Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following order is incorrect? (1) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{SbH}_{3}<\mathrm{NH}_{3}\) (Melting point) (2) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{NH}_{3}<\mathrm{SbH}_{3}\) (Boiling point) (3) \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}>\mathrm{BiH}_{3}\) (Dipole moment) (a) only 1 (b) only 2 (c) both (1) and (2) (d) none is incorrect here

Step-by-Step Solution

Answer

Only (2) is incorrect.

Step 1: Identify Property Trends

Each order given in the problem corresponds to a different property: melting point, boiling point, and dipole moment. Chemical properties like these tend to follow periodic trends, but they can vary significantly based on molecular structure and interactions.

Step 2: Analyze Melting Point Order

For the melting point order: (1) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{SbH}_{3}<\mathrm{NH}_{3}\). Generally, for group 15 hydrides, the melting points increase down the group except for \(\mathrm{NH}_{3}\) due to hydrogen bonding, which can cause a higher melting point than the heavier group 15 hydrides. This order is generally true, as \(\mathrm{NH}_{3}\) has a much higher melting point due to strong hydrogen bonding.

Step 3: Evaluate Boiling Point Order

For the boiling point order: (2) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{NH}_{3}<\mathrm{SbH}_{3}\). \(\mathrm{NH}_{3}\) has significant hydrogen bonding, raising its boiling point above those of heavier hydrides \(\mathrm{PH}_{3}\) and \(\mathrm{AsH}_{3}\), but \(\mathrm{SbH}_{3}\), being heavier, should generally have a higher boiling point than \(\mathrm{NH}_{3}\). The given order in (2) is incorrect because \(\mathrm{NH}_{3}\)'s boiling point should be higher than \(\mathrm{SbH}_{3}\) due to the hydrogen bonding effects.

Step 4: Consider Dipole Moment Order

For the dipole moment order: (3) \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}>\mathrm{BiH}_{3}\). The dipole moment order largely depends on the electronegativity of the central atom and molecular symmetry. \(\mathrm{NH}_{3}\) has the highest dipole moment due to its geometry and nitrogen's electronegativity, confirming the correctness of this order.

Step 5: Determine the Incorrect Order

After evaluating all the options, it's clear that order (2) for boiling points is incorrect. \(\mathrm{NH}_{3}\) should have a higher boiling point than \(\mathrm{SbH}_{3}\) because of the effective hydrogen bonding in \(\mathrm{NH}_{3}\).

Key Concepts

Melting Point Trends

The melting point of group 15 hydrides changes as you move down the group. Generally speaking, one might expect the melting points to increase due to the heavier mass of the elements. However, ammonia (\(\mathrm{NH}_3\)) disrupts this trend due to its ability to form strong hydrogen bonds.

These hydrogen bonds significantly elevate the melting point of \(\mathrm{NH}_3\) compared to the other group 15 hydrides like phosphine (\(\mathrm{PH}_3\)), arsine (\(\mathrm{AsH}_3\)), stibine (\(\mathrm{SbH}_3\)), and bismuthine (\(\mathrm{BiH}_3\)). In summary, the increasing strength of hydrogen bonding in ammonia causes its melting point to be higher than what might be expected from its molecular weight alone.

The correct order of melting points, taking hydrogen bonding into account, is indeed as follows: \(\mathrm{PH}_3 < \mathrm{AsH}_3 < \mathrm{SbH}_3 < \mathrm{NH}_3\).

Boiling Point Trends

When considering boiling points, particularly in group 15 hydrides, the mass of the molecules plays an important role, yet intermolecular forces such as hydrogen bonding significantly influence the outcome.

In this group, ammonia (\(\mathrm{NH}_3\)) again stands out due to its hydrogen bonding capabilities, which enhance its boiling point despite its relatively low mass.

Although heavier hydrides like stibine (\(\mathrm{SbH}_3\)) have larger masses, the role of hydrogen bonding in \(\mathrm{NH}_3\) is more significant, suggesting that ammonia should have a higher boiling point than stibine, as it takes more energy to disrupt these bonds.

This insight corrects the original statement, indicating that the comparison made was incorrect. The revised and correct boiling point order should be: \(\mathrm{PH}_3 < \mathrm{AsH}_3 < \mathrm{SbH}_3 < \mathrm{NH}_3\).

Dipole Moments

The dipole moment of a molecule can be understood through the lens of electronegativity and molecular shape. In group 15 hydrides, nitrogen in \(\mathrm{NH}_3\) has the highest electronegativity, forming a highly polar molecule.

Thus, \(\mathrm{NH}_3\) exhibits the highest dipole moment among the group, due to the electronegativity difference between nitrogen and hydrogen, and its trigonal pyramidal structure that ensures a strong net dipole.

As we move down the group to phosphine (\(\mathrm{PH}_3\)), arsine (\(\mathrm{AsH}_3\)), stibine (\(\mathrm{SbH}_3\)) and bismuthine (\(\mathrm{BiH}_3\)), the dipole moment decreases. This happens because the central atom's electronegativity decreases and their bonds become less polar.

Thus, the order of dipole moments, \(\mathrm{NH}_3 > \mathrm{PH}_3 > \mathrm{AsH}_3 > \mathrm{SbH}_3 > \mathrm{BiH}_3\), is accurate.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that significantly affects the physical properties of molecules, like boiling and melting points.

In group 15 hydrides, \(\mathrm{NH}_3\) is the unique member capable of hydrogen bonding, facilitated by nitrogen's high electronegativity and small size. These bonds are much stronger than usual van der Waals forces present in other hydrides.

Hydrogen bonds result in \(\mathrm{NH}_3\) having unusually high melting and boiling points compared to other group 15 hydrides. These effects are sturdy enough to overrule the otherwise prevailing trend of increasing boiling and melting points with mass in the group.

This explains why ammonia is often an anomaly in ordering trends based solely on molecular weight.

Chemical Properties of Hydrides

The chemical properties of group 15 hydrides are influenced by the periodic nature of the elements involved. Each hydride exhibits characteristic traits based on its central atom's electronegativity and molecular geometry.

For example, \(\mathrm{NH}_3\) is basic due to the nitrogen's ability to donate its lone pair, whereas heavier hydrides like \(\mathrm{BiH}_3\) tend to be less basic.

The reactivity also varies down the group. As we move from \(\mathrm{NH}_3\) to \(\mathrm{BiH}_3\), the bond strength between the central atom and hydrogen decreases due to larger atomic radii and weaker overlap of orbitals.

This affects both chemical behaviors and their stability, making heavier hydrides generally less stable. Understanding these properties is essential for predicting the behavior of these compounds in different chemical contexts.