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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

Credit: Courtesy of NASA/JPL-Caltech
Source: http://commons.wikimedia.org/wiki/File:PIA00072.jpg

#### Don’t breathe too deeply.

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### 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.

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

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.

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

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:

 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).

### Summary

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

### Review

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

### Notes/Highlights Having trouble? Report an issue.

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