Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

Conjugate acid of \(\mathrm{NH}_{2}^{-}\)is (a) \(\mathrm{NH}_{4}^{+}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{NH}_{2}\) (d) \(\mathrm{NH}\)

Step-by-Step Solution

Answer

The conjugate acid of \(\mathrm{NH}_{2}^{-}\) is \(\mathrm{NH}_{3}\) (option b).

Step 1: Understanding Conjugate Acid Concept

The conjugate acid of a base is formed when the base gains a proton (H⁺). Here, we need to determine the conjugate acid of \(\mathrm{NH}_{2}^{-}\). Identify \(\mathrm{NH}_{2}^{-}\) as the base, and then figure out what results when \(\mathrm{NH}_{2}^{-}\) gains a proton.

Step 2: Add a Proton to the Base

Next, add a proton (H⁺) to \(\mathrm{NH}_{2}^{-}\). Adding a proton to \(\mathrm{NH}_{2}^{-}\) results in the following chemical reaction: \(\mathrm{NH}_{2}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{NH}_{3}\).

Step 3: Identify the Conjugate Acid

The resulting molecule from Step 2 is \(\mathrm{NH}_{3}\). Therefore, \(\mathrm{NH}_{3}\) is the conjugate acid of \(\mathrm{NH}_{2}^{-}\).

Step 4: Choose the Correct Option

According to the question, among the given options: (a) \(\mathrm{NH}_{4}^{+}\), (b) \(\mathrm{NH}_{3}\), (c) \(\mathrm{NH}_{2}\), (d) \(\mathrm{NH}\); option (b) \(\mathrm{NH}_{3}\) is the correct conjugate acid of \(\mathrm{NH}_{2}^{-}\).

Key Concepts

Proton Transfer

Proton transfer is a fundamental process in chemistry, especially within the realm of acid-base reactions. It involves the transfer of a hydrogen ion, or proton (H⁺), from one molecule to another. When a base accepts a proton, it becomes a conjugate acid. Let's consider the specific case of \(\mathrm{NH}_{2}^{-}\). This is an anion known as the amide ion, and it acts as a base. During a proton transfer, \(\mathrm{NH}_{2}^{-}\) accepts a proton to form \(\mathrm{NH}_{3}\) (ammonia).

• Proton transfer is central to defining acid and base behavior.
• In the instance of \(\mathrm{NH}_{2}^{-}\), gaining a proton readily demonstrates this process.

Proton transfer not only illustrates the dynamic nature of chemical reactions but also facilitates changes in molecular structure, leading to the formation of conjugate acid-base pairs. Understanding this concept is crucial when studying various chemical reactions.

Ammonia Molecule

Ammonia (\(\mathrm{NH}_{3}\)) is a small, simple molecule with significant importance in both nature and industry. It is created when a proton is added to the amide ion \(\mathrm{NH}_{2}^{-}\), as we see in acid-base chemistry. In this context, ammonia acts as the conjugate acid.

• Ammonia easily participates in reactions due to its ability to donate and accept protons.
• Its structure consists of one nitrogen atom bonded to three hydrogen atoms, showcasing a trigonal pyramidal shape.

Ammonia plays a pivotal role in numerous biological systems and atmospheric cycles. Moreover, it serves as a base in many chemical processes. Its ability to engage in proton transfer underscores its versatility as both an acid and a base, depending on the reaction context. This duality accentuates ammonia's vital function in various chemical environments.

Acid-Base Reactions

Acid-base reactions are essential interactions where acids donate protons and bases accept protons. These reactions often lead to the formation of conjugate acid-base pairs. In our specific example, the amide ion \(\mathrm{NH}_{2}^{-}\) (acting as a base) accepts a proton to become ammonia \(\mathrm{NH}_{3}\), which is the conjugate acid.

• An acid, in the Brønsted-Lowry sense, is any species that donates a proton.
• A base is any species that accepts a proton.

This framework helps us understand how substances react in different chemical environments. Through acid-base reactions, substances often change roles, transitioning between acids and bases as they gain or lose protons. Observing and predicting these interactions help in discussions about chemical equilibrium, buffering solutions, and other applications in chemistry.