Question

Complete the following equations in those instances in which a reaction (hydrolysis) will occur. If no reaction occurs, so state. (a) \(\mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{NO}_{3}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O} \longrightarrow\) (b) \(\mathrm{Na}^{+}(\mathrm{aq})+\mathrm{NO}_{2}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O} \longrightarrow\) (c) \(\mathrm{K}^{+}(\mathrm{aq})+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O} \longrightarrow\) (d) \(\mathrm{K}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})+\mathrm{Na}^{+}(\mathrm{aq})+\mathrm{I}^{-}(\mathrm{aq})+\) (e) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{2} \mathrm{O} \longrightarrow\)

Short answer

(a) \( NH_{4}^{+}(aq) + H_{2}O \longrightarrow NH_{3}(aq) + H_{3}O^{+}(aq) \), (b) No reaction, (c) \( C_{6}H_{5}COO^{-}(aq) + H_{2}O \longrightarrow C_{6}H_{5}COOH(aq) + OH^{-}(aq) \), (d) No reaction, (e) \( C_{6}H_{5}NH_{3}^{+}(aq) + H_{2}O \longrightarrow C_{6}H_{5}NH_{2}(aq) + H_{3}O^{+}(aq) \)

Step-by-step solution

Checking for Hydrolysis - \( NH_{4}^{+} \)

Ammonium ion \( NH_{4}^{+} \) can undergo hydrolysis to form ammonia and hydronium ions. So, the reaction would be \( NH_{4}^{+}(aq) + H_{2}O(current) \longrightarrow NH_{3}(aq) + H_{3}O^{+}(aq) \)

Checking for Hydrolysis - \( Na^{+} \)

Sodium ion \( Na^{+} \) is from a strong base (sodium hydroxide), and it doesn't undergo hydrolysis. Hence, no reaction occurs with water.

Checking for Hydrolysis - \( K^{+} \) and \( C_{6}H_{5}COO^{-} \)

Potassium ion \( K^{+} \) cannot undergo hydrolysis as it is from a strong base (potassium hydroxide). The benzoate ion \( C_{6}H_{5}COO^{-} \) is the conjugate base of a weak acid (benzoic acid) so it undergoes hydrolysis to form a benzoic acid and hydroxide ion. The reaction is \( C_{6}H_{5}COO^{-} (aq) + H_{2}O \longrightarrow C_{6}H_{5}COOH(aq) + OH^{-}(aq) \)

Checking for Hydrolysis - \( K^{+}, Cl^{-}, Na^{+}, I^{-} \)

None of the ions from this set can undergo hydrolysis because all these ions are from strong acids or bases, such as Hydrochloric acid (for \( Cl^{-} \)), Sodium hydroxide (for \( Na^{+} \)), Potassium hydroxide (for \( K^{+} \)) or hydroiodic acid (for \( I^{-} \)). Therefore, no reaction occurs in this case.

Checking for Hydrolysis - \( C_{6}H_{5}NH_{3}^{+} \)

The benzenamine ion \( C_{6}H_{5}NH_{3}^{+} \) is the conjugate acid of a weak base, so it undergoes hydrolysis to form a weak base and hydronium ion. The reaction will be \( C_{6}H_{5}NH_{3}^{+}(aq) + H_{2}O \longrightarrow C_{6}H_{5}NH_{2}(aq) + H_{3}O^{+}(aq) \)

Key concepts

Chemical Equilibrium

Chemical equilibrium plays a pivotal role in hydrolysis reactions. Understanding this concept is essential when predicting the outcome of reactions in aqueous solutions. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, meaning that the concentrations of the reactants and products remain constant over time. This state is dynamic, not static, as reactants continue to form products and vice-versa, but without any net change in concentration.

For example, in the provided exercise, the ammonium ion (NH_{4}^{+}) reacts with water to form ammonia and hydronium ions. Over time, these ions can also recombine to form NH_{4}^{+} and H_{2}O. When the rate at which the ammonia and hydronium ions are forming equals the rate at which they are recombining, the reaction has reached chemical equilibrium. In practical terms, when studying hydrolysis, students should keep in mind that even when a reaction is predicted to occur, it may only proceed until equilibrium is reached, not necessarily to completion.

Acid-Base Conjugates

In the context of hydrolysis, acid-base conjugates are critically important. Conjugate acids and bases are pairs that differ by the presence of a single proton. When an acid donates a proton, it becomes its conjugate base, while when a base accepts a proton, it becomes its conjugate acid.

The exercise brings this into focus through the benzoate ion (C_{6}H_{5}COO^{-}), which is the conjugate base of benzoic acid. It readily reacts with water to re-form benzoic acid and a hydroxide ion (OH^{-}). Similarly, the benzenamine ion (C_{6}H_{5}NH_{3}^{+}) represents a conjugate acid, which upon hydrolysis, forms a weak base (benzenamine) and a hydronium ion (H_{3}O^{+}). These pairs illustrate how understanding conjugate relationships helps predict the direction of hydrolysis reactions.

Properties of Aqueous Solutions

The behavior of ions in aqueous solutions further aids in predicting whether a hydrolysis reaction will take place. The properties of these solutions, such as pH, conductivity, and reactivity, depend on the ions present and their concentrations. In a strong acid or base solution, like that of Cl^{-} from hydrochloric acid or Na^{+} from sodium hydroxide, the ions do not readily undergo hydrolysis because they are stable in water. Their conjugate pairs are strong bases or acids that do not significantly react with water.

On the other hand, the ions derived from weak acids or bases can significantly affect the properties of an aqueous solution because they can undergo hydrolysis, altering the solution's pH. For instance, the benzoate ion (C_{6}H_{5}COO^{-}) from a weak acid increases the pH upon hydrolysis by producing hydroxide ions. Awareness of these properties enables students to predict the outcomes of reactions in aqueous solutions, which is essential for sufficiently understanding the exercise.