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Acids are generally defined as those chemical substances which release hydrogen ions on dissolving in water. Although acids are most often thought of as proton (H+) or hydronium ion ([H3O]+) donors, as defined by the Brønstad-Lowry theory, the Lewis acid-base theory is more comprehensive and defines acids as electron pair acceptors. As such, acids are the opposite of bases in chemistry. Acids dissolved in otherwise pure water have a pH less than 7 at 25°C. Compounds, substances, or materials which overall have the nature of an acid can be called acidic; for example, aqueous solutions (solutions of water) having a pH less than 7 can be considered acidic. The degree to which a substance or material is acidic is called acidity; for aqueous solutions, acidity is quantified on the pH scale.

Acids play an integral role in the metabolic functions of animals, in chemistry, material science, and the food industry. Acids, such as the twenty common amino acids and carboxylic acids (including valeric acid, and propionic acid), are necessary for life.

A distinction is made between strong and weak acids. Strong acids, e.g. hydrochloric acid (HCl), perchloric acid (HClO4), are completely dissociated in a solution. This means for solutions of equivalent concentration, there will be a higher concentration of H+ ions for a strong acid than a weak acid. Weak acids, e.g. ethanoic acid or acetic acid (CH3COOH), do not dissociate completely but release only a small fraction of their hydrogens as H+ ions. There are also intermediate-strength acids.

Arrhenius acid

According to a theory proposed by Svante Arrhenius, an acid is defined as a species that provides or releases a proton (H+ ion). For an acid, this is very similar to the Brønstad-Lowry definition. Conversely according to the Arrhenius theory, a base is defined as a species that provides or releases a hydroxide ion (OH-).

Brønstad-Lowry acids

According to the theory proposed by Brønstad and Lowry, an acid is defined as a species which donates a proton (H+ ion) or a hydronium ([H3O]+) ion. For example, hydrochloric acid (HCl) donates a H+ ion. Therefore it is an acid. Conversely, a base is defined as a species which accepts a proton (H+ ion). Sodium hydroxide (NaOH), by contrast, donates a OH- ion, and so, it is considered a base, not an acid. Not all hydrogen atoms in a molecule will dissociate (leave the molecule) as H+ or H3O+ ions, so those hydrogen atoms are not considered acidic. For example, hydrogens on the -CH3 group of acetic acid practically will not come off as H+ ions, and therefore only the hydrogen on the -COOH group is considered acidic.
There is a drawback in this theory: acids which do not release protons or hydronium ions are not considered acids. For example, in the following reaction, boron trifluoride does not donate a proton or hydronium ion, so it is not a Brønstad-Lowry acid.

Yet by nature BF3 is an acid.
Consequently, this definition of acids has been discarded in favor of Lewis' definition.

Lewis acids

According to the Lewis definition, an acid is a compound that accepts an electron pair. Since the compound BF3 can accept or combine with the electron pair of a compound like NH3, BF3 is considered an acid by the Lewis definition. The Lewis definition encompasses the Arrhenius and Brønstad-Lowry definitions of an acid. A base, according to the Lewis definition, is a compound that donates an electron pair. NH3 would be considered the base in the preceding example.

For a discussion of Lewis acids and Lewis bases, see Lewis acid-base theory.