Problem 133 - Chemistry

Question

Like water, liquid ammonia undergoes autoionization: $$ \mathrm{NH}_{3}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{NH}_{2}^{-} $$ (a) Identify the Brönsted acids and Bronsted bases in this reaction. (b) What species correspond to \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{OH}^{-},\) and what is the condition for a neutral solution?

Step-by-Step Solution

Answer

(a) Brönsted acid: \(\text{NH}_4^+\); Brönsted base: \(\text{NH}_2^-\). (b) \(\text{NH}_4^+\) is \(\text{H}_3\text{O}^+\), \(\text{NH}_2^-\) is \(\text{OH}^-\); neutral when \(\text{NH}_4^+\) = \(\text{NH}_2^-\).

Step 1: Understand the Reaction Components

The given reaction is an autoionization reaction of ammonia: \(\text{NH}_3 + \text{NH}_3 \rightleftharpoons \text{NH}_4^+ + \text{NH}_2^-\). In this process, ammonia molecules interact with each other, producing an ammonium ion (\(\text{NH}_4^+\)) and an amide ion (\(\text{NH}_2^-\)).

Step 2: Identify Brönsted Acid and Base

In this reaction, one \(\text{NH}_3\) molecule donates a proton to another \(\text{NH}_3\) molecule. The \(\text{NH}_3\) that donates a proton becomes \(\text{NH}_2^-\), acting as a Brönsted base. The \(\text{NH}_3\) that accepts a proton becomes \(\text{NH}_4^+\), which is a Brönsted acid.

Step 3: Analogy to Water's Autoionization

In terms of water's autoionization, \(\text{H}_2\text{O} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{OH}^-\), the \(\text{NH}_4^+\) corresponds to \(\text{H}_3\text{O}^+\) as the proton donor, and \(\text{NH}_2^-\) corresponds to \(\text{OH}^-\) as the proton acceptor.

Step 4: Condition for Neutral Solution

A neutral ammonia solution is achieved when the concentration of \(\text{NH}_4^+\) equals that of \(\text{NH}_2^-\), analogous to \(\text{H}_3\text{O}^+\) and \(\text{OH}^-\) concentrations being equal in water.

Key Concepts

Brönsted acids and bases

In the world of chemistry, Brönsted acids and bases play a crucial role in proton exchange reactions. A Brönsted acid is a substance that donates a proton ( ext{H}^+). In the autoionization of ammonia, the ext{NH}_4^+ ion acts as a Brönsted acid because it carries an extra proton after accepting it from another ammonia molecule. On the other hand, a Brönsted base is a substance that accepts a proton. During this same reaction, an ammonia molecule accepts a proton from another ext{NH}_3 molecule, transforming into the ext{NH}_2^- ion, thus behaving as a Brönsted base. To summarize:

• Brönsted Acid: Proton donor (e.g., ext{NH}_4^+ in the reaction).
• Brönsted Base: Proton acceptor (e.g., ext{NH}_2^- in the reaction).

Understanding these roles helps in predicting the behavior of substances during chemical reactions.

neutral solution

A solution is defined as neutral when it has an equal concentration of hydrogen ions and hydroxide ions. In pure water, neutral occurs where concentrations of ext{H}_3 ext{O}^+ and ext{OH}^- ions are equal (both are 10^{-7} mol/L at 25°C). Similarly, in the context of ammonia autoionization, a neutral ammonia solution is achieved when the concentration of ext{NH}_4^+ ions equals that of ext{NH}_2^- ions. This balance between the proton-giving and proton-receiving species ensures that the solution is neither acidic nor basic.

• Neutral Solution in Water: ext{[H}_3 ext{O}^+] = ext{[OH}^-]
• Neutral Solution in Ammonia: ext{[NH}_4^+] = ext{[NH}_2^-]

Maintaining this equilibrium is vital in chemical formulations and reactions involving ammonia.

ammonium ion

The ammonium ion ( ext{NH}_4^+) is a positively charged polyatomic ion. It forms when an ammonia molecule ( ext{NH}_3) accepts a proton, thus acting as a Brönsted acid in the context of autoionization. This transformation happens when the lone pair of the nitrogen atom in ext{NH}_3 captures an extra hydrogen ion. Key characteristics of the ammonium ion:

• It's a cation, meaning it carries a positive charge.
• Acts as a proton donor in ammonia autoionization.
• Stable in both solution and solid form, often found in various ammonium salts like ammonium chloride ( ext{NH}_4 ext{Cl}).

Understanding the behavior of the ammonium ion is fundamental in predicting the outcome of many chemical reactions involving ammonia.

amide ion

The amide ion ( ext{NH}_2^-) is the conjugate base formed from ammonia ( ext{NH}_3) when it donates a proton. This ion is negatively charged, capable of acting as a powerful Brönsted base, meaning it has a strong tendency to accept a proton. Characteristics of the amide ion include:

• It's an anion, which means it carries a negative charge.
• Functions as a proton acceptor in reactions.
• Commonly found in organic chemistry, particularly in the production of amines and other nitrogen-containing organic compounds.

Recognizing the amide ion’s role in chemical reactions can help in understanding the broader scope of nitrogen chemistry and its applications.