Problem 15 - Chemistry The Molecular Science

Question

Which are conjugate acid-base pairs? (a) \(\mathrm{NH}_{2}^{-}\) and \(\mathrm{NH}_{4}^{+}\) (b) \(\mathrm{NH}_{3}\) and \(\mathrm{NH}_{2}^{-}\) (c) \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{OH}^{-}\) and \(\mathrm{O}^{2-}\) (e) \(\mathrm{H}_{3} \mathrm{O}^{+}\) and \(\mathrm{OH}^{-}\)

Step-by-Step Solution

Answer

Conjugate pairs: (b) \\(\mathrm{NH}_{3}\\) and \\(\mathrm{NH}_{2}^{-}\\); (c) \\(\mathrm{H}_{3} \mathrm{O}^{+}\\) and \\(\mathrm{H}_{2} \mathrm{O}\\).

Step 1: Concept Understanding

A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton (H⁺). The acid in the pair has one extra hydrogen ion compared to its corresponding base.

Step 2: Identify Pairs (a)

In pair (a), \(\mathrm{NH}_{2}^{-}\) and \(\mathrm{NH}_{4}^{+}\), \(\mathrm{NH}_{4}^{+}\) is the acid form, and it could lose an H⁺ to become \(\mathrm{NH}_{3}\), but \(\mathrm{NH}_{2}^{-}\) would be two steps away (skipping \(\mathrm{NH}_{3}\)). Therefore, they are not conjugate acid-base pairs.

Step 3: Identify Pairs (b)

In pair (b), \(\mathrm{NH}_{3}\) and \( ext{NH}_2^-\), \( ext{NH}_3\) loses a proton to become \( ext{NH}_2^-\), demonstrating that they form a conjugate acid-base pair.

Step 4: Identify Pairs (c)

Looking at (c), \( ext{H}_3 ext{O}^+\) and \( ext{H}_2 ext{O}\), \( ext{H}_3 ext{O}^+\) can donate an H⁺ to form \( ext{H}_2 ext{O}\). These are a conjugate acid-base pair because the conversion involves a single proton transfer.

Step 5: Identify Pairs (d)

For pair (d), \( ext{OH}^-\) and \( ext{O}^{2-}\), converting \( ext{OH}^-\) to \( ext{O}^{2-}\) requires the loss of one H⁺ and an extra electron instead of a proton. Thus, this is not a conjugate pair because electron transfer is also involved.

Step 6: Identify Pairs (e)

In pair (e), \( ext{H}_3 ext{O}^+\) and \( ext{OH}^-\) are part of the complete ionization of water, forming H⁺ and OH⁻, but do not directly convert into each other by a simple single proton transfer. They are not a conjugate acid-base pair.

Key Concepts

Proton Transfer

In the realm of chemistry, proton transfer is a fundamental concept involving the movement of protons (H⁺) from one molecule to another. This transfer is crucial in various chemical reactions, particularly in acid-base chemistry. Without the movement of these protons, many reactions would not proceed.

To understand proton transfer, envision it as a process where a proton is shuttled between two substances. This can transform an acid into its conjugate base, or a base into its conjugate acid. In the provided exercise, for example, when \( \mathrm{NH}_{3} \) loses a proton, it turns into \( \mathrm{NH}_{2}^{-} \), showcasing a clear example of proton transfer.

During reactions, understanding which direction the proton will move is important. It typically moves from a point of higher concentration, the acid, to one of lower concentration, the base, until equilibrium is reached. Thus, observing proton transfers helps us better grasp the nature of reactions in solutions.

Bronsted-Lowry Acid-Base Theory

Bronsted-Lowry Acid-Base Theory is a significant model in chemistry that describes acids and bases based on their ability to donate or accept protons (H⁺). In this theory, an acid is any substance that can donate a proton to another substance. Conversely, a base is any substance that can accept a proton.

According to this theory, every acid has a corresponding base, known as a conjugate base. Similarly, every base has a conjugate acid. The distinction between these pairs lies in a single proton difference. For example, \( \mathrm{H}_3 \mathrm{O}^{+} \) and \( \mathrm{H}_2 \mathrm{O} \) showcase an acid with its conjugate base, where \( \mathrm{H}_3 \mathrm{O}^{+} \) donates a proton to form \( \mathrm{H}_2 \mathrm{O} \).

This concept helps in predicting the products of acid-base reactions and understanding the behavior of different substances in various environments. Bronsted-Lowry also broadens the definition of acids and bases beyond the more limited Arrhenius theory, making it very useful for solving and predicting chemical reactions involving acids and bases.

Chemistry Problem Solving

Solving chemistry problems requires a systematic approach to analyzing and understanding the scenarios presented. When you encounter questions like those about conjugate acid-base pairs, it's crucial to keep a few strategies in mind.

• **Understand the Concepts**: Before beginning a problem, ensure you understand the underlying concepts, such as proton transfer and acid-base definitions. This will make identifying correct relationships easier.
• **Identify Given Data**: Pay attention to what is provided in the problem. Knowing the molecular formulas and charges can help in mapping out the transfer of protons.
• **Apply Theories**: Use concepts like the Bronsted-Lowry Theory to determine relationships between substances. Recognition of conjugate acid-base pairs is essential.
• **Check Consistency**: After solving, review your logic and calculations to ensure the solution makes sense chemically.

By grasping these strategies, students will find it easier, as demonstrated in the exercise, to determine correct conjugate pairs like \( \mathrm{NH}_{3} \) and \( \mathrm{NH}_{2}^{-} \), or \( \mathrm{H}_3 \mathrm{O}^{+} \) and \( \mathrm{H}_2 \mathrm{O} \). Developing problem-solving skills in this way not only assists in tackling textbook exercises but also prepares students for more complex chemical inquiries.