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 requires an analysis of the boiling points of ammonia (NH₃) and phosphine (PH₃) in the context of their intermolecular forces, as described in an assertion and a corresponding reason.

Concept Used:

The boiling point of a substance is the temperature at which its vapor pressure equals the pressure surrounding the liquid, and the liquid changes into a vapor. The boiling point is directly related to the strength of the intermolecular forces (IMFs) between the molecules. Stronger IMFs require more energy (and thus a higher temperature) to overcome, leading to a higher boiling point.

The primary types of intermolecular forces relevant here are:

1. Hydrogen Bonding: A special, strong type of dipole-dipole attraction that occurs when hydrogen is bonded to a highly electronegative atom (like Nitrogen, Oxygen, or Fluorine). The hydrogen atom carries a significant partial positive charge and is attracted to the lone pair of electrons on an adjacent electronegative atom.
2. Van der Waals Forces: These are weaker forces that include dipole-dipole interactions and London dispersion forces. They exist in all molecules but are the dominant forces in nonpolar molecules or molecules that cannot form hydrogen bonds.

Step-by-Step Solution:

Step 1: Analyze the Assertion (A).

The assertion states: "PH₃ has lower boiling point than NH₃."

Let's compare the experimentally determined boiling points of these two compounds:

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

Since -87.7 °C is a lower temperature than -33.34 °C, the boiling point of PH₃ is indeed lower than that of NH₃. Therefore, the statement made in Assertion (A) is true.

Step 2: Analyze the Reason (R).

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

Let's examine the intermolecular forces present in each liquid:

• In liquid NH₃: Nitrogen is a small and highly electronegative atom. The N-H bonds are very polar. Consequently, NH₃ molecules can form strong intermