10.2 Brønsted-Lowry Acids and Bases

Learning Objectives

By the end of this module, you will be able to:

• Identify acids, bases, and conjugate acid-base pairs according to the Brønsted-Lowry definition
• Use the ion-product constant for water to calculate hydronium and hydroxide ion concentrations

Acids and bases have been known for a long time. When Robert Boyle characterized them in 1680, he noted that acids dissolve many substances, change the color of certain natural dyes (for example, they change litmus from blue to red), and lose these characteristic properties after coming into contact with alkalis (bases). In the eighteenth century, it was recognized that acids have a sour taste, react with limestone to liberate a gaseous substance (now known to be CO2), and interact with alkalis to form neutral substances. In 1815, Humphry Davy contributed greatly to the development of the modern acid-base concept by demonstrating that hydrogen is the essential constituent of acids. Around that same time, Joseph Louis Gay-Lussac concluded that acids are substances that can neutralize bases and that these two classes of substances can be defined only in terms of each other. The significance of hydrogen was reemphasized in 1884 when Carl Axel Arrhenius defined an acid as a compound that dissolves in water to yield hydrogen cations (now recognized to be hydronium ions) and a base as a compound that dissolves in water to yield hydroxide anions.

In an earlier module, we defined acids and bases as Arrhenius did: We identified an acid as a compound that dissolves in water to yield hydronium ions (H3O+) and a base as a compound that dissolves in water to yield hydroxide ions (OH). This definition is not wrong; it is simply limited.

We extend on the Arrhenius definition of an acid or a base using the more general definition proposed in 1923 by the Danish chemist Johannes Brønsted and the English chemist Thomas Lowry. Their definition centers on the proton, H+. A proton is what remains when a normal hydrogen atom, ${}_{1}{}^{1}\text{H}$, loses an electron. A compound that donates a proton to another compound is called a Brønsted-Lowry acid, and a compound that accepts a proton is called a Brønsted-Lowry base. An acid-base reaction is the transfer of a proton from a proton donor (acid) to a proton acceptor (base).

Acids may be compounds such as HCl or H2SO4, organic acids like acetic acid (CH3COOH) or ascorbic acid (vitamin C), or H2O. Anions (such as ${\text{HSO}}_{4}^{-}$, ${\text{H}}_{2}{\text{PO}}_{4}^{-}$, HS, and ${\text{HCO}}_{3}^{-}$) may also act as acids. Bases fall into the same two categories. Bases may be neutral molecules (such as H2O, NH3, and CH3NH2), anions (such as OH, HS, ${\text{HCO}}_{3}^{-}$, ${\text{CO}}_{3}^{2-}$, F, and ${\text{PO}}_{4}^{3-}$). The most familiar bases are ionic compounds such as NaOH and Ca(OH)2, which contain the hydroxide ion, OH. The hydroxide ion in these compounds accepts a proton from acids to form water:

$\large{\text{H}}^{\text{+}}+{\text{OH}}^{-}\longrightarrow {\text{H}}_{2}\text{O}$

We call the product that remains after an acid donates a proton the conjugate base of the acid. This species is a base because it can accept a proton (to re-form the acid):

$\large\begin{array}{l}\text{acid}\rightarrow \text{proton}+\text{conjugate base}\\ \text{HF}\rightarrow {\text{H}}^{\text{+}}+{\text{F}}^{-}\\ {\text{H}}_{2}{\text{SO}}_{4}\rightarrow {\text{H}}^{+}+{\text{HSO}}_{4}^{-}\\ {\text{H}}_{2}\text{O}\rightarrow {\text{H}}^{\text{+}}+{\text{OH}}^{-}\\ {\text{HSO}}_{4}^{-}\rightarrow {\text{H}}^{\text{+}}+{\text{SO}}_{4}^{2-}\\ {\text{NH}}_{4}^{+}\rightarrow {\text{H}}^{+}+{\text{NH}}_{3}\end{array}$

We call the product that results when a base accepts a proton the base’s conjugate acid. This species is an acid because it can give up a proton (and thus re-form the base):

$\large\begin{array}{l}\text{base}+\text{proton}\rightarrow \text{conjugate acid}\\ {\text{OH}}^{-}+{\text{H}}^{\text{+}}\rightarrow {\text{H}}_{2}\text{O}\\ {\text{H}}_{2}\text{O}+{\text{H}}^{\text{+}}\rightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}\\ {\text{NH}}_{3}+{\text{H}}^{\text{+}}\rightarrow {\text{NH}}_{4}{}^{\text{+}}\\ {\text{S}}^{2-}+{\text{H}}^{\text{+}}\rightarrow {\text{HS}}^{-}\\ {\text{CO}}_{3}{}^{2-}+{\text{H}}^{\text{+}}\rightarrow {\text{HCO}}_{3}{}^{-}\\ {\text{F}}^{-}+{\text{H}}^{\text{+}}\rightarrow \text{HF}\end{array}$

In these two sets of equations, the behaviors of acids as proton donors and bases as proton acceptors are represented in isolation. In reality, all acid-base reactions involve the transfer of protons between acids and bases. For example, consider the acid-base reaction that takes place when ammonia is dissolved in water. A water molecule (functioning as an acid) transfers a proton to an ammonia molecule (functioning as a base), yielding the conjugate base of water, OH, and the conjugate acid of ammonia, ${\text{NH}}_{4}{}^{\text{+}}:$

Example 1: Bronsted-Lowry Acids and Bases

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

$\large{\text{C}_{6}\text{H}}_{5}\text{OH}\left(l\right)+{\text{NH}_{2}}^{-}\left(aq\right)\longrightarrow {\text{C}_{6}\text{H}}_{5}\text{O}^{-}\left(aq\right)+{\text{NH}}_{3}\left(aq\right)$

Example 2: Conjugate Acids and Conjugate Bases

Identify the conjugate acid-base pairs in this equation:

$\large{(\text{C}\text{H}}_{3})_{3}\text{N}\left(l\right)+{\text{H}_{2}}\text{O}\left(l\right)\longrightarrow {(\text{C}\text{H}}_{3})_{3}\text{NH}^{+}\left(aq\right)+{\text{OH}}^{-}\left(aq\right)$

Identify the conjugate acid-base pairs in this equation:

$\large{\text{NH}}_{2}^{-}\left(aq\right)+{\text{H}_{2}}\text{O}\left(l\right)\longrightarrow {\text{NH}}_{3}\left(aq\right)+{\text{OH}}^{-}\left(aq\right)$

Key Concepts and Summary

A compound that can donate a proton (a hydrogen ion) to another compound is called a Brønsted-Lowry acid. The compound that accepts the proton is called a Brønsted-Lowry base. The species remaining after a Brønsted-Lowry acid has lost a proton is the conjugate base of the acid. The species formed when a Brønsted-Lowry base gains a proton is the conjugate acid of the base. Thus, an acid-base reaction occurs when a proton is transferred from an acid to a base, with formation of the conjugate base of the reactant acid and formation of the conjugate acid of the reactant base.

Exercises

1. Write equations that show NH3 as both a conjugate acid and a conjugate base.
2. Write equations that show ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}$ acting both as an acid and as a base.
3. What is the conjugate acid of each of the following? What is the conjugate base of each?
1. OH
2. H2O
3. ${\text{HCO}}_{3}{}^{-}$
4. NH3
5. ${\text{HSO}}_{4}{}^{-}$
6. H2O2
7. HS
8. ${\text{H}}_{5}{\text{N}}_{2}{}^{\text{+}}$
4. What is the conjugate acid of each of the following? What is the conjugate base of each?
1. H2S
2. ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}$
3. PH3
4. HS
5. ${\text{HSO}}_{3}{}^{-}$
6. ${\text{H}}_{3}{\text{O}}_{2}{}^{\text{+}}$
7. H4N2
8. CH3OH
5. Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:
1. ${\text{HNO}}_{3}+{\text{H}}_{2}\text{O}\longrightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}+{\text{NO}}_{3}{}^{-}$
2. ${\text{CN}}^{-}+{\text{H}}_{2}\text{O}\longrightarrow \text{HCN}+{\text{OH}}^{-}$
3. ${\text{H}}_{2}{\text{SO}}_{4}+{\text{Cl}}^{-}\longrightarrow \text{HCl}+{\text{HSO}}_{4}{}^{-}$
4. ${\text{HSO}}_{4}{}^{-}+{\text{OH}}^{-}\longrightarrow {\text{SO}}_{4}{}^{\text{2-}}+{\text{H}}_{2}\text{O}$
5. ${\text{O}}^{2-}+{\text{H}}_{2}\text{O}\longrightarrow 2{\mathrm{OH}}^{-}$
6. ${\text{H}}_{2}\text{S}+{\text{NH}}_{2}{}^{-}\longrightarrow {\text{HS}}^{-}+{\text{NH}}_{3}$
6. Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:
1. ${\text{NO}}_{2}{}^{-}+{\text{H}}_{2}\text{O}\longrightarrow {\text{HNO}}_{2}+{\text{OH}}^{-}$
2. $\text{HBr}+{\text{H}}_{2}\text{O}\longrightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}+{\text{Br}}^{-}$
3. ${\text{HS}}^{-}+{\text{H}}_{2}\text{O}\longrightarrow {\text{H}}_{2}\text{S}+{\text{OH}}^{-}$
4. ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}+{\text{OH}}^{-}\longrightarrow {\text{HPO}}_{4}{}^{\text{2-}}+{\text{H}}_{2}\text{O}$
5. ${\text{H}}_{2}{\text{PO}}_{4}{}^{-}+\text{HCl}\longrightarrow {\text{H}}_{3}{\text{PO}}_{4}+{\text{Cl}}^{-}$
6. ${\text{CH}}_{3}\text{OH}+{\text{H}}^{-}\longrightarrow {\text{CH}}_{3}{\text{O}}^{-}+{\text{H}}_{2}$

Glossary

Brønsted-Lowry acid: proton donor

Brønsted-Lowry base: proton acceptor

conjugate acid: substance formed when a base gains a proton

conjugate base: substance formed when an acid loses a proton