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Chapter 17: Problem 19

What reaction occurs when each of the following is dissolved in water? a. HF b. \(\mathrm{NaF}\) c. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) d. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{Cl}\)

Short Answer

Expert verified
a. HF partially ionizes; b. NaF fully dissociates; c. C鈧咹鈧匩H鈧� partially ionizes; d. C鈧咹鈧匩H鈧僀l fully dissociates.

Step by step solution

01

Dissolution of HF in Water

When hydrofluoric acid (HF) is dissolved in water, it partially ionizes into hydrogen ions (H鈦�) and fluoride ions (F鈦�). This process is described by the equilibrium reaction: \[ \mathrm{HF} \rightleftharpoons \mathrm{H}^{+} + \mathrm{F}^{-} \] HF is a weak acid, so it does not completely dissociate in water.
02

Dissolution of NaF in Water

Sodium fluoride (NaF) is an ionic compound that completely dissociates in water into sodium ions (Na鈦�) and fluoride ions (F鈦�). The dissolution reaction is:\[ \mathrm{NaF} \rightarrow \mathrm{Na}^{+} + \mathrm{F}^{-} \] Since NaF completely dissociates, it forms free ions in solution.
03

Dissolution of C鈧咹鈧匩H鈧� in Water

Aniline (C鈧咹鈧匩H鈧�) is a weak base and when dissolved in water, it undergoes a partial reaction with water to produce phenylammonium ions (C鈧咹鈧匩H鈧冣伜) and hydroxide ions (OH鈦�). This can be represented as: \[ \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} + \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+} + \mathrm{OH}^{-} \] Aniline is considered a weak base as it does not completely ionize in water.
04

Dissolution of C鈧咹鈧匩H鈧僀l in Water

Phenylammonium chloride (C鈧咹鈧匩H鈧僀l) is an ionic compound that dissociates in water into phenylammonium ions (C鈧咹鈧匩H鈧冣伜) and chloride ions (Cl鈦�). The chloride ion does not react further in water, while the presence of C鈧咹鈧匩H鈧冣伜 indicates that the compound originates from a weak base and a strong acid, which slightly lowers the pH of the solution. The dissociation is:\[ \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3} \mathrm{Cl} \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+} + \mathrm{Cl}^{-} \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Ionization
Ionization is a fundamental concept in aqueous reactions. In chemistry, ionization refers to the process where a molecule separates into its ions when dissolved in water or another solvent. This can happen either completely or partially, depending on the strength of the acid or base involved. When a substance ionizes in water, it releases ions, leading to the formation of a solution that can conduct electricity.
For example, consider hydrofluoric acid (HF). When HF is dissolved in water, it ionizes partially:
  • The equilibrium reaction is: \[ \text{HF} \rightleftharpoons \text{H}^{+} + \text{F}^{-} \]
  • This means only some HF molecules release hydrogen ions (H鈦�), while others remain intact.
  • This equilibrium state demonstrates its partial ionization.
Ionization can differ based on whether a substance is a weak acid, weak base, or ionic compound.
Weak Acids
Weak acids are acids that do not completely ionize in solution. Instead, they only partially donate hydrogen ions (H鈦�) to the surrounding water, resulting in an equilibrium between ionized and non-ionized forms. The degree of ionization for a weak acid is generally low, meaning most of the acid remains in its molecular form in solution. This characteristic leads to a relatively high pH compared to strong acids.
Hydrofluoric acid (HF) is a classic example of a weak acid:
  • When HF is dissolved in water, it forms some hydrogen ions and fluoride ions: \[ \text{HF} \rightleftharpoons \text{H}^{+} + \text{F}^{-} \]
  • However, a significant portion of HF remains un-ionized due to its weak nature.
  • Weak acids have a specific equilibrium constant (Ka) that indicates their strength and how much they ionize.
Weak Bases
Weak bases are similar to weak acids, but instead of donating protons, they accept protons to a limited extent. They only partially ionize in water, producing hydroxide ions (OH鈦�) and the corresponding cations, which result in a higher pH compared to strong bases but lower than neutral solutions.
An example of a weak base is aniline (\( \text{C}_{6} \text{H}_{5} \text{NH}_{2} \)):
  • When aniline is dissolved in water, it reacts slightly to form phenylammonium ions and hydroxide ions:\[ \text{C}_{6} \text{H}_{5} \text{NH}_{2} + \text{H}_{2}\text{O} \rightleftharpoons \text{C}_{6} \text{H}_{5} \text{NH}_{3}^{+} + \text{OH}^{-} \]
  • Only a small portion of aniline is converted into phenylammonium ions, indicating its weak base status.
  • The fraction of ionization is limited, characterized by an equilibrium constant known as Kb.
The greater the Kb value, the stronger the weak base, meaning more extensive ionization.
Ionic Compounds
Ionic compounds are formed from the electrostatic attraction between oppositely charged ions, typically resulting from a metal combining with a non-metal. These compounds dissociate into their respective ions when dissolved in water, which means they split completely to form ions. This results in solutions that readily conduct electricity due to the abundance of free ions.
For example, sodium fluoride (\( \text{NaF} \)) is an ionic compound:
  • When NaF is dissolved in water, it separates entirely into sodium ions and fluoride ions:\[ \text{NaF} \rightarrow \text{Na}^{+} + \text{F}^{-} \]
  • No NaF molecules remain intact; all are separated into ions that are free to move.
  • This complete dissociation makes ionic compounds like NaF different from weak acids and bases, which only partially ionize.
Ionic compounds typically have high melting and boiling points, reflecting the strong attraction between ions in the solid state.

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Most popular questions from this chapter

Each of the following statements concerns a \(0.10 M\) solution of a weak organic base, B. Briefly describe why each statement is either true or false. a. \([\mathrm{B}]\) is approximately equal to \(0.10 \mathrm{M}\). b. [B] is much greater than \(\left[\mathrm{HB}^{+}\right]\). c. \(\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\) is greater than \(\left[\mathrm{HB}^{+}\right]\). d. The \(\mathrm{pH}\) is 13 . e. \(\left[\mathrm{HB}^{+}\right]\) is approximately equal to \(\left[\mathrm{OH}^{-}\right]\). f. \(\left[\mathrm{OH}^{-}\right]\) equals \(0.10 \mathrm{M}\).

Write chemical equations for the acid ionizations of each of the following weak acids (express these in terms of \(\left.\mathrm{H}_{3} \mathrm{O}^{+}\right)\) a. \(\mathrm{HCO}_{2} \mathrm{H}\) (formic acid) b. HF (hydrofluoric acid) c. \(\mathrm{HN}_{3}\) (hydrazoic acid) d. HOCN (cyanic acid)

Cyanoacetic acid, \(\mathrm{CH}_{2} \mathrm{CNCOOH}\), is used in the manufacture of barbiturate drugs. An aqueous solution containing \(5.0 \mathrm{~g}\) in a liter of solution has a pH of \(1.89\). What is the value of \(K_{a} ?\)

You have the following solutions, all of the same molar concentrations: \(\mathrm{KBr}, \mathrm{HBr}, \mathrm{CH}_{3} \mathrm{NH}_{2}\), and \(\mathrm{NH}_{4} \mathrm{Cl}\). Rank them from the lowest to the highest hydroxide-ion concentrations.

Find the \(\mathrm{pH}\) of the solution obtained when \(25 \mathrm{~mL}\) of \(0.065 M\) benzylamine, \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{NH}_{2}\), is titrated to the equivalence point with \(0.050 M\) hydrochloric acid. \(K_{b}\) for benzylamine is \(4.7 \times 10^{-10}\)
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