Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

The strongest base is (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{PH}_{3}\) (c) \(\mathrm{AsH}_{3}\) (d) \(\mathrm{SbH}_{3}\)

Step-by-Step Solution

Answer

The strongest base is \(\mathrm{NH}_{3}\).

Step 1: Understand Basicity

Basicity refers to the ability of a compound to accept protons. In the context of this problem, we are comparing the basicity of different hydrides of Group 15 elements: ammonia (\(\text{NH}_3\)), phosphine (\(\text{PH}_3\)), arsine (\(\text{AsH}_3\)), and stibine (\(\text{SbH}_3\)).

Step 2: Analyze the Group 15 Trend

In Group 15 of the periodic table, as we move down the group from nitrogen to antimony, the atomic size increases. As the size increases, the lone pair of electrons becomes less available for proton acceptance due to being held less tightly by the nucleus.

Step 3: Determine Basic Strength

Ammonia (\(\text{NH}_3\)) is known to be the strongest base among these compounds. This is because nitrogen is smaller than the other elements, resulting in a stronger availability of its lone pair of electrons to accept protons compared to the larger phosphorus, arsenic, and antimony atoms.

Key Concepts

Group 15 Hydrides

Group 15 of the periodic table contains elements like nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). The hydrides formed by these elements—namely ammonia ( N_{}), phosphine ( P_{}), arsine ( As_{}), stibine ( Sb_{}), and bismuthine ( BiH_{})—are known for their capacity to act as bases.
These compounds are called hydrides because they contain hydrogen atoms bonded to a more electronegative element. Each of these hydrides has distinct chemical properties, largely influenced by the element to which hydrogen is attached.

• Ammonia ( N_{}) is the smallest and most widely studied in this group.
• Phosphine ( P_{}) and arsine ( As_{}) are less commonly encountered but important in various industrial applications.
• Stibine ( Sb_{}) and bismuthine are relatively rare and less stable.

Understanding the behavior of these hydrides is essential for assessing their basicity and their role in chemical reactions.

Periodic Table Trends

One of the key concepts in understanding chemical properties is recognizing trends in the periodic table. These trends significantly influence the behavior and reactivity of elements and their compounds.
In Group 15, as you move from the top (nitrogen) to the bottom (antimony), several trends are evident:

• Atomic Size: Each element has an increasing atomic size due to the addition of electron shells.
• Electronegativity: Typically decreases, meaning the ability to attract electrons weakens down the group.
• Bond Strength: The strength of the bond between hydrogen and the element weakens as the size of the element increases.

These trends affect how these elements form bonds and their ability to accept protons, influencing their basicity. For instance, smaller atoms like nitrogen can hold electrons more tightly, which is why ammonia is a stronger base compared to others in the group.

Proton Acceptance in Bases

The core of basicity lies in a compound's ability to accept protons (H^{+}). In case of Group 15 hydrides, this is primarily determined by the lone pair of electrons available on the central atom, ready to bond with a proton.
Factors influencing a base's ability to accept protons include:

• Atomic Size: Larger atomic size usually results in a less concentrated electron cloud, weakening the lone pair's ability to accept protons.
• Electronegativity: More electronegative elements hold their lone pairs more tightly, facilitating easier proton acceptance.
• Availability of Lone Pairs: Compounds with more readily available lone pairs on a compact and electronegative atom tend to be stronger bases.

Among the Group 15 hydrides, ammonia is the strongest base. This is because the nitrogen atom is relatively small and highly electronegative, making its lone pair readily available for proton acceptance compared to phosphorus, arsenic, and antimony.