Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : PH₃ has lower boiling point than NH₃.
Reason (R) : In liquid state NH₃ molecules are associated through vander waal’s forces, but PH₃ molecules are associated through hydrogen bonding.
In the light of the above statements.
Choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is not the correct explanation of (A)
• B. (A) is not correct but (R) is correct
• C. Both (A) and (R) are correct but (R) is the correct explanation of (A)
• D. (A) is correct but (R) is not correct

Answer 1

The Correct Option is D

This question involves an analysis of the boiling points of ammonia (NH₃) and phosphine (PH₃) based on their intermolecular forces, presented as an assertion and a reason.

Concept Used:

Boiling point is the temperature at which a substance's vapor pressure equals the surrounding pressure, leading to its transition from liquid to vapor. Boiling point is directly proportional to the strength of intermolecular forces (IMFs). Stronger IMFs require more energy, thus a higher temperature, to overcome.

The relevant IMFs are:

1. Hydrogen Bonding: A strong dipole-dipole attraction occurring when hydrogen is bonded to a highly electronegative atom (N, O, or F). The partially positive hydrogen is attracted to a lone pair on an adjacent electronegative atom.
2. Van der Waals Forces: Weaker forces including dipole-dipole interactions and London dispersion forces, present in all molecules. They are dominant in nonpolar molecules or those lacking hydrogen bonding capabilities.

Step-by-Step Solution:

Step 1: Analyze the Assertion (A).

Assertion: "PH₃ has lower boiling point than NH₃."

Boiling points:

• Ammonia (NH₃) = -33.34 °C (239.81 K)
• Phosphine (PH₃) = -87.7 °C (185.45 K)

Since -87.7 °C is lower than -33.34 °C, PH₃ has a lower boiling point than NH₃. Assertion (A) is true.

Step 2: Analyze the Reason (R).

Reason: "In liquid state NH₃ molecules are associated through vander waal’s forces, but PH₃ molecules are associated through hydrogen bonding."

Intermolecular forces:

• NH₃: Nitrogen's high electronegativity and small size allow for strong intermolecular hydrogen bonds. Van der Waals forces are also present but secondary to hydrogen bonding.
• PH₃: Phosphorus's lower electronegativity and larger size result in less polar P-H bonds, precluding hydrogen bonding. The dominant IMFs are weaker van der Waals forces.

The Reason incorrectly assigns van der Waals forces to NH₃ and hydrogen bonding to PH₃. Reason (R) is false.

Step 3: Conclude the relationship between Assertion and Reason.

Assertion (A) is true, and Reason (R) is false. The true explanation for Assertion (A) is that NH₃'s higher boiling point is due to strong hydrogen bonding, which PH₃ lacks.

Final Result:

Assertion (A) is correct, and Reason (R) is incorrect.

Therefore, the appropriate answer is: (A) is true but (R) is false.