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Ch.17 - Acids and Bases
Tro - Chemistry: A Molecular Approach 5th Edition
Tro5th EditionChemistry: A Molecular ApproachISBN: 9780134874371Not the one you use?Change textbook
All textbooksTro 5th EditionCh.17 - Acids and BasesProblem 88c
Chapter 17, Problem 88c

Write equations showing how each weak base ionizes water to form OH–. Also write the corresponding expression for Kb. c. C2H5NH2

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Identify the weak base: Ethylamine, \( \text{C}_2\text{H}_5\text{NH}_2 \), is a weak base.
Write the ionization equation: When ethylamine ionizes in water, it accepts a proton from water, forming its conjugate acid and hydroxide ions: \( \text{C}_2\text{H}_5\text{NH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{C}_2\text{H}_5\text{NH}_3^+ + \text{OH}^- \).
Define the base ionization constant \( K_b \): The base ionization constant is a measure of the strength of a base in solution.
Write the expression for \( K_b \): The expression for the base ionization constant \( K_b \) is given by the concentrations of the products over the reactants: \( K_b = \frac{[\text{C}_2\text{H}_5\text{NH}_3^+][\text{OH}^-]}{[\text{C}_2\text{H}_5\text{NH}_2]} \).
Understand the significance of \( K_b \): A larger \( K_b \) value indicates a stronger base, meaning it ionizes more in solution.

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Key Concepts

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

Weak Bases

Weak bases are substances that partially ionize in water to produce hydroxide ions (OH-). Unlike strong bases, which completely dissociate, weak bases establish an equilibrium between the undissociated base and its ions in solution. This characteristic is crucial for understanding their behavior in aqueous solutions and their effect on pH.
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Ionization of Water

The ionization of water refers to the process where water molecules dissociate into hydrogen ions (H+) and hydroxide ions (OH-). This equilibrium is essential for understanding how weak bases interact with water, as they accept protons from water, leading to the formation of OH- ions, which increases the solution's basicity.
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Base Dissociation Constant (Kb)

The base dissociation constant (Kb) quantifies the strength of a weak base in solution. It is defined as the equilibrium constant for the reaction of the base with water, representing the ratio of the concentration of the products (OH- and the conjugate acid) to the concentration of the reactants (the weak base). A higher Kb value indicates a stronger weak base.
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Related Practice
Textbook Question

Caffeine (C8H10N4O2) is a weak base with a pKb of 10.4. Calculate the pH of a solution containing a caffeine concentration of 455 mg/L.

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Amphetamine (C9H13N) is a weak base with a pKb of 4.2. Calculate the pH of a solution containing an amphetamine concentration of 225 mg/L.