## The Arrhenius Definition

#### Learning Objective

• Recall the Arrhenius acid definition and its limitations.

#### Key Points

• An Arrhenius acid increases the concentration of hydrogen (H+) ions in an aqueous solution, while an Arrhenius base increases the concentration of hydroxide (OH) ions in an aqueous solution.
• The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions and refer to the concentration of the solvent ions.
• The universal aqueous acid–base definition of the Arrhenius concept is described as the formation of a water molecule from a proton and hydroxide ion. Therefore, in Arrhenius acid–base reactions, the reaction between an acid and a base is a neutralization reaction.

#### Terms

• hydroniumThe hydrated hydrogen ion ( $H_3O^+$ ).
• aciditya measure of the overall concentration of hydrogen ions in solution
• alkalinitya measure of the overall concentration of hydroxide ions in solution

## The Arrhenius Definition

An acid-base reaction is a chemical reaction that occurs between an acid and a base. Several concepts exist that provide alternative definitions for the reaction mechanisms involved and their application in solving related problems. Despite several differences in definitions, their importance as different methods of analysis becomes apparent when they are applied to acid-base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent.

The Arrhenius definition of acid-base reactions, which was devised by Svante Arrhenius, is a development of the hydrogen theory of acids. It was used to provide a modern definition of acids and bases, and followed from Arrhenius’s work with Friedrich Wilhelm Ostwald in establishing the presence of ions in aqueous solution in 1884. This led to Arrhenius receiving the Nobel Prize in Chemistry in 1903.

As defined by Arrhenius:

• An Arrhenius acid is a substance that dissociates in water to form hydrogen ions (H+). In other words, an acid increases the concentration of H+ ions in an aqueous solution. This protonation of water yields the hydronium ion (H3O+); in modern times, H+ is used as a shorthand for H3O+ because it is now known that a bare proton (H+) does not exist as a free species in aqueous solution.
• An Arrhenius base is a substance that dissociates in water to form hydroxide (OH) ions. In other words, a base increases the concentration of OH ions in an aqueous solution.

## Limitations of the Arrhenius Definition

The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions and refer to the concentration of the solvated ions. Under this definition, pure H2SO4 or HCl dissolved in toluene are not acidic, despite the fact that both of these acids will donate a proton to toluene. In addition, under the Arrhenius definition, a solution of sodium amide (NaNH2) in liquid ammonia is not alkaline, despite the fact that the amide ion ($\text{NH}_2^-$) will readily deprotonate ammonia. Thus, the Arrhenius definition can only describe acids and bases in an aqueous environment.

## Arrhenius Acid-Base Reaction

An Arrhenius acid-base reaction is defined as the reaction of a proton and an hydroxide ion to form water:

$H^++OH^-\rightarrow H_2O$

Thus, an Arrhenius acid base reaction is simply a neutralization reaction.