Textbook Question
Write equations showing how each weak base ionizes water to form OH–. Also write the corresponding expression for Kb. a. NH3 b. HCO3– c. CH3NH2
Ch.16 - Acids and Bases
Chapter 16, Problem 88b
Write equations showing how each weak base ionizes water to form OH–. Also write the corresponding expression for Kb. b. C6H5NH2
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Identify the weak base and its formula. In this case, the weak base is aniline, which has the chemical formula C6H5NH2.
Write the ionization reaction of the weak base with water. Aniline reacts with water to form aniline ion and hydroxide ion: C6H5NH2 + H2O ⇌ C6H5NH3+ + OH-.
Identify the products and reactants in the ionization reaction. Reactants are C6H5NH2 and H2O, and the products are C6H5NH3+ (anilinium ion) and OH- (hydroxide ion).
Write the expression for the base ionization constant (Kb). The Kb expression is derived from the equilibrium concentrations of the products and reactants: Kb = [C6H5NH3+][OH-] / [C6H5NH2].
Note that in the Kb expression, the concentration of water is typically omitted because it is a liquid and its concentration remains relatively constant.
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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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ICE Charts of Weak Bases
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. The extent of this ionization is influenced by the strength of the base.
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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 undissociated base. A higher Kb value indicates a stronger weak base, as it suggests a greater degree of ionization.
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Related Practice
Textbook Question
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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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