Understanding the Three Models of Acids and Bases
The Arrhenius, Brønsted-Lowry, and Lewis theories are three different models used to define acids and bases. Each theory offers a progressively broader and more inclusive definition, moving from the behavior of substances in water to the transfer of protons, and finally to the exchange of electron pairs.
The Arrhenius Theory: Focus on Water
The Arrhenius theory, the most restrictive model, defines an acid as a substance that increases the concentration of hydrogen ions (H+) when dissolved in water. An Arrhenius base is a substance that increases the concentration of hydroxide ions (OH-) in water. This theory is limited because it only applies to aqueous solutions.
The Brønsted-Lowry Theory: Focus on Protons
The Brønsted-Lowry theory provides a more general definition. It defines an acid as a proton (H+) donor and a base as a proton acceptor. This model is not restricted to aqueous solutions and introduces the concept of conjugate acid-base pairs, where a base becomes its conjugate acid after accepting a proton, and vice versa.
The Lewis Theory: Focus on Electron Pairs
The Lewis theory is the most general of the three. It defines an acid as an electron pair acceptor and a base as an electron pair donor. This broad definition includes substances that do not contain hydrogen, such as boron trifluoride (BF3), and explains a wider range of chemical reactions, including the formation of coordinate covalent bonds.