Problem 53 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Based on the compounds of group 15 elements, the correct statement(s) is (are) [Adv. 2018](a) \(\mathrm{Bi}_{2} \mathrm{O}_{5}\) is more basic than \(\mathrm{N}_{2} \mathrm{O}_{5}\) (b) \(\mathrm{NF}_{3}\) is more covalent than \(\mathrm{BiF}_{3}\) (c) \(\mathrm{PH}_{3}\) boils at lower temperature than \(\mathrm{NH}_{3}\) (d) The \(\mathrm{N}-\mathrm{N}\) single bond is stronger than the P-P single bond

Step-by-Step Solution

Answer

(a), (b), and (c) are correct statements.

Step 1: Analyzing Bi2O5 and N2O5

Group 15 elements generally form oxides that change from acidic to basic as we move down the group. Nitrogen, being at the top, forms acidic oxides like \( \mathrm{N}_{2}\mathrm{O}_{5} \), whereas bismuth, at the bottom, forms more basic compounds like \( \mathrm{Bi}_{2}\mathrm{O}_{5} \). Thus, \( \mathrm{Bi}_{2} \mathrm{O}_{5} \) is more basic than \( \mathrm{N}_{2} \mathrm{O}_{5} \). Hence, statement (a) is correct.

Step 2: Covalency of NF3 and BiF3

Covalency is influenced by electronegativity and the size of the central atom. Nitrogen is smaller and more electronegative compared to bismuth, leading \( \mathrm{NF}_{3} \) to be more covalent due to better overlap and electron sharing compared to the ionic character of \( \mathrm{BiF}_{3} \). Hence, statement (b) is correct.

Step 3: Boiling Points of PH3 and NH3

Boiling points are affected by molecular interactions. \( \mathrm{NH}_{3} \) has hydrogen bonding due to the high electronegativity of nitrogen, resulting in a higher boiling point compared to \( \mathrm{PH}_{3} \), where hydrogen bonds are absent. Therefore, \( \mathrm{PH}_{3} \) boils at a lower temperature than \( \mathrm{NH}_{3} \). Statement (c) is correct.

Step 4: N-N vs P-P Bond Strength

The single bond between heavier atoms tends to be weaker due to diminished orbital overlap. Thus, the \( \mathrm{P-P} \) bond, involving larger atoms compared to \( \mathrm{N-N} \), is weaker. The \( \mathrm{N-N} \) bond is indeed stronger, making statement (d) incorrect.

Key Concepts

Acidic and Basic Oxides

In the realm of Group 15 elements, oxides exhibit a fascinating trend where their properties change as one moves down the group. Oxides of these elements transition from acidic to basic. At the top of the group is nitrogen, which forms acidic oxides such as \( \mathrm{N}_2\mathrm{O}_5 \). This is because nitrogen oxides generally contain a higher charge density and a tendency to form particular acids, thus showcasing their acidic nature.

On the contrary, as we descend the group, the oxides of heavier elements like bismuth become more basic. The compound \( \mathrm{Bi}_2\mathrm{O}_5 \) is an example of this basic characteristic. The increase in basicity can be attributed to the larger atomic size making it less electronegative and the reduced oxidation state of bismuth, leading \( \mathrm{Bi}_2\mathrm{O}_5 \) to show metallic character. As a result, bismuth's oxides react with acids more readily, highlighting their basic nature. This transition from acidic to basic is a common property observed as the metallic character increases down the group.

Covalency and Electronegativity

Understanding covalency in Group 15 elements requires a look at electronegativity and atomic size. Covalent bonds form more readily between elements that exhibit similar electronegativities, allowing atoms to share electrons equally and hence form stable covalent compounds.

Nitrogen, which is small and highly electronegative, readily forms covalent bonds, as seen in \( \mathrm{NF}_3 \). This compound is more covalent than \( \mathrm{BiF}_3 \) because the overlap of orbitals between nitrogen and fluorine allows better electron sharing. Bismuth, being less electronegative and larger, forms bonds with greater ionic character, making \( \mathrm{BiF}_3 \) less covalent. The transition from covalent to more ionic character reflects the decreasing electronegativity and increasing atomic size down the group.

Boiling Points and Intermolecular Forces

Boiling points are crucial for understanding the molecular interactions in Group 15 hydrides. The key to boiling point variations between \( \mathrm{PH}_3 \) and \( \mathrm{NH}_3 \) lies in the nature of intermolecular forces. \( \mathrm{NH}_3 \) exhibits hydrogen bonding due to nitrogen's high electronegativity. This interaction is a strong intermolecular force that raises \( \mathrm{NH}_3 \)'s boiling point significantly compared to other Group 15 hydrides.

In contrast, \( \mathrm{PH}_3 \) lacks such strong hydrogen bonds due to phosphorus being less electronegative and larger, causing it to rely on weaker van der Waals forces for intermolecular attraction. These weaker interactions in \( \mathrm{PH}_3 \) result in a lower boiling point compared to \( \mathrm{NH}_3 \), as less energy is required to break these weak intermolecular forces and convert \( \mathrm{PH}_3 \) from liquid to gas.

Bond Strength and Orbital Overlap

In exploring bond strength within Group 15, particularly the single bond's strength, orbital overlap is a pivotal factor. Strong bonds are characterized by effective orbital overlap, which is easier to achieve with smaller atoms such as nitrogen. Therefore, the \( \mathrm{N-N} \) single bond is stronger than the \( \mathrm{P-P} \) bond.

The reduced bond strength of \( \mathrm{P-P} \) is due to phosphorus's larger atomic size. Larger atoms have more diffused orbitals, which do not overlap as efficiently as those of smaller atoms. This inefficiency leads to weaker bonding. As elements in Group 15 increase in size, the overlap efficiency decreases, showing a trend of weakened single bond strengths as we move from \( \mathrm{N-N} \) to \( \mathrm{P-P} \). This concept highlights why stronger bonding is usually associated with smaller, more electronegative atoms.