Arrhenius acids and bases

Definition of Arrhenius acids and bases, and Arrhenius acid-base reactions .

Key points

• An Arrhenius acid is any species that increases the concentration of $\text{H}^+$ in aqueous solution.
• An Arrhenius base is any species that increases the concentration of $\text{OH}^-$ in aqueous solution.
• In aqueous solution, $\text{H}^+$ ions immediately react with water molecules to form hydronium ions, $\text{H}_3\text{O}^+$.
• In an acid-base or neutralization reaction, an Arrhenius acid and base usually react to form water and a salt.

Introduction

From the vinegar in your kitchen cabinet to the soap in your shower, acids and bases are everywhere! But what does it mean to say that something is acidic or basic? In order to answer this question, we need to examine some of the theories describing acids and bases. In this article, we will focus on the Arrhenius theory.

Arrhenius acids

The Arrhenius theory of acids and bases was originally proposed by the Swedish chemist Svante Arrhenius in 1884. He suggested classifying certain compounds as acids or bases based on what kind of ions formed when the compound was added to water.
Photograph of two Ruby Red grapefruits, one whole and one cut into three pieces.
Citrus fruits—such as grapefruit—contain high amounts of citric acid, a common organic acid. Image credit: Wikimedia Commons, CC BY-SA 2.5
An Arrhenius acid is any species that increases the concentration of $\greenD{\text{H}^+}$ ions—or protons—in aqueous solution. For example, let's consider the dissociation reaction for hydrochloric acid, $\text{HCl}$, in water:
$\greenD{\text{H}}\text {Cl}(aq)\rightarrow \greenD{\text{H}^+}(aq)+\text{Cl}^-(aq)$
When we make an aqueous solution of hydrochloric acid, $\greenD{\text{H}}\text{Cl}$ dissociates into $\greenD{\text{H}^+}$ ions and $\text{Cl}^-$ ions. Since this results in an increase in the concentration of $\greenD{\text{H}^+}$ ions in solution, hydrochloric acid is an Arrhenius acid.

Hydrogen or hydronium ions?

Let's say we made a 2 M aqueous solution of hydrobromic acid, $\text{HBr}$, which is an Arrhenius acid. Does that mean we have 2 M of $\text H^+$ ions in our solution?
Actually, no. In practice, the positively charged protons react with the surrounding water molecules to form hydronium ions, $\text{H}_3\text{O}^+$. This reaction can be written as follows:
$\text{H}^+(aq)+\text{H}_2\text{O}(l)\rightarrow\text{H}_3\text{O}^+(aq)$
Even though we often write acid dissociation reactions showing the formation of $\text H^+(aq)$, there are no free $\text{H}^+$ ions floating around in an aqueous solution. Instead, there are primarily $\text{H}_3\text{O}^+$ ions, which form immediately when an acid dissociates in water. The following picture illustrates the formation of hydronium from water and hydrogen ions using molecular models:
Picture of a proton, represented by a dot, reacting with a water molecule to form hydronium.
When an acid dissociates in water to form $\text{H}^+$ ions, protons, the $\text{H}^+$ ions immediately react with water to form $\text{H}_3\text{O}^+$. Thus, chemists talk about the concentrations of hydrogen ions and hydronium ions interchangeably. Image credit: UC Davis Chemwiki, CC BY-NC-SA 3.0 US
In practice, most chemists talk about the concentration of $\text{H}^+$ and the concentration of $\text{H}_3\text{O}^+$ interchangeably. When we want to be more accurate—and less lazy!—we can write the dissociation of hydrobromic acid to explicitly show the formation of hydronium instead of protons:
\begin{aligned}\text{HBr}(aq)+\text{H}_2\text{O}(l) &\rightarrow\text{H}_3\text{O}^+(aq)+\text{Br}^-(aq)~~~~~~~~{\text{More accurate}}\\ \\ &\text{vs.} \\ \\ \text{HBr}(aq) &\rightarrow\text{H}^+(aq)+\text{Br}^-(aq)~~~~\text{Shorter and easier to write!}\end{aligned}
In general, either description is acceptable for showing the dissociation of an Arrhenius acid.

