Problem 41 - A Complete Resource Book in Chemistry for JEE Main

Question

The conjugate acid of \(\mathrm{NH}_{2}^{-}\)is (a) \(\mathrm{NH}_{4}^{+}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{NH}_{2} \mathrm{OH}\) (d) \(\mathrm{N}_{2} \mathrm{H}_{4}\)

Step-by-Step Solution

Answer

The conjugate acid of \(\mathrm{NH}_{2}^{-}\) is \(\mathrm{NH}_{3}\), so the answer is (b).

Step 1: Understand the Concept of Conjugate Acid

The conjugate acid of a base is formed when the base gains a proton (H⁺). In this exercise, we are asked to find the conjugate acid of \(\mathrm{NH}_{2}^{-}\).

Step 2: Identify the Base

Here, the base given is \(\mathrm{NH}_{2}^{-}\). This is an anion (negatively charged ion) and can accept a proton to form its conjugate acid.

Step 3: Add a Proton to the Base

The conjugate acid of \(\mathrm{NH}_{2}^{-}\) is formed by adding a proton (H⁺) to it. So, \(\mathrm{NH}_{2}^{-} + \mathrm{H}^{+} = \mathrm{NH}_{3}\).

Step 4: Select the Correct Answer

Among the given options, \(\mathrm{NH}_{3}\) is the result when \(\mathrm{NH}_{2}^{-}\) gains a proton, making it the conjugate acid. So, the correct answer is (b) \(\mathrm{NH}_{3}\).

Key Concepts

Acid-Base Reactions

Acid-base reactions involve the transfer of protons between reactants. These reactions can be seen as a tug-of-war for hydrogen ions (H⁺). In these reactions, acids donate protons and bases accept them.
The product that forms after a base gains a proton is called the conjugate acid. Conversely, when an acid loses a proton, its product is known as the conjugate base.
Conjugate pairs, such as the one involving \( \text{NH}_2^- \) and \( \text{NH}_3 \), help us understand how species interact during these reactions. They emphasize the balance of protons being traded back and forth, keeping the reaction balanced.

• An example: The hydroxide ion (OH⁻) can accept an H⁺, forming water (H₂O) as its conjugate acid.
• Similarly, when acids like hydrochloric acid (HCl) donate a proton, chloride ions (Cl⁻) are left as conjugate bases.

Knowing these kinds of pairs enhances your understanding of how substances transform during chemical reactions.

Ammonia

Ammonia (\( \text{NH}_3 \)) is a colorless gas with a distinct, sharp smell. It plays a significant role in many acid-base reactions due to its ability to both accept and donate protons, illustrating its amphoteric nature.
As a weak base, ammonia can accept a proton to form \( \text{NH}_4^+ \), the ammonium ion. This is particularly relevant in reactions with stronger acids, where ammonia acts as a base and neutralizes the acid.
Ammonia's capacity to donate a proton in reactions is lesser known but showcases its versatile behavior. Its conjugate, \( \text{NH}_4^+ \), highlights this process by acting as an acid in specific circumstances.

• In the reaction \( \text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+ \), ammonia acts as a base.
• Conversely, if \( \text{NH}_4^+ \) were to donate a proton, it reverts back to \( \text{NH}_3 \).

This exchange demonstrates the dynamic balancing act in chemical reactions involving ammonia.

Proton Transfer Reactions

Proton transfer reactions are fundamental to understanding how acids and bases behave. These reactions define the movement of protons (H⁺) from one molecule to another, altering the chemical structure and properties of the involved species.
The core idea is simple: one species acts as a donor (acid), and the other as an acceptor (base). When acids lose a proton, they form their conjugate bases. Meanwhile, when bases gain a proton, they form their conjugate acids.

• Consider the classic example of the bicarbonate ion (HCO₃⁻), which can act either as an acid or a base, depending on its chemical surroundings.
• Such flexibility demonstrates the key role of protons in dictating molecular behavior during reactions.

Proton transfer reactions not only reveal the transformation within single molecules but also underline the broader catalyst for change in chemical reactions. These microscopic shifts depend heavily on the transfer of protons, showcasing their significance in chemistry.