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Chapter 15: Problem 114

Classify each of the following ions according to whether they react with water to give a neutral, acidic, or basic solution: (a) \(\mathrm{F}^{-}\) (b) \(\mathrm{Br}^{-}\) (c) \(\mathrm{NH}_{4}^{+}\) (d) \(\mathrm{K}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{+}\) (e) \(\mathrm{SO}_{3}^{2-}\) (f) \(\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\)

Short Answer

Expert verified
(a) Basic (b) Neutral (c) Acidic (d) Neutral (e) Basic (f) Acidic.

Step by step solution

01

Identifying Neutral Ions

Start by identifying ions that do not affect the pH of a solution when dissolved in water. Typically, these are the conjugate bases of strong acids or salts of strong alkalis. - (b) \(\mathrm{Br}^{-}\) is the conjugate base of the strong acid \( \mathrm{HBr} \), thus it remains neutral in water.- (d) \(\mathrm{K}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{+}\) is formed by a strong alkali metal cation, \(\mathrm{K}^+\) which does not significantly react with water. It remains neutral.
02

Identifying Acidic Ions

Acidic ions are typically those that release \(\mathrm{H}^+\) ions when dissolved in water. This can include the cations of weak bases or some transition metal ions.- (c) \(\mathrm{NH}_{4}^{+}\) is the conjugate acid of the weak base \(\mathrm{NH}_3\), so it increases the [\(\mathrm{H}^+\)] concentration in solution, making it acidic.- (f) \(\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) is a hydrated transition metal cation and can undergo hydrolysis, releasing \(\mathrm{H}^+\) ions and thus acts acidic.
03

Identifying Basic Ions

Basic ions can accept \(\mathrm{H}^+\) ions or release \(\mathrm{OH}^-\) when dissolved in water. These are often the anions of weak acids.- (a) \(\mathrm{F}^{-}\) is the conjugate base of the weak acid \(\mathrm{HF} \). It can react with water to form \(\mathrm{OH}^-\) ions, thus making the solution basic.- (e) \(\mathrm{SO}_{3}^{2-}\) is derived from \(\mathrm{SO}_2\), known for forming \(\mathrm{H}_2\mathrm{SO}_3\), a weak acid. Hence, \(\mathrm{SO}_{3}^{2-}\) can increase the concentration of \(\mathrm{OH}^-\) when reacting with water, making it basic.

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Key Concepts

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

Acidic Ions
Acidic ions play a crucial role in influencing the acidity of an aqueous solution. When these ions dissolve in water, they increase the concentration of hydrogen ions (\(\mathrm{H}^+\)), causing a decrease in pH. This reaction is commonly observed in cations derived from weak bases or certain transition metals.

For example, \(\mathrm{NH}_4^+\) is the conjugate acid of the weak base \(\mathrm{NH}_3\). When introduced into water, \(\mathrm{NH}_4^+\) donates \(\mathrm{H}^+\) ions, increasing the solution's acidity. Similarly, the hydrated ion \(\mathrm{Cr(\mathrm{H}_2\mathrm{O})}_6^{3+}\) can undergo hydrolysis, where it reacts with water and releases \(\mathrm{H}^+\), further indicating its acidic nature.
  • **Key Characteristics of Acidic Ions:**
    • Derived from weak bases or transition metals.
    • Increase \(\mathrm{H}^+\) concentration in water.
    • Lower the pH of the solution.
Basic Ions
Basic ions function by either accepting \(\mathrm{H}^+\) ions or releasing hydroxide ions (\(\mathrm{OH}^-\)) into an aqueous solution, thereby raising the pH. This behavior is typically exhibited by the anions of weak acids.

Take \(\mathrm{F}^-\) for instance. It originates from hydrofluoric acid (\(\mathrm{HF}\)), which is a weak acid. Due to this, \(\mathrm{F}^-\) ions have a tendency to react with water, forming \(\mathrm{OH}^-\) ions, thus increasing the solution's basicity. Similarly, \(\mathrm{SO}_3^{2-}\) ions can react with water to form \(\mathrm{OH}^-\), indicating its basic characteristic.
  • **Key Traits of Basic Ions:**
    • Often found as anions of weak acids.
    • Release \(\mathrm{OH}^-\) ions into the solution.
    • Increase the pH, making solutions more basic.
Neutral Ions
Neutral ions are unique in that they do not alter the pH of a solution. This trait is often associated with ions formed from strong acids or bases, where their conjugates are too weak to influence the pH significantly.

For instance, \(\mathrm{Br}^-\) is the conjugate base of the strong acid hydrobromic acid (\(\mathrm{HBr}\)), and thus, it remains unreactive with water, maintaining the solution鈥檚 neutrality. Another example is \(\mathrm{K(\mathrm{H}_2\mathrm{O})}_6^+\), which includes \(\mathrm{K}^+\)鈥攁 cation of a strong alkali that does not affect the water's pH.
  • **Characteristics of Neutral Ions:**
    • Involve conjugate bases of strong acids or cations of strong bases.
    • No significant impact on \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) concentrations.
    • Maintain the neutrality of the solution's pH.

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Most popular questions from this chapter

Morpholine \(\left(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO}\right)\) is a weak organic base with \(\mathrm{p} K_{\mathrm{b}}=5.68\). Calculate the pH and the concentrations of all species present \(\left(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{NO}, \mathrm{HC}_{4} \mathrm{H}_{9} \mathrm{NO}^{+}\right.\), and \(\left.\mathrm{OH}^{-}\right)\) in a \(0.0100 \mathrm{M}\) morpholine solution.

Beginning with the equilibrium equation for the dissociation of a weak acid \(H A\), show that the percent dissociation varies directly as the square root of \(K_{\mathrm{a}}\) and inversely as the square root of the initial concentration of HA when the concentration of HA that dissociates is negligible compared with its initial concentration.

At \(0^{\circ} \mathrm{C}\), the density of liquid water is \(0.9998 \mathrm{~g} / \mathrm{mL}\) and the value of \(K_{w}\) is \(1.14 \times 10^{-15}\). What fraction of the molecules in liquid water are dissociated at \(0^{\circ} \mathrm{C}\) ? What is the percent dissociation at \(0^{\circ} \mathrm{C}\) ? What is the \(\mathrm{pH}\) of a neutral solution at \(0^{\circ} \mathrm{C}\) ?

Give the formula for the conjugate acid of each of the following Bronsted- Lowry bases: (a) \(\mathrm{SO}_{3}^{2-}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (d) \(\mathrm{OH}\) (e) \(\mathrm{HCO}_{3}^{-}\) (f) \(\mathrm{H}\)

Aqueous solutions of hydrogen sulfide contain \(\mathrm{H}_{2} \mathrm{~S}, \mathrm{HS}^{-}, \mathrm{S}^{2-}\). \(\mathrm{H}_{3} \mathrm{O}^{+}, \mathrm{OH}^{-}\), and \(\mathrm{H}_{2} \mathrm{O}\) in varying concentrations. Which of these species can act only as an acid? Which can act only as a base? Which can act both as an acid and as a base?
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