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

(b) \(\mathrm{NH}_{3}\)

Step 1: Understanding the Conjugate Acid

To find the conjugate acid of a base, you must add a proton (\(\text{H}^+\)) to the base. The base in this problem is \(\text{NH}_{2}^{-}\).

Step 2: Adding a Proton to the Base

Adding a proton (\(\text{H}^+\)) to \(\text{NH}_{2}^{-}\) gives you \(\text{NH}_{3}\). This is because the negative charge of \(\text{NH}_{2}^{-}\) will be neutralized by the added proton.

Step 3: Identifying the Conjugate Acid

The species obtained after adding a proton, \(\text{NH}_{3}\), is the conjugate acid of the base \(\text{NH}_{2}^{-}\).

Step 4: Choosing the Correct Answer

From the choices given (a) \(\text{NH}_{4}^{+}\), (b) \(\text{NH}_{3}\), (c) \(\text{NH}_{2}\text{OH}\), (d) \(\text{N}_{2}\text{H}_{4}\), the correct conjugate acid for \(\text{NH}_{2}^{-}\) is \(\text{NH}_{3}\), which corresponds to option (b).

Key Concepts

Proton Transfer

The concept of proton transfer is central to understanding many acid-base reactions. In chemistry, a proton transfer refers to the movement of a hydrogen ion (H\(^+\)) from one molecule to another. This process is integral in forming conjugate acid-base pairs.

When a base, like \( ext{NH}_2^-\), accepts a proton, it transforms into its conjugate acid, \( ext{NH}_3\). This transfer of a proton is what distinguishes one member of the pair from the other.

• Base (before proton transfer): \( ext{NH}_2^-\)
• Proton (what's transferred): \( ext{H}^+\)
• Conjugate Acid (after proton transfer): \( ext{NH}_3\)

The ease of proton transfer is influenced by the strengths of the acids and bases involved. Stronger bases readily accept protons, while weaker acids donate them more willingly. Understanding these dynamics helps predict the outcomes of many chemical reactions.

Amino Group

An amino group, chemically denoted as \( ext{-NH}_2\), is a basic functional group commonly found in organic molecules. Its basicity arises primarily from the lone electron pair on the nitrogen atom. This pair can readily accept a proton, rendering the amino group a potential site for proton attachment.

When the amino group \( ext{-NH}_2\) receives a proton, it forms ammonium \(\text{-NH}_3\). Here’s how the process occurs:

• The nitrogen atom's lone pair interacts with the incoming proton \(\text{H}^+\).
• As a proton is accepted, the electron pair forms a new N-H bond.
• Once protonated, the group transitions into a positively charged ammonium ion.

In biological systems, amino groups play critical roles in forming amino acids, the building blocks of proteins. Their ability to react with acids and bases significantly influences biochemical pathways.

Acid-Base Reactions

Acid-base reactions are fundamental to the field of chemistry and involve the transfer of protons between reactants. In the Bronsted-Lowry theory, which is commonly referenced, an acid donates a proton to a base.

The reaction between \(\text{NH}_2^-\) and \(\text{H}^+\) showcases a classic acid-base interaction:

• The base, \(\text{NH}_2^-\), accepts the proton \(\text{H}^+\), forming the conjugate acid, \(\text{NH}_3\).
• The original acid (often a hydronium ion or water) loses a proton, turning into its conjugate base.

This relationship is vital because it allows chemists to predict the direction and extent of chemical reactions, showing whether a reaction will proceed forward or reverse.

Overall, acid-base reactions are not just limited to the laboratory but occur throughout nature, affecting processes in digestion, cellular respiration, and even buffering systems in the human body.