5.11: Conjugate Acids and Bases

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 Learning Objectives

• Identify conjugate acid-base pairs.
• Determine the formula of the conjugate acid or conjugate base.

Conjugate Acid-Base Pairs

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, \(\ce{OH^-}\), and the conjugate acid of ammonia, \(\ce{NH4+}\):

 

This figure has three parts in two rows. In the first row, two diagrams of acid-base pairs are shown. On the left, a space filling model of H subscript 2 O is shown with a red O atom at the center and two smaller white H atoms attached in a bent shape. Above this model is the label “H subscript 2 O (acid)” in purple. An arrow points right, which is labeled “Remove H superscript plus.” To the right is another space filling model with a single red O atom to which a single smaller white H atom is attached. The label in purple above this model reads, “O H superscript negative (conjugate base).” Above both of these red and white models is an upward pointing bracket that is labeled “Conjugate acid-base pair.” To the right is a space filling model with a central blue N atom to which three smaller white H atoms are attached in a triangular pyramid arrangement. A label in green above reads “N H subscript 3 (base).” An arrow labeled “Add H superscript plus” points right. To the right of the arrow is another space filling model with a blue central N atom and four smaller white H atoms in a tetrahedral arrangement. The green label above reads “N H subscript 3 superscript plus (conjugate acid).” Above both of these blue and white models is an upward pointing bracket that is labeled “Conjugate acid-base pair.” The second row of the figure shows the chemical reaction, H subscript 2 O ( l ) is shown in purple, and is labeled below in purple as “acid,” plus N H subscript 3 (a q) in green, labeled below in green as “base,” followed by a double sided arrow arrow and O H superscript negative (a q) in purple, labeled in purple as “conjugate base,” plus N H subscript 4 superscript plus (a q)” in green, which is labeled in green as “conjugate acid.” The acid on the left side of the equation is connected to the conjugate base on the right with a purple line. Similarly, the base on the left is connected to the conjugate acid on the right side.

In the reaction of ammonia with water to give ammonium ions and hydroxide ions, ammonia acts as a base by accepting a proton from a water molecule, which in this case means that water is acting as an acid. In the reverse reaction, an ammonium ion acts as an acid by donating a proton to a hydroxide ion, and the hydroxide ion acts as a base. The conjugate acid–base pairs for this reaction are \(NH_4^+/NH_3\) and \(H_2O/OH^−\).

Figure \(\PageIndex{1}\). The pairing of parent acids and bases with conjugate acids and bases.

Figure \(\PageIndex{1}\): The Relative Strengths of Some Common Conjugate Acid-Base Pairs. The strongest acids are at the bottom left, and the strongest bases are at the top right. The conjugate base of a strong acid is a very weak base, and, conversely, the conjugate acid of a strong base is a very weak acid.

Example \(\PageIndex{1}\)

Identify the conjugate acid-base pairs in this equilibrium.

\[\ce{CH3CO2H + H2O <=> H3O^{+} + CH3CO2^{-}} \nonumber \]

Solution

Similarly, in the reaction of acetic acid with water, acetic acid donates a proton to water, which acts as the base. In the reverse reaction, \(H_3O^+\) is the acid that donates a proton to the acetate ion, which acts as the base.

Once again, we have two conjugate acid-base pairs:

• the parent acid and its conjugate base (\(CH_3CO_2H/CH_3CO_2^−\)) and
• the parent base and its conjugate acid (\(H_3O^+/H_2O\)).

Example \(\PageIndex{2}\)

Identify the conjugate acid-base pairs in this equilibrium.

\[\ce{(CH_{3})_{3}N + H_{2}O <=> (CH_{3})_{3}NH^{+} + OH^{-}} \nonumber \]

Solution

One pair is H₂O and OH⁻, where H₂O has one more H⁺ and is the conjugate acid, while OH⁻ has one less H⁺ and is the conjugate base.

The other pair consists of (CH₃)₃N and (CH₃)₃NH⁺, where (CH₃)₃NH⁺ is the conjugate acid (it has an additional proton) and (CH₃)₃N is the conjugate base.

Exercise \(\PageIndex{1}\)

Identify the conjugate acid-base pairs in this equilibrium.

\[\ce{NH2^{-} + H2O\rightleftharpoons NH3 + OH^{-}} \nonumber \]

Answer

H₂O (acid) and OH⁻ (base); NH₂⁻ (base) and NH₃ (acid)

 Formulas of Conjugate Acids and Bases

Notice in the examples above that an acid and its conjugate base are only different by a proton (H⁺), similarly, a base and its conjugate acid also only differ by a proton.

The formula of the conjugate base is the acid MINUS a proton. For example, the conjugate base of H₂O is OH⁻, and the conjugate base of NH₃ is NH₂⁻.

The formula of the conjugate acid is the base PLUS a proton. For example, the conjugate acid of NH₃ is NH₄⁺, and the conjugate acid of H₂O is H₃O⁺.

Exercise \(\PageIndex{2}\)

Identify the conjugate base of the following:

a. H₂CO₃

b. HCN

c. H₂PO₄⁻

Answer

a. HCO₃⁻   b. CN⁻   c. HPO₄²⁻

Exercise \(\PageIndex{3}\)

Identify the conjugate acid of the following:

a. NO₃⁻

b. PO₄³⁻

c. HSO₄⁻

Answer

a. HNO₃   b. HPO₄²⁻      c. H₂SO₄