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What Are Acid Donators?

To a large extent, all acids are donators, owing to the fact that they donate protons to a proton-accepting base. In acids, this proton is a highly reactive hydrogen ion, which is accepted by water molecules to produce hydronium ions. Acids vary in their ability to donate protons, as well as their characterization based on the Lowry-Brønsted and Lewis theories.

  1. Strong and Weak Acid Donators

    • Acids are strong donators when all hydrogen ions are dissociated and the acid is completely ionized. Weaker donators remain partially dissociated and do not give up protons easily. Among the stronger donators is hydrogen iodide, which fully dissociates into hydrogen and iodide ions. Among the weakest is the hydroxide ion, which readily accepts protons rather than giving them away. These are called conjugate bases. More acid donator properties means less conjugate base properties and vice versa.

    Lowry-Brønsted Acids

    • The Lowry-Brønsted theory defines an acid as a proton donor. According to this model, in the reaction between hydrogen chloride and water, to produce hydronium and chloride ions, hydrogen chloride is described as a Brønsted acid, being the proton donor; water is the Brønsted base, being the proton acceptor. On the product side, hydronium ions are the Brønsted acids, while chloride ions are the Brønsted bases.

    Lewis Acid

    • The Lewis theory categorizes acids based on electron pairs rather than protons. A Lewis acid is an electron pair acceptor. In the same reaction described above, hydrogen chloride is the Lewis acid because it accepts electrons into its empty orbital, donated by the water molecule. This is in accordance with the frontier molecular orbital (FMO) theory, which classifies Lewis acids as interacting via their lowest unoccupied molecular orbital.

    Relationship between Brønsted-Lowry and Lewis Theory

    • The Brønsted-Lowry and Lewis theories are intertwined. Both define the same compound as acids, however, a Brønsted acid donates hydrogen ions, while the hydrogen ion itself is a Lewis acid. Brønsted acidity is considered the same as proton movement between Lewis bases. Also, Brønsted acids are proton donators because they do not possess any spare electron pairs. All Lewis acids are not Brønsted acids, as some may not even have protons to donate.

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