Arrhenius bases

An Arrhenius base is defined as any species that increases the concentration of hydroxide ions, $\redD{\text{OH}^-}$, in aqueous solution. An example of an Arrhenius base is the highly soluble sodium hydroxide, $\text{NaOH}$. Sodium hydroxide dissociates in water as follows:
$\text{Na} \redD{\text{OH}}(aq)\rightarrow\text{Na}^+(aq)+\redD{\text{OH}^-}(aq)$
In water, sodium hydroxide fully dissociates to form $\redD{\text{OH}^-}$ and $\text{Na}^+$ ions, resulting in an increase in the concentration of hydroxide ions. Therefore, $\text{NaOH}$ is an Arrhenius base. Common Arrhenius bases include other Group 1 and Group 2 hydroxides such as $\text{LiOH}$ and $\text{Ba(OH)}_2$.
Beaker with water molecules, sodium cations, and hydroxide anions.
An aqueous solution of sodium hydroxide, an Arrhenius base, contains dissociated sodium and hydroxide ions.
Even insoluble compounds such as $\text{Ca(OH)}_2$ are often classified as Arrhenius bases because they have some tiny fraction that can dissolve in water, and the fraction that is in solution dissociates to form $\text{OH}^-$ ions.
Another way to think about this is that complete solubility is not a requirement for something to be an Arrhenius base! As long as some of it gets into solution and increases the concentration of $\text{OH}^-$ ions, we can classify it as an Arrhenius base.
Note that depending on your class—or textbook or teacher—non-hydroxide-containing bases may or may not be classified as Arrhenius bases. Some textbooks define an Arrhenius base more narrowly: a substance that increases the concentration of $\text{OH}^-$ in aqueous solution and also contains at least one unit of $\text{OH}^-$ in the chemical formula. While that doesn't change the classification of the Group 1 and 2 hydroxides, it can get a little confusing with compounds such as methylamine, $\text {CH}_3 \text {NH}_2$.
When methylamine is added to water, the following reaction occurs:
$\text {CH}_3 \text {NH}_2(aq)+\text H_2 \text O(l) \leftrightharpoons \text {CH}_3 \text {NH}_3^+(aq)+\redD{\text {OH}^-}(aq)$
Based on our first definition, methylamine would be an Arrhenius base since the $\text {OH}^-$ ion concentration increases in the solution. By the second definition, however, it would not count as an Arrhenius base since the chemical formula does not include hydroxide.

Acid-base reactions: Arrhenius acid + Arrhenius base = water + salt

When an Arrhenius acid reacts with an Arrhenius base, the products are usually water plus a salt. These reactions are also sometimes called neutralization reactions. For example, what happens when we combine aqueous solutions of hydrofluoric acid, $\text{HF}$, and lithium hydroxide, $\text{LiOH}$?
If we think about the acid solution and base solution separately, we know the following:
• An Arrhenius acid increases the concentration of $\greenD{\text H^+}(aq)$:
$\greenD{\text{H}}\text{F}(aq) \leftrightharpoons \greenD{\text{H}^+}(aq)+\text{F}^-(aq)$
• An Arrhenius base increases the concentration of $\redD{\text{OH}^-}(aq)$:
$\text{Li}\redD{\text{OH}}(aq) \rightarrow \text{Li}^+(aq)+\redD{\text{OH}^-}(aq)$
When the acid and base combine in solution, $\text H_2 \text O$ is produced from the reaction between hydrogen ions and hydroxide ions, while the other ions form the salt $\text{LiF}(aq)$:
$\greenD{\text H^+}(aq)+\redD{\text{OH}^-}(aq) \rightarrow \text{H}_2 \text O(l)~~~~~~~~~~~~~\text{Formation of water}$
$\text{Li}^+(aq)+\text{F}^-(aq) \rightarrow\text{LiF}(aq)~~~~~~~~~~~~~~~~\text{Formation of salt}$
If we add the reactions for the formation of water and the formation of salt, we get our overall neutralization reaction between hydrofluoric acid and lithium hydroxide:
$\greenD{\text{H}}\text{F}(aq) + \text{Li}\redD{\text{OH}}(aq) \rightarrow \text{H}_2 \text O(l)+\text{LiF}(aq)$

Limitations of the Arrhenius definition

The Arrhenius theory is limited in that it can only describe acid-base chemistry in aqueous solutions. Similar reactions can also occur in non-aqueous solvents, however, as well as between molecules in the gas phase. As a result, modern chemists usually prefer the Brønsted-Lowry theory, which is useful in a broader range of chemical reactions. The Brønsted-Lowry theory of acids and bases will be discussed in a separate article!

Summary

• An Arrhenius acid is any species that increases the concentration of $\text{H}^+$ in aqueous solution.
• An Arrhenius base is any species that increases the concentration of $\text{OH}^-$ in aqueous solution.
• In aqueous solution, $\text{H}^+$ ions immediately react with water molecules to form hydronium ions, $\text{H}_3\text{O}^+$.
• In an acid-base or neutralization reaction, an Arrhenius acid and base usually react to form water and a salt.