7.2: Acids and Bases

Learning Objectives

• Identify an Arrhenius acid and an Arrhenius base.
• Identify a Brønsted-Lowry acid and a Brønsted-Lowry base.
• Write chemical reactions between an Arrhenius acid and an Arrhenius base and between Brønsted-Lowry acid and a Brønsted-Lowry base.

There are three major classifications of substances known as acids or bases. The Arrhenius definition states that an acid produces H⁺ in solution and a base produces OH^-. This theory was developed by Svante Arrhenius in 1883. Later, two more sophisticated and general theories were proposed. These are the Brønsted-Lowry and the Lewis definitions of acids and bases. The Lewis theory is discussed elsewhere.

Figure \(\PageIndex{1}\) Acid base reaction theories as superset and subset models.

The Arrhenius Theory of Acids and Bases

In 1884, the Swedish chemist Svante Arrhenius proposed two specific classifications of compounds, termed acids and bases. When dissolved in an aqueous solution, certain ions were released into the solution. An Arrhenius acid is a compound that increases the concentration of \(\ce{H^{+}}\) ions that are present when added to water. These H⁺ ions form the hydronium ion (H₃O⁺) when they combine with water molecules. This process is represented in a chemical equation by adding H₂O to the reactants side.

\[ \ce{HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq) } \nonumber \]

In this reaction, hydrochloric acid (\(HCl\)) dissociates completely into hydrogen (H⁺) and chlorine (Cl^-) ions when dissolved in water, thereby releasing H⁺ ions into solution. Formation of the hydronium ion equation:

\[\ce{ HCl(aq) + H_2O(l) \rightarrow H_3O^{+}(aq) + Cl^{-}(aq)} \nonumber \]

An Arrhenius base is a compound that increases the concentration of \(\ce{OH^{-}}\) ions that are present when added to water. The dissociation is represented by the following equation:

\[\ce{ NaOH \; (aq) \rightarrow Na^{+} \; (aq) + OH^{-} \; (aq) } \nonumber \]

In this reaction, sodium hydroxide (NaOH) disassociates into sodium (\(Na^+\)) and hydroxide (\(OH^-\)) ions when dissolved in water, thereby releasing OH^- ions into solution.

Arrhenius acids are substances which produce hydrogen ions in solution and Arrhenius bases are substances which produce hydroxide ions in solution.

Limitations to the Arrhenius Theory

The Arrhenius theory has many more limitations than the other two theories. The theory does not explain the weak base ammonia (NH₃), which in the presence of water, releases hydroxide ions into solution, but does not contain OH- itself. Also, the Arrhenius definition of acid and base is limited to aqueous (i.e., water) solutions.

The Brønsted-Lowry Theory of Acids and Bases

In 1923, Danish chemist Johannes Brønsted and English chemist Thomas Lowry independently proposed new definitions for acids and bases, ones that focus on proton transfer. A Brønsted-Lowry acid is any species that can donate a proton (H⁺) to another molecule. A Brønsted-Lowry base is any species that can accept a proton from another molecule. In short, a Brønsted-Lowry acid is a proton donor (PD), while a Brønsted-Lowry base is a proton acceptor (PA).

A Brønsted-Lowry acid is a proton donor, while a Brønsted-Lowry base is a proton acceptor.

Let us use the reaction of ammonia in water to demonstrate the Brønsted-Lowry definitions of an acid and a base. Ammonia and water molecules are reactants, while the ammonium ion and the hydroxide ion are products:

\[\ce{NH3(aq) + H2O (ℓ) <=> NH^{+}4(aq) + OH^{−}(aq) }\label{Eq1} \]

What has happened in this reaction is that the original water molecule has donated a hydrogen ion to the original ammonia molecule, which in turn has accepted the hydrogen ion. We can illustrate this as follows (Figure \(\PageIndex{2}\)):

Figure \(\PageIndex{2}\) reaction of ammonia and water.

Because the water molecule donates a hydrogen ion to the ammonia, it is the Brønsted-Lowry acid, while the ammonia molecule—which accepts the hydrogen ion—is the Brønsted-Lowry base. Thus, ammonia acts as a base in both the Arrhenius sense and the Brønsted-Lowry sense.

Is an Arrhenius acid like hydrochloric acid still an acid in the Brønsted-Lowry sense? Yes, but it requires us to understand what really happens when HCl is dissolved in water. Recall that the hydrogen atom is a single proton surrounded by a single electron. To make the hydrogen ion, we remove the electron, leaving a bare proton. Do we really have bare protons floating around in aqueous solution? No, we do not. What really happens is that the H⁺ ion attaches itself to H₂O to make H₃O⁺, which is called the hydronium ion. For most purposes, H⁺ and H₃O⁺ represent the same species, but writing H₃O⁺ instead of H⁺ shows that we understand that there are no bare protons floating around in solution. Rather, these protons are actually attached to solvent molecules.

The Hydronium Ion

A proton in aqueous solution may be surrounded by more than one water molecule, leading to formulas like \(\ce{H5O2^{+}}\) or \(\ce{H9O4^{+}}\) rather than \(\ce{H3O^{+}}\). It is simpler, however, to use \(\ce{H3O^{+}}\) to represent the hydronium ion.

Figure \(\PageIndex{3}\) reaction of hydrogen and water to form hydronium. (CC BY-NC 3.0 

With this in mind, how do we define HCl as an acid in the Brønsted-Lowry sense? Consider what happens when HCl is dissolved in H₂O:

\[\ce{HCl(g) + H_2O (ℓ) \rightarrow H_3O^{+}(aq) + Cl^{−}(aq) }\label{Eq2} \]

We can depict this process using Lewis electron dot diagrams (Figure \(\PageIndex{4}\)):

Figure \(\PageIndex{4}\) Lewis electron dot diagram of HCl dissolved in H₂O

Now we see that a hydrogen ion is transferred from the HCl molecule to the H₂O molecule to make chloride ions and hydronium ions. As the hydrogen ion donor, HCl acts as a Brønsted-Lowry acid; as a hydrogen ion acceptor, H₂O is a Brønsted-Lowry base. So HCl is an acid not just in the Arrhenius sense but also in the Brønsted-Lowry sense. Moreover, by the Brønsted-Lowry definitions, H₂O is a base in the formation of aqueous HCl. So the Brønsted-Lowry definitions of an acid and a base classify the dissolving of HCl in water as a reaction between an acid and a base—although the Arrhenius definition would not have labeled H₂O a base in this circumstance.

A summary of the acid-base definitions based on the Arrhenius and Bronstes-Lowry theories is given in Table \(\PageIndex{1}\).

Table \(\PageIndex{1}\) Acid-Base Definitions

Type	Acid	Base
Arrhenius	\(\ce{H^+}\) ions in solution	\(\ce{OH^-}\) ions in solution
Brønsted-Lowry	\(\ce{H^+}\) donor	\(\ce{H^+}\) acceptor

All Arrhenius acids and bases are Brønsted-Lowry acids and bases as well. However, not all Brønsted-Lowry acids and bases are Arrhenius acids and bases.

Example \(\PageIndex{1}\)

Aniline (C₆H₅NH₂) is slightly soluble in water. It has a nitrogen atom that can accept a hydrogen ion from a water molecule just like the nitrogen atom in ammonia does. Write the chemical equation for this reaction and identify the Brønsted-Lowry acid and base.

Solution

C₆H₅NH₂ and H₂O are the reactants. When C₆H₅NH₂ accepts a proton from H₂O, it gains an extra H and a positive charge and leaves an OH⁻ ion behind. The reaction is as follows:

\[\ce{C6H5NH2(aq) + H2O(ℓ) <=> C6H5NH3^{+}(aq) + OH^{−}(aq)} \nonumber \]

Because C₆H₅NH₂ accepts a proton, it is the Brønsted-Lowry base. The H₂O molecule, because it donates a proton, is the Brønsted-Lowry acid.

Exercise \(\PageIndex{1}\)

Identify the Brønsted-Lowry acid and the Brønsted-Lowry base in this chemical equation.

\[\ce{H2PO4^{-} + H_2O <=> HPO4^{2-} + H3O^{+}} \nonumber \]

Answer

Brønsted-Lowry acid: H₂PO₄^-; Brønsted-Lowry base: H₂O

Exercise \(\PageIndex{2}\)

Which of the following compounds is a Bronsted-Lowry base?

1. HCl
2. HPO₄^2-
3. H₃PO₄
4. NH₄⁺
5. CH₃NH₃⁺

Answer:

A Brønsted-Lowry Base is a proton acceptor, which means it will take in an H⁺. This eliminates \(\ce{HCl}\), \(\ce{H3PO4}\), \(\ce{NH4^{+}}\) and \(\ce{CH_3NH_3^{+}}\) because they are Bronsted-Lowry acids. They all give away protons. In the case of \(\ce{HPO4^{2-}}\), consider the following equation:

\[\ce{HPO4^{2-} (aq) + H2O (l) \rightarrow PO4^{3-} (aq) + H3O^{+}(aq) } \nonumber \]

Here, it is clear that HPO₄^2- is the acid since it donates a proton to water to make H₃O⁺ and PO₄^3-. Now consider the following equation:

\[ \ce{ HPO4^{2-}(aq) + H2O(l) \rightarrow H2PO4^{-} + OH^{-}(aq)} \nonumber \]

In this case, HPO₄^2- is the base since it accepts a proton from water to form H₂PO₄^- and OH^-. Thus, HPO₄^2- is an acid and base together, making it amphoteric.

Since HPO₄^2- is the only compound from the options that can act as a base, the answer is (b) HPO₄^2-.

Summary

• An Arrhenius acid is a compound that increases the H⁺ ion concentration and an Arrhenius base is a compound that increases the OH⁻ ion concentration in aqueous solution.
• A Brønsted-Lowry acid is a proton donor; a Brønsted-Lowry base is a proton acceptor.
• All Arrhenius acids and bases are Brønsted-Lowry acids and bases as well. However, not all Brønsted-Lowry acids and bases are Arrhenius acids and bases.

Contributors and Attributions

• Libretext: The Basics of GOB Chemistry (Ball et al.)

• Marisa Alviar-Agnew (Sacramento City College)

• Libretext: Chemistry for Allied Health (Soult)