therefore, the transformations that occur during a Brønsted-Lowry acid/base reaction, which can be described as a decomposition reaction/combination reaction sequence, are classified as chemical changes.  Recall that, in order for the composition of a chemical to be changed, the bonds that exist within that molecule must also be altered.  Pre-existing bonds, which are each comprised of two electrons, must be broken, so that the associated atoms can separate and rearrange, and, finally, new bonds are created.

    In 1923, Gilbert Lewis, who is most well-known for visually-representing covalent molecules as two-dimensional pictures that became known as Lewis structures, proposed that, since the stability and, therefore, the reactivity, of a particle is ultimately dependent on its electron configuration, the sequence in which bonds are made and broken during a chemical reaction was actually driven by the transfer of electrons between particles.  Lewis correctly hypothesized that, during a Brønsted-Lowry acid/base reaction, the pair of electrons that bond the hydrogen atom to the remaining portion of the solute must be lost by, or transferred away from, the hydrogen atom, which then ionizes as a hydrogen ion, H+1.  Because an H+1 ion consists, by definition, of only a single proton, this charged particle is extremely unstable, as it does not contain any electrons.  Therefore, in order to stabilize a hydrogen ion, a nearby solvent molecule must donate two electrons to the generated H+1 particle.  By accepting this electron density, the H+1 ion is "absorbed by" the solvent molecule, and a new bond between these species is produced.

    This electron-transfer-driven reactivity model revolutionalized the study of all chemical reactions, not only those that occur between acids and bases.  Therefore, in honor of his significant scientific contribution, particles that participated in an acid/base reaction through the acceptance and donation of electron density became known as Lewis acids and bases, respectively.  Formally, a Lewis acid is defined as an electron-pair acceptor, and a Lewis base is defined as an electron-pair donor.  Therefore, a Lewis acid/base reaction involves the transfer of two electrons from a solvent molecule, which is classified as a Lewis base, to a hydrogen ion, which is categorized as a Lewis acid.  Due to the synergistic relationship that exists between Lewis acids and bases, this classification system is inherently more sophisticated than the Arrhenius system, in which two ions are generated and analyzed as independent chemical entities.

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