Problem 30 - Chemistry The Science in Context

Question

Liquid ammonia at a temperature of \(223 \mathrm{K}\) undergoes autoionization. The value of the equilibrium constant for the autoionization of ammonia is considerably less than that of water. Write an equation for the autoionization of ammonia and suggest a reason why the value of \(K\) for the process is less than that of water.

Step-by-Step Solution

Answer

Question: Write the equation for the autoionization of ammonia and explain why the equilibrium constant (K) for this process is less than that of water's autoionization. Answer: The equation for the autoionization of ammonia is 2NH₃ ⇌ NH₄⁺ + NH₂⁻. The value of K for ammonia's autoionization is less than that of water's autoionization (2H₂O ⇌ H₃O⁺ + OH⁻) because the nitrogen-hydrogen (N-H) bond in ammonia is less polar than the oxygen-hydrogen (O-H) bond in water. The weaker polarity results in weaker intermolecular attractions and a lower tendency for ammonia to undergo autoionization compared to water.

Step 1: Understand autoionization

Autoionization is a process where a substance reacts with itself to produce ions. In liquid ammonia and water, autoionization results in the formation of ions from the reaction between two molecules of the substance.

Step 2: Write the autoionization equation for ammonia

In the case of ammonia (NH₃), when two ammonia molecules react with each other, one of them donates a proton (H⁺) to the other, forming ammonium (NH₄⁺) and amide (NH₂⁻) ions. The chemical equation can be written as: 2NH₃ ⇌ NH₄⁺ + NH₂⁻

Step 3: Compare ammonia's autoionization with water's autoionization

Water also undergoes autoionization, producing hydronium (H₃O⁺) and hydroxide (OH⁻) ions. The chemical equation for autoionization of water is: 2H₂O ⇌ H₃O⁺ + OH⁻

Step 4: Identify the reason K is less for ammonia

The reason why the value of K for the autoionization of ammonia is less than that of water can be attributed to the difference in their molecular structures and bonding strength. In water, the oxygen-hydrogen (O-H) bond is highly polar, leading to a stronger attraction between water molecules and thus a greater tendency for autoionization. On the other hand, in ammonia, the nitrogen-hydrogen (N-H) bond is less polar compared to the O-H bond, resulting in weaker intermolecular attractions and a lower tendency for autoionization. Therefore, the equilibrium constant K for ammonia's autoionization is smaller than that of water.

Key Concepts

Equilibrium Constant

The equilibrium constant, often denoted as \( K \), is a crucial concept in chemistry. It provides a measure of the extent to which a chemical reaction proceeds to form products at equilibrium. In the case of ammonia undergoing autoionization, the equation is written as:\[ 2 ext{NH}_3 ightleftharpoons ext{NH}_4^+ + ext{NH}_2^- \]The equilibrium constant expression for this reaction can be represented as:\[ K = \frac{[ ext{NH}_4^+][ ext{NH}_2^-]}{[ ext{NH}_3]^2} \]In chemistry, a smaller value of \( K \) indicates that the equilibrium lies more to the left, meaning that the reactants are favored over the products. This is precisely why ammonia's \( K \) value is lower than that of water during their respective autoionization processes.

• Water has a highly polar O-H bond, leading to more substantial autoionization.
• Ammonia has a less polar N-H bond, resulting in less tendency to ionize.

Thus, the equilibrium constant helps us understand the behavior of chemical systems at equilibrium, showing why ammonia does not ionize as easily as water.

Ammonium Ion

The ammonium ion, \( ext{NH}_4^+ \), is an essential component formed during the autoionization of ammonia. This ion is created when one ammonia molecule donates a proton \( ext{H}^+ \) to another.Ammonium is much like the conjugate acid of ammonia. During the autoionization process:

• One \( ext{NH}_3 \) molecule accepts a proton, transforming into an \( ext{NH}_4^+ \) ion.
• The process is reversible, meaning \( ext{NH}_4^+ \) can release a proton back to become ammonia again.

This back and forth between the formation and dissolution of ammonium ions contributes to the equilibrium state of the reaction.
Ammonium possesses distinct properties by comparison to ammonia, influencing various biological and chemical systems. Understanding its formation and behavior helps clarify overall reaction dynamics in systems involving ammonia as a solvent.

Amide Ion

Amide ions, denoted as \( ext{NH}_2^- \), are the other product of ammonia's autoionization who couple with the formation of ammonium ions. During the reaction, one molecule of ammonia loses a proton, becoming the amide ion.

• These ions carry a negative charge and represent the conjugate base of ammonia.
• They partake in the equilibrium process, complementing ammonium ions.

The stability of the amide ion plays a role in determining equilibrium dynamics.
Amide ions are less stable than their ammonium counterparts, largely influencing why ammonia's equilibrium constant is significantly lower than water’s. The difference in the molecular structure and bond polarity of ammonia compared to water accounts for the decreased stability of amide ions through less effective delocalization of charge.
Recognizing the behavior and formation of amide ions helps in the comprehensive understanding of ammonia's autoionization process, particularly in fields like organic chemistry and synthetic applications.