An Arrhenius acid dissociates in water to form hydrogen ions, while an
Arrhenius base dissociates in water to form hydroxide ions.
LEARNING OBJECTIVE [ edit ]
Recall the Arrhenius acid definition and its limitations.
KEY POINTS [ edit ]
An Arrhenius acid increases the concentration of hydrogen (H+) ions in an aqueous solution,
while an Arrhenius baseincreases 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 moleculefrom a proton and hydroxide ion. Therefore, in Arrhenius acid–
base reactions, the reaction between an acid and a base is a neutralization reaction.
TERMS [ edit ]
alkalinity
a measure of the overall concentration of hydroxide ions in solution
acidity
a measure of the overall concentration of hydrogen ions in solution
hydronium
+
The hydrated hydrogen ion ( H 3 O ).
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The Arrhenius Definition
An acid­base reaction is a chemical reaction that occurs between an acid and a base. Several
concepts exist that provide alternative definitions for the reaction mechanismsinvolved 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
acid­base reactions for gaseous or liquidspecies, or when acid or base character may be
somewhat less apparent.
The Arrhenius definition of acid­base 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'swork 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 hydroniumion (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
−
(NH 2 ) will readily deprotonate ammonia. Thus, the Arrhenius definition can only describe
acids and bases in an aqueous environment.
Arrhenius Acid­Base Reaction
An Arrhenius acid­base reaction is defined as the reaction of a proton and an hydroxide ion
to form water:
H + + OH − → H 2 O
Thus, an Arrhenius acid base reaction is simply a neutralization reaction.
Chemistry 12.1 What are Acids and Bases? (Part 1 of 2) ­ YouTube
This introduction to acids and bases discusses their general properties and explains the Arrhenius
definitions for acids and bases.