Arrhenius acids and bases are the first types of acids and bases most students learn about in chemistry class. Partly this is because Arrhenius acid-base theory is the first modern explanation of acids and bases based on molecules and ions. Svante Arrhenius’s hydrogen theory of acids in bases in 1884 earned him the Nobel Prize in Chemistry in 1903. The other reason people learn about Arrhenius acids and bases is because the theory offers the simplest explanation and is a good starting point for understanding Brønsted–Lowry acids and bases and Lewis acids and bases.
- Svante Arrhenius proposed the first modern definition of acids and bases.
- An Arrhenius acid dissociates in water to form hydrogen ions or increase the H+ concentration in aqueous solution.
- An Arrhenius base dissociates in water to form hydroxide ions or increase the OH– concentration in aqueous solution.
- A neutralization reaction occurs when an Arrhenius acid and base react to form water and a salt.
Arrhenius Acid Definition
An Arrhenius acid is a chemical species that increases the concentration of the hydrogen ion (H+) in aqueous solution. The general form of the chemical reaction for Arrhenius acid dissociation is:
HA(aq) → H+(aq) + A–(aq)
For example, hydrochloric acid is an Arrhenius acid that dissociates in water to form the hydrogen ion and the chloride ion:
HCl(aq) → H+(aq) + Cl–(aq)
Hydrogen Ions or Hydronium Ions
The original Arrhenius definition of an acid concerned hydrogen ion concentration, but in reality, free hydrogen ions attach themselves to water molecules and form the hydronium ion, H3O+.
H+(aq) + H2O(l) → H3O+(aq)
So, a more accurate equation for the dissociation of hydrochloric acid is:
HCl(aq) + H2O(l) → H3O+(aq) + Cl−(aq)
It does not really matter whether you define Arrhenius acids according to hydrogen ions or hydronium ions.
Examples of Arrhenius Acids
Arrhenius acids contain one or more hydrogen atoms in their chemical formulas. But, not every molecule containing hydrogen is an acid. For example, methane (CH4) is not an Arrhenius acid because it is a nonpolar molecule containing only slightly polar covalent bonds. For a species to be an acid, the molecule must be polar and the bond between hydrogen and another atom must be polar.
| Name | Formula |
|---|---|
| acetic acid | CH3COOH |
| chloric acid | HClO3 |
| hydrochloric acid | HCl |
| hydrobromic acid | HBr |
| hydroiodic acid | HI |
| hydrofluoric acid | HF |
| nitric acid | HNO3 |
| oxalic acid | H2C2O4 |
| perchloric acid | HClO4 |
| phosphoric acid | H3PO4 |
| sulfuric acid | H2SO4 |
| sulfurous acid | H2SO3 |
Arrhenius Base Definition
An Arrhenius base is a chemical species that increases the concentration of the hydroxide ion (OH–) in aqueous solution. The general form of the chemical equation for Arrhenius base dissociation is:
BOH(aq) → B+(aq) + OH–(aq)
For example, sodium hydoxide (NaOH) dissociates in water and forms the sodium ion and hydroxide ion:
NaOH(aq) → Na+(aq) + OH–(aq)
Are All Arrhenius Bases Hydroxides?
You may wonder whether it’s necessary for a substance to be a hydroxide to be an Arrhenius base. The answer is that it depends who you’re asking.
Some textbooks and instructors narrowly define an Arrhenius base as species that increases OH– concentration in aqueous solution and has at least one “OH” in its chemical formula.
| Name | Formula |
|---|---|
| lithium hydroxide | LiOH |
| sodium hydroxide | NaOH |
| potassium hydroxide | KOH |
| rubidium hydroxide | RbOH |
| cesium hydroxide | CsOH |
| *calcium hydroxide | Ca(OH)2 |
| *strontium hydroxide | Sr(OH)2 |
| *barium hydroxide | Ba(OH)2 |
| *only dissociate at concentrations of 0.01M or less |
However, other chemists define an Arrhenius base simply as any species that increases hydroxide ion concentration. Under this definition, methylamine is an Arrhenius base because it forms hydroxide ions, even though its chemical formula does not contain them.
CH3NH2(aq) + H2O(l) ⇌ CH3NH3+(aq) + OH−(aq)
Arrhenius Acid-Base Reaction (Neutralization)
An Arrhenius acid and an Arrhenius base react usually reaction with each other in a neutralization reaction that forms water and a salt. The hydrogen ion from the acid and hydroxide ion from the base combine to form water, while the cation from the dissociation of the base and the anion from the dissociation of the acid combine to form a salt.
acid + base → water + salt
Consider, for example, the reaction between hydrofluoric acid (an Arrhenius acid) and lithium hydroxide (an Arrhenius base).
HF(aq) ⇌ H+(aq) + F−(aq)
LiOH(aq) → Li+(aq) + OH−(aq)
The overall reaction is:
HF(aq) + LiOH(aq) → H2O(l) + LiF(aq)
Limitations of Arrhenius Acid-Base Theory
The Arrhenius definitions of acids and bases describe the behavior of most common acids and bases, but the definitions don’t apply when the solvent is anything besides water or when chemical reactions occur between gases. Although the Arrhenius theory has its uses, most chemists use the Brønsted-Lowry theory of acids and bases because it takes a more generalized approach to the concept.
References
- Finston, H.L.; Rychtman, A.C. (1983). A New View of Current Acid-Base Theories. New York: John Wiley & Sons. doi:10.1002/ciuz.19830170211
- Meyers, R. (2003). The Basics of Chemistry. Greenwood Press. ISBN 978-0313316647.
- Miessler G.L.; Tarr D.A. (1999). Inorganic Chemistry (2nd ed.). Prentice-Hall. ISBN 0-13-841891-8.
- Murray, Kermit K.; et al. (June 2013) [2006]. “Standard definition of terms relating to mass spectrometry recommendations”. Pure and Applied Chemistry. 85 (7): 1515–1609. doi:10.1351/PAC-REC-06-04-06