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Arrhenius Acids

Defines acids as compounds that ionize to produce protons and illustrates common uses of bases.

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Arrhenius Acids

Venus has a highly acidic atmosphere

Credit: Courtesy of NASA/JPL-Caltech
Source: http://commons.wikimedia.org/wiki/File:PIA00072.jpg
License: CC BY-NC 3.0

Don’t breathe too deeply.

Venus (pictured above) is the planet nearest to us, but has a very different and hostile environment. It has a surface temperature that averages around 450°C. The atmosphere is composed of carbon dioxide, but clouds of sulfuric acid move through the upper atmosphere, helping to create the extremely unfriendly conditions. Because of these conditions, Venus is not a place you want to visit on vacation.

Arrhenius Acids

Swedish chemist Svante Arrhenius (1859-1927) was the first to propose a theory to explain the observed behavior of acids and bases. Because of their ability to conduct a current, he knew that both acids and bases contained ions in solution. An Arrhenius acid is a compound, which ionizes to yield hydrogen ions (H+) in aqueous solution.

Acids are molecular compounds with ionizable hydrogen atoms. Only hydrogen atoms that are part of a highly polar covalent bond are ionizable. Hydrogen chloride (HCl) is a gas at room temperature and under normal pressure. The H-Cl bond in hydrogen chloride is a polar bond. The hydrogen atom is electron deficient because of the higher electronegativity of the chlorine atom. Consequently, the hydrogen atom is attracted to the lone pair of electrons in a water molecule when HCl is dissolved in water. The result is that the H-Cl bond breaks, with both bonding electrons remaining with the Cl, forming a chloride ion. The H+ ion attaches to the water molecule, forming a polyatomic ion called the hydronium ion. The hydronium ion (H3O+) can be thought of as a water molecule with an attached hydrogen ion.

Hydrochloric acid ionizes to create hydronium ions in water

Credit: Ben Mills (Wikimedia: Benjah-bmm27)
Source: HCl: http://commons.wikimedia.org/wiki/File:Hydrogen-chloride-3D-vdW.png; H2O: http://commons.wikimedia.org/wiki/File:Water-3D-vdW.png; H3O+: http://commons.wikimedia.org/wiki/File:Hydronium-3D-vdW.png; http://commons.wikimedia.org/wiki/File:Chloride-ion-3D-vdW.png
License: CC BY-NC 3.0

Formation of a hydronium ion.[Figure2]

Equations showing the ionization of an acid in water are frequently simplified by omitting the water molecule:

\begin{align*}\text{HCl}(g) \rightarrow \text{H}^+(aq) + \text{Cl}^-\end{align*}

This is merely a simplification of the previous equation, but it is commonly used. Any hydrogen ions in an aqueous solution will be attached to water molecules as hydronium ions.

Not all hydrogen atoms in molecular compounds are ionizable. In methane (CH4), the hydrogen atoms are covalently bonded to carbon in bonds that are only slightly polar. The hydrogen atoms are not capable of ionizing and methane has no acidic properties. Acetic acid (CH3COOH) belongs to a class of acids called organic acids. There are four hydrogen atoms in the molecule, but only the one hydrogen that is bonded to an oxygen atom is ionizable.

Acetic acid generates hydronium ions when dissolved in water

Credit: Ben Mills (Wikimedia: Benjah-bmm27)
Source: http://commons.wikimedia.org/wiki/File:Acetic-acid-2D-flat.png
License: CC BY-NC 3.0

The O-H bond can be ionized to yield the H+ ion and the acetate ion. The other hydrogen atoms in this molecule are not acidic.[Figure3]

Table below lists some of the more common acids:

Common Acids
Acid Name Formula
Hydrochloric acid HCl
Nitric acid HNO3
Sulfuric acid H2SO4
Phosphoric acid H3PO4
Acetic acid CH3COOH
Hypochlorous acid HClO

A monoprotic acid is an acid that contains only one ionizable hydrogen. Hydrochloric acid and acetic acid are monoprotic acids. A polyprotic acid is an acid that contains multiple ionizable hydrogens. Most common polyprotic acids are either diprotic (such as H2SO4) or triprotic (such as H3PO4).


  • Arrhenius acid is defined.
  • Examples of Arrhenius acids are given.


  1. What is an Arrhenius acid?
  2. What is a hydronium ion?
  3. Is H2SO4 a monoprotic or a polyprotic acid?

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Image Attributions

  1. [1]^ Credit: Courtesy of NASA/JPL-Caltech; Source: http://commons.wikimedia.org/wiki/File:PIA00072.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: HCl: http://commons.wikimedia.org/wiki/File:Hydrogen-chloride-3D-vdW.png; H2O: http://commons.wikimedia.org/wiki/File:Water-3D-vdW.png; H3O+: http://commons.wikimedia.org/wiki/File:Hydronium-3D-vdW.png; http://commons.wikimedia.org/wiki/File:Chloride-ion-3D-vdW.png; License: CC BY-NC 3.0
  3. [3]^ Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Acetic-acid-2D-flat.png; License: CC BY-NC 3.0

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