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

Step-by-Step Solution

Answer

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

Step 1: Identify the Base

To find the conjugate acid, we need to first identify the base in question. The given base is \( \mathrm{NH}_{2}^{-} \).

Step 2: Understand the Conjugate Acid-Base Pair Concept

A conjugate acid is formed when a base gains a proton (\( \mathrm{H}^{+} \)). Therefore, we need to add one proton to \( \mathrm{NH}_{2}^{-} \) to form its conjugate acid.

Step 3: Add a Proton to the Base

Add a proton (\( \mathrm{H}^{+} \)) to \( \mathrm{NH}_{2}^{-} \). The chemical reaction is: \[ \mathrm{NH}_{2}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{NH}_{3} \]

Step 4: Identify the Resulting Conjugate Acid

The addition of a proton to the base \( \mathrm{NH}_{2}^{-} \) results in the formation of \( \mathrm{NH}_{3} \). Thus, \( \mathrm{NH}_{3} \) is the conjugate acid of \( \mathrm{NH}_{2}^{-} \).

Step 5: Select the Correct Answer Choice

After identifying \( \mathrm{NH}_{3} \) as the conjugate acid, compare it with the given options. Option (b) \( \mathrm{NH}_{3} \) matches the correct conjugate acid.

Key Concepts

Proton Transfer

Proton transfer is the fundamental process that occurs in acid-base chemistry. This simple yet crucial mechanism involves a proton, which is essentially a hydrogen ion (\(\mathrm{H}^{+}\)), being transferred from one molecule to another. This exchange is what defines an acid-base reaction.
In our context, the base \(\mathrm{NH}_{2}^{-}\) is capable of accepting a proton to become \(\mathrm{NH}_{3}\).
Think of a proton as a ball being passed between two people, where \(\mathrm{NH}_{2}^{-}\) holds its hands out to catch the ball. This transfer leads to the formation of the conjugate acid, which is the base plus the proton.
A quick summary of proton transfer involves:

• A base acquiring a proton.
• The formation of a conjugate acid from that base.

Acid-Base Reactions

Acid-base reactions are all about the transfer of protons. In these reactions, you have two main players: acids and bases. An acid is a substance that donates a proton, while a base is one that accepts it.
These reactions are essential because they help to form what's called a conjugate acid-base pair.
Let's look at this in our example:

• The base, \(\mathrm{NH}_{2}^{-}\), accepts a proton from an acid, forming \(\mathrm{NH}_{3}\)
• The acid doesn’t need to be specified but plays an essential role in providing that proton.

This results in creating a conjugate acid-base pair, demonstrating the balance and exchange at the heart of acid-base chemistry.

Chemical Equations

Chemical equations are a concise way of representing chemical reactions on paper. For acid-base reactions, they show the movement of protons clearly. In our example, we see the chemical equation:\[\mathrm{NH}_{2}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{NH}_{3}\]
This equation tells us a few things:

• It begins with reactants, \(\mathrm{NH}_{2}^{-}\) and \(\mathrm{H}^{+}\), on the left side.
• These reactants interact to form products, like \(\mathrm{NH}_{3}\), on the right side.

Chemical equations not only highlight what substances you start and end with but also make it easy to track changes during the reaction, such as the gaining or losing of protons.