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4.12: Molecular Definitions of Acids and Bases

  • Page ID
    333659
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    Learning Objectives

    • Identify an Arrhenius acid and an Arrhenius base.
    • Identify a Brønsted-Lowry acid and a Brønsted-Lowry base.
    • Identify conjugate acid-base pairs in an acid-base reaction.

    There are three major classifications of substances known as acids or bases. The theory developed by Svante Arrhenius in 1883, the Arrhenius definition, states that an acid produces H+ in solution and a base produces OH-. Later, two more sophisticated and general theories were proposed. These theories are the Brønsted-Lowry and Lewis definitions of acids and bases. This section will cover the Arrhenius and Brønsted-Lowry theories; the Lewis theory is discussed elsewhere.

    The Arrhenius Theory of Acids and Bases

    In 1884, the Swedish chemist Svante Arrhenius proposed two specific classifications of compounds, termed acids and bases. When dissolved in an aqueous solution, certain ions were released into the solution. An Arrhenius acid is a compound that increases the concentration of \(\ce{H^{+}}\) ions that are present when added to water. These H+ ions form the hydronium ion (H3O+) when they combine with water molecules. This process is represented in a chemical equation by adding H2O to the reactants side.

    \[ \ce{HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq) }\]

    In this reaction, hydrochloric acid (\(HCl\)) dissociates completely into hydrogen (H+) and chlorine (Cl-) ions when dissolved in water, thereby releasing H+ ions into solution. Formation of the hydronium ion equation:

    \[\ce{ HCl(aq) + H_2O(l) \rightarrow H_3O^{+}(aq) + Cl^{-}(aq)} \]

    An Arrhenius base is a compound that increases the concentration of \(\ce{OH^{-}}\) ions that are present when added to water. The dissociation is represented by the following equation:

    \[\ce{ NaOH \; (aq) \rightarrow Na^{+} \; (aq) + OH^{-} \; (aq) }\]

    In this reaction, sodium hydroxide (NaOH) disassociates into sodium (\(Na^+\)) and hydroxide (\(OH^-\)) ions when dissolved in water, thereby releasing OH- ions into solution.

    Arrhenius acids are substances which produce hydrogen ions in solution and Arrhenius bases are substances which produce hydroxide ions in solution.

    Limitations to the Arrhenius Theory

    The Arrhenius theory has many more limitations than other, broader theories. The theory does not explain the weak base ammonia (NH3), which in the presence of water, releases hydroxide ions into solution, but does not contain OH- itself. The Arrhenius definition of acid and base is also limited to aqueous (i.e., water) solutions. However, this theory will be enough for now.

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