Arrhenius acids and bases (article) | Khan Academy (2023)

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.

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.

An Arrhenius acid is any species that increases the concentration of $\greenD{\text{H}^+}$H+start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54 ions—or protons—in aqueous solution. For example, let's consider the dissociation reaction for hydrochloric acid, $\text{HCl}$HClstart text, H, C, l, end text, in water:

When we make an aqueous solution of hydrochloric acid, $\greenD{\text{H}}\text{Cl}$HClstart color #1fab54, start text, H, end text, end color #1fab54, start text, C, l, end text dissociates into $\greenD{\text{H}^+}$H+start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54 ions and $\text{Cl}^-$Cl−start text, C, l, end text, start superscript, minus, end superscript ions. Since this results in an increase in the concentration of $\greenD{\text{H}^+}$H+start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54 ions in solution, hydrochloric acid is an Arrhenius acid.

Let's say we made a $2\; M$2M2, M aqueous solution of hydrobromic acid, $\text{HBr}$HBrstart text, H, B, r, end text, which is an Arrhenius acid. Does that mean we have $2\; M$2M2, M of $\text H^+$H+start text, H, end text, start superscript, plus, end superscript ions in our solution?

Actually, no. In practice, the positively charged protons react with the surrounding water molecules to form hydronium ions, $\text{H}_3\text{O}^+$H3​O+start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript. This reaction can be written as follows:

Even though we often write acid dissociation reactions showing the formation of $\text H^+(aq)$H+(aq)start text, H, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis, there are no free $\text{H}^+$H+start text, H, end text, start superscript, plus, end superscript ions floating around in an aqueous solution. Instead, there are primarily $\text{H}_3\text{O}^+$H3​O+start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript 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:

In practice, most chemists talk about the concentration of $\text{H}^+$H+start text, H, end text, start superscript, plus, end superscript and the concentration of $\text{H}_3\text{O}^+$H3​O+start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript 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:

In general, either description is acceptable for showing the dissociation of an Arrhenius acid.

An Arrhenius base is defined as any species that increases the concentration of hydroxide ions, $\redD{\text{OH}^-}$OH−start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, in aqueous solution. An example of an Arrhenius base is the highly soluble sodium hydroxide, $\text{NaOH}$NaOHstart text, N, a, O, H, end text. Sodium hydroxide dissociates in water as follows:

In water, sodium hydroxide fully dissociates to form $\redD{\text{OH}^-}$OH−start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39 and $\text{Na}^+$Na+start text, N, a, end text, start superscript, plus, end superscript ions, resulting in an increase in the concentration of hydroxide ions. Therefore, $\text{NaOH}$NaOHstart text, N, a, O, H, end text is an Arrhenius base. Common Arrhenius bases include other Group 1 and Group 2 hydroxides such as $\text{LiOH}$LiOHstart text, L, i, O, H, end text and $\text{Ba(OH)}_2$Ba(OH)2​start text, B, a, left parenthesis, O, H, right parenthesis, end text, start subscript, 2, end subscript.

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 $\text{OH}^-$OH−start text, O, H, end text, start superscript, minus, end superscript in aqueous solution and also contains at least one unit of $\text{OH}^-$OH−start text, O, H, end text, start superscript, minus, end superscript 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, $\text {CH}_3 \text {NH}_2$CH3​NH2​start text, C, H, end text, start subscript, 3, end subscript, start text, N, H, end text, start subscript, 2, end subscript.

When methylamine is added to water, the following reaction occurs:

Based on our first definition, methylamine would be an Arrhenius base since the $\text {OH}^-$OH−start text, O, H, end text, start superscript, minus, end superscript 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.

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, $\text{HF}$HFstart text, H, F, end text, and lithium hydroxide, $\text{LiOH}$LiOHstart text, L, i, O, H, end text?

If we think about the acid solution and base solution separately, we know the following:

• An Arrhenius acid increases the concentration of $\greenD{\text H^+}(aq)$H+(aq)start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54, left parenthesis, a, q, right parenthesis:

• An Arrhenius base increases the concentration of $\redD{\text{OH}^-}(aq)$OH−(aq)start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, left parenthesis, a, q, right parenthesis:

When the acid and base combine in solution, $\text H_2 \text O$H2​Ostart text, H, end text, start subscript, 2, end subscript, start text, O, end text is produced from the reaction between hydrogen ions and hydroxide ions, while the other ions form the salt $\text{LiF}(aq)$LiF(aq)start text, L, i, F, end text, left parenthesis, a, q, right parenthesis:

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:

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!