8.2: Acid/Base Classifications: Arrhenius Acids and Bases

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Learning Objectives

Define Arrhenius acid.
Define Arrhenius base.
Explain why a hydrogen ion, H⁺¹, is known as "a proton."

The terms "acid" and "base" were first utilized in 1884 by a Swedish chemist named Svante Arrhenius, who studied the ionizations of strong and weak electrolytes in aqueous solutions, which are defined as homogeneous mixtures that are prepared by dissolving one or more solutes in water. Recall that the chemical that is referenced after the indicator word "in" in the description of a solution is classified as the solvent in that solution. Therefore, by definition, water is the solvent that must be used to prepare an aqueous mixture. Furthermore, while the solvent and solutes that are present in a solution do not react with one another, a solute that is classified as either a strong or a weak electrolyte will dissociate, or separate, into its constituent cations and anions during the solvation process.

During his extensive investigations of water-soluble electrolytes, Arrhenius observed that hydrogen ions, H⁺¹, or hydroxide ions, OH^–1, were generated upon the hydration and subsequent dissociation of an unexpectedly-high number of electrolytes. Due to the frequency with which these charged particles were detected in the aqueous solutions that he prepared, Arrhenius regarded the production of H⁺¹ and OH^–1 ions as a chemically-significant phenomenon and classified the electrolytes from which these particles were generated as "acids" and "bases," respectively. Formally, an Arrhenius acid is defined as a proton, H⁺¹, donor in water, and an Arrhenius base is defined as a hydroxide ion, OH^–1, donor in water.

In the given definition of an Arrhenius acid, the ion that is symbolized as "H⁺¹" is identified as a proton, due to the chemical composition of this charged particle. Hydrogen, H, has an atomic number of 1 and, therefore, contains one proton. In order to "cancel out" and yield a net neutral overall charge, the number of positively-charged protons and negatively-charged electrons in an atom must be equal. Therefore, a hydrogen atom also contains one electron. The number of neutrons that are present in an atom can be calculated by subtracting the number of protons in that atom from the mass number of the isotope that is being considered. The atomic mass average of hydrogen, 1.0079, indicates that the most common isotope of this element has a mass number of 1 and, therefore, does not contain any neutrons. However, the hydrogen ion, H⁺¹, is a cation, which is a positively-charged particle that is generated after a neutral atom loses valence electrons. While the non-metals that are found on the right side of the periodic table gain electrons and, consequently, ionize to form anions, hydrogen is unique, in that it is the only non-metal that is located on the left side of the periodic table. Consequently, a hydrogen atom loses its only electron and, as a result, ionizes to form a cation that bears a +1 charge, as is characteristic of all of the elements that are present in Group 1A on the periodic table. As stated above, a neutral hydrogen atom is comprised of one proton and one electron. Therefore, since the only subatomic particle that remains after the ionization of this element is a single proton, the resultant H⁺¹ ion can be identified as "a proton."