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 \(\mathrm{NH}_{3}\), which matches option (b).

Step 1: Identify the Base

First, identify the given base in the problem, which is \(\mathrm{NH}_{2}^{-}\). This is the amide ion, and we need to find its conjugate acid.

Step 2: Concept of Conjugate Acids and Bases

Recall that the conjugate acid of a base is formed by adding a proton (\(\mathrm{H}^{+}\)) to the base. For the base \(\mathrm{NH}_{2}^{-}\), we will add \(\mathrm{H}^{+}\) to find its conjugate acid.

Step 3: Add a Proton to the Base

Add one proton (\(\mathrm{H}^{+}\)) to \(\mathrm{NH}_{2}^{-}\). This forms \(\mathrm{NH}_{3}\). The reaction can be represented as: \(\mathrm{NH}_{2}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{NH}_{3}\).

Step 4: Verify the Solution Against Options

Check which option from the list matches with \(\mathrm{NH}_{3}\). Option (b) \(\mathrm{NH}_{3}\) is the matching answer.

Key Concepts

Amide Ion

The amide ion, denoted as \( \mathrm{NH}_{2}^{-} \), is an important concept in acid-base chemistry. It is considered a strong base, meaning it has a high tendency to accept protons (\( \mathrm{H}^{+} \)). This ion consists of a nitrogen atom bonded to two hydrogen atoms, with an additional shared pair of electrons giving it a negative charge. This negative charge is significant because it makes the nitrogen atom more eager to bond with a proton. If you were to examine its structure, the nitrogens’ lone pair of electrons is what primarily gives the amide ion its basic properties.

• The amide ion is commonly found in reactions where a strong base is needed.
• It is often used in organic chemistry to initiate various nucleophilic reactions.

Understanding how the amide ion behaves gives you insight into how it can both attract and attach a proton, forming its conjugate acid.

Proton Transfer

Proton transfer is a central mechanism in many acid-base reactions, where a proton \((\mathrm{H}^{+})\) moves from an acid to a base. In the context of the amide ion, this process involves the transfer of a proton to \( \mathrm{NH}_{2}^{-} \), resulting in the formation of \( \mathrm{NH}_{3} \) (ammonia). Consider this reaction:\[\mathrm{NH}_{2}^{-} + \mathrm{H}^{+} \rightarrow \mathrm{NH}_{3}\]This simple step, though small, is fundamental in chemistry as it transforms the properties of the substances involved. Proton transfer can occur quickly, and it's the basis for the concept of a conjugate acid-base pair where the initial substance gains a proton, becoming its conjugate acid.

• Proton transfer reactions are often reversible.
• They are crucial in maintaining the balance of reactions in biological and environmental systems.

Grasping the concept of proton transfer is essential for understanding the behavior of acids and bases in various contexts.

Acid-Base Reaction

Acid-base reactions involve the transfer of protons between reactants. In these reactions, the substance donating a proton is the acid, while the one accepting it is the base. The reaction between the amide ion \( \mathrm{NH}_{2}^{-} \) and a proton \( \mathrm{H}^{+} \) to form ammonia \( \mathrm{NH}_{3} \) is a classic example. This is how conjugate acid-base pairs are formed - one base gains a proton and becomes a conjugate acid.Understanding these reactions can be simplified by recognizing:

• The stronger the acid or base, the more readily they donate or accept protons.
• Conjugate acids and bases differ by one proton.

This broader view allows us to predict the outcomes of reactions, as well as understand their importance in chemical processes occurring in nature and industry. Recognizing acid-base reactions helps chemists devise methods for creating new compounds and materials.