Arrhenius Acids

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 (H ₃ O ⁺ ) can be thought of as a water molecule with an attached hydrogen ion.

Figure 1. Formation of a hydronium ion.

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

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 (CH ₄ ), 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 (CH ₃ COOH) 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.

The Table below lists some of the more common acids:

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 H ₂ SO ₄ ) or triprotic (such as H ₃ PO ₄ ).

Arrhenius Bases

Arrhenius Bases

An Arrhenius base is a compound, which ionizes to yield hydroxide ions (OH⁻ ) in aqueous solution. The Table below lists several of the more common bases:

All of the bases listed in the table are solids at room temperature. Upon dissolving in water, each dissociates into a metal cation and the hydroxide ion.

Figure 2. This foot has severe burns due to prolonged contact with a solution of sodium hydroxide, also known as lye.

Sodium hydroxide is a very caustic substance also known as lye. Lye is used as a rigorous cleaner and is an ingredient in the manufacture of soaps. Care must be taken with strong bases like sodium hydroxide, as exposure can lead to severe burns (see Figure 2 ).

Sodium belongs to the group of elements called the alkali metals. An alkaline solution is another name for a solution that is basic. All alkali metals react readily with water to produce the metal hydroxide and hydrogen gas. The resulting solutions are basic.

Bases that consist of an alkali metal cation and the hydroxide anion are all very soluble in water. Compounds of the Group 2 metals (the alkaline earth metals) are also basic. However, these compounds are generally not as soluble in water. Therefore the dissociation reactions for these compounds are shown as equilibrium reactions.

These relatively insoluble hydroxides were some of the compounds discussed in the context of the solubility product constant . The solubility of magnesium hydroxide is 0.0084 g per liter of water at 25°C. Because of its low solubility, magnesium hydroxide is not as dangerous as sodium hydroxide. In fact, magnesium hydroxide is the active ingredient in a product called milk of magnesia, which is used as an antacid or a mild laxative.

Glossary