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

The incorrect order is in statement (1).

Step 1: Understanding the Physical Properties

To solve this exercise, we need to evaluate the correctness of the given trends for melting point, boiling point, and dipole moment. Each property is affected by different molecular characteristics.

Step 2: Analyzing Melting Points

Melting point depends on the molecular mass and intermolecular forces. Typically, larger molecules with greater mass tend to have higher melting points, unless hydrogen bonding significantly alters the trend.

Step 3: Evaluating Statement 1

The given order for melting point is \(\text{PH}_3 < \text{AsH}_3 < \text{SbH}_3 < \text{NH}_3\). This is incorrect, as \(\text{NH}_3\) has a higher melting point than the others due to hydrogen bonding, but the correct trend should have \(\text{SbH}_3\) as higher than \(\text{NH}_3\) due to its larger mass and weaker hydrogen bonding in \(\text{NH}_3\). The correct order should be \(\text{NH}_3 > \text{SbH}_3 > \text{AsH}_3 > \text{PH}_3\).

Step 4: Analyzing Boiling Points

Boiling points generally increase with molecular weight, but NH3's hydrogen bonding makes it an exception. Checking the order \(\text{PH}_3 < \text{AsH}_3 < \text{NH}_3 < \text{SbH}_3\), hydrogen bonding gives \(\text{NH}_3\) a higher boiling point than would otherwise be expected. In this context, the order is correct as \(\text{NH}_3\) should indeed come before \(\text{SbH}_3\) in boiling point due to strong hydrogen bonds.

Step 5: Evaluating Statement 3

Dipole moments are influenced by the electronegativity differences and molecular shape. The order \(\text{NH}_3 > \text{PH}_3 > \text{AsH}_3 > \text{SbH}_3 > \text{BiH}_3\) is correct as \(\text{NH}_3\) has the highest dipole moment due to the high electronegativity of nitrogen and its trigonal pyramidal shape enhancing dipole moment, compared to the other molecules.

Step 6: Conclusion

Statement 1 is clearly incorrect as the order stated does not account for the significant increase in melting point due to \(\text{NH}_3\)'s hydrogen bonding. Statement 2 and 3, however, are correct when considering the specific influences on boiling point and dipole moment.

Key Concepts

Melting Point

The melting point of a substance is the temperature at which it transitions from a solid to a liquid. One key factor affecting melting point is molecular mass. Generally, as molecular mass increases, so does the melting point. However, another crucial player is inter molecular forces, like hydrogen bonding.

For compounds like ammonia (\( ext{NH}_3\)) and the hydrogen chalcogenides (\( ext{PH}_3, ext{AsH}_3, ext{SbH}_3\)), this means that although ammonia might have a lower molecular mass, its strong hydrogen bonds significantly raise its melting point. Compare that to larger but less interactive molecules like \( ext{SbH}_3\), where mass contributes more dominantly than hydrogen bonding.

• This explains why ammonia’s melting point is actually higher than that of \( ext{SbH}_3\), despite its smaller size.

Understanding these dynamics helps correct the misconception in statement 1 of the original exercise. The order should reflect ammonia's influential hydrogen bonds, placing \( ext{NH}_3\) higher than \( ext{SbH}_3\) due to this overarching effect.

Boiling Point

Boiling point determines when a liquid changes into gas. Several factors affect it, including molecular weight and inter molecular forces. While higher molecular weight can mean higher boiling points, strong inter molecular forces might change that expectation. Hydrogen bonding serves as a prime example of this effect.

In the case of \( ext{NH}_3\), its ability to form hydrogen bonds plays a substantial role. Hydrogen bonds make it more energy-intensive to break molecular forces and transition to the gaseous state. Therefore, ammonia, despite having a lower molecular mass than compounds like \( ext{SbH}_3\), exhibits a high boiling point due to this bonding.

• It's why the trend in statement 2, where \( ext{NH}_3\) comes before \( ext{SbH}_3\), holds true and showcases hydrogen bonding's influence.

With hydrogen bonds affecting ammonia so distinctly, we learn to appreciate boiling point not just in mass terms but as a reflection of molecular force interactions.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. Molecules with asymmetric shapes and significant differences in electronegativity between bonded atoms tend to have larger dipole moments.

Take \( ext{NH}_3\) (ammonia), for example, it has a pronounced dipole moment due to its trigonal pyramidal shape and significant electronegativity of nitrogen. The shape causes the electrons to focus more on one side, creating a distinct positive and negative end. Other molecules like \( ext{PH}_3, ext{AsH}_3\), and \( ext{SbH}_3\) lack this combination of shape and electronegativity interplay.

• The order from the problem reflects these concepts well, acknowledging ammonia's exceptional dipole moment compared to the heavier and less electronegative counterparts.

This insight into dipole moments also emphasizes how molecular geometry and element properties define a molecule's polarity and its chemical behavior.