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Ch.17 Carboxylic Acids and Their Derivatives
McMurry - Fundamentals of GOB 8th Edition
McMurry8th EditionFundamentals of GOBISBN: 9780134015187Not the one you use?Change textbook
All textbooksMcMurry 8th EditionCh.17 Carboxylic Acids and Their DerivativesProblem 38
Chapter 17, Problem 38

Write the equation for the ionization of hexanoic acid in water at pH 7.4.

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Hexanoic acid (C₆H₁₂O₂) is a weak acid, so it partially ionizes in water. The general reaction for the ionization of a weak acid is: HA + H₂O ⇌ H₃O⁺ + A⁻, where HA is the weak acid and A⁻ is its conjugate base.
In this case, hexanoic acid (C₆H₁₂O₂) acts as HA. When it reacts with water, it donates a proton (H⁺) to water, forming hydronium ion (H₃O⁺) and the conjugate base, hexanoate ion (C₆H₁₁O₂⁻).
The chemical equation for the ionization of hexanoic acid in water is: C₆H₁₂O₂ + H₂O ⇌ H₃O⁺ + C₆H₁₁O₂⁻.
At pH 7.4, the solution is slightly basic, but hexanoic acid will still partially ionize because it is a weak acid. The extent of ionization depends on its acid dissociation constant (Ka).
To fully understand the behavior of hexanoic acid at pH 7.4, you would compare the pH to the pKa of hexanoic acid. If pH > pKa, the conjugate base (C₆H₁₁O₂⁻) will predominate; if pH < pKa, the undissociated acid (C₆H₁₂O₂) will predominate.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionization of Acids

Ionization refers to the process by which an acid donates a proton (H+) to water, resulting in the formation of its conjugate base and hydronium ions (H3O+). For weak acids like hexanoic acid, this process is not complete, and the equilibrium between the undissociated acid and its ions is established.
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pH and Acid-Base Equilibrium

pH is a measure of the hydrogen ion concentration in a solution, influencing the degree of ionization of weak acids. At a specific pH, such as 7.4, the ratio of ionized to unionized forms of the acid can be determined using the Henderson-Hasselbalch equation, which relates pH to pKa and the concentrations of the acid and its conjugate base.

Hexanoic Acid Structure

Hexanoic acid, a six-carbon fatty acid, has the chemical formula C6H12O2. Its structure includes a carboxylic acid group (-COOH) that is responsible for its acidic properties. Understanding its molecular structure is essential for writing the correct ionization equation and predicting its behavior in aqueous solutions.
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Related Practice
Textbook Question

Draw the structures of the following compounds and use dashed lines to indicate where they form hydrogen bonds to other molecules of the same kind: (ii) methyl formate

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Draw the structures of the following compounds and use dashed lines to indicate where they form hydrogen bonds to other molecules of the same kind: (i) formic acid

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Arrange these compounds in order of increasing boiling points and explain your rationale for the order.

(i) formic acid

(ii) methyl formate

(iii) formamide.

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Suppose you have a sample of benzoic acid dissolved in water.

b. Now assume that aqueous NaOH is added to the benzoic acid solution until pH 12 is reached. Draw the structure of the major organic species present.

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Suppose you have a sample of benzoic acid dissolved in water.

c. Finally, assume that aqueous HCl is added to the solution from (b) until pH 2 is reached. Draw the structure of the major organic species present.

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Give systematic names for the following carboxylic acids:

d.

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