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: Assess Melting Points

For statement (1) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{SbH}_{3}<\mathrm{NH}_{3}\), these phosphine group hydrides have their melting point primarily influenced by molecular weight, except for ammonia \(\mathrm{NH}_3\) due to hydrogen bonding. Hydrogen bonding in \(\mathrm{NH}_3\) makes it have a significantly higher melting point than expected by molecular weight alone, making the statement correct, as \(\mathrm{PH}_3, \, \mathrm{AsH}_3, \, \mathrm{SbH}_3\) should logically follow the general trend of increasing melting point with heavier molecular weight, except for the spike at \(\mathrm{NH}_3\).

Step 2: Review Boiling Points

Evaluate statement (2) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{NH}_{3}<\mathrm{SbH}_{3}\). The boiling points are influenced heavily by hydrogen bonding and molecular weight. Thus, \(\mathrm{NH}_3\) with significant hydrogen bonding, boils at higher temperatures than \(\mathrm{PH}_3\) and \(\mathrm{AsH}_3\). However, \(\mathrm{SbH}_3\), being larger and having more dispersion forces, should actually have a higher boiling point than all others. This order is incorrect because \(\mathrm{NH}_3\) should come last or second last after \(\mathrm{SbH}_3\).

Step 3: Check Dipole Moments

For statement (3), \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}>\mathrm{BiH}_{3}\), this order is correct according to their molecular geometry and electronegativity differences contributing to dipole moment. \(\mathrm{NH}_3\) has a strong dipole due to its hydrogen bonding and geometry, while the rest have weaker dipole moments primarily increased by decreasing electronegativity down the group.

Key Concepts

Melting Points

The melting point of a substance indicates the temperature at which it transitions from solid to liquid. For hydrides such as those in the phosphine group like \( \mathrm{PH}_3, \ \mathrm{AsH}_3, \ \mathrm{SbH}_3, \ \mathrm{NH}_3 \), several factors influence their melting points:

• **Molecular Weight:** Typically, the melting point increases with molecular weight. This is because molecules with a higher molecular weight have stronger intermolecular forces due to larger electron clouds.
• **Hydrogen Bonding:** This is especially relevant for \( \mathrm{NH}_3 \) (ammonia). Hydrogen bonds are a type of strong intermolecular attraction that significantly raises the melting point. Ammonia, having hydrogen bonds, has a much higher melting point than other phosphine-like hydrides.

The melting point order involves understanding that while heavier molecules have higher melting points, the presence of strong hydrogen bonds in ammonia defies this trend by significantly spiking the melting point above others in its group.

Boiling Points

The boiling point is the temperature at which a substance transitions from liquid to gas. The boiling points of hydrides in the same group can be affected by:

• **Molecular Size and Weight:** As molecular size and weight increase, so do the boiling points due to greater dispersion forces.
• **Hydrogen Bonding:** Substances like \( \mathrm{NH}_3 \) exhibit hydrogen bonding, making them boil at higher temperatures compared to similar molecular weights without hydrogen bonds.

For the given hydrides, the expected order should place \( \mathrm{NH}_3 \) at a higher boiling point due to its hydrogen bonding, despite the heavier molecular weight of \( \mathrm{SbH}_3 \) suggesting it should boil at an even higher temperature. This discrepancy points out an incorrect order if \( \mathrm{NH}_3 \) does not precede \( \mathrm{SbH}_3 \) in the boiling point sequence.

Dipole Moment

The dipole moment is an indicator of the polarity of a molecule, measured by the product of the charge difference and the distance between charges. For hydrides:

• **Molecular Shape:** This affects the dipole moment as symmetrical molecules have no net dipole.
• **Electronegativity:** The larger the difference in electronegativity between the central atom and hydrogen, the larger the dipole moment.

Ammonia \( \mathrm{NH}_3 \) has the greatest dipole moment due to its trigonal pyramidal shape and significant electronegativity difference with hydrogen. Other hydrides in the list see a decrease in dipole moment correlating with both the change in geometry and the decreasing electronegativity of the central atoms as one moves down the group. This order remains correct since \( \mathrm{NH}_3 \) exhibits the strongest relative dipole moment due to its hydrogen bonding capacity and molecular shape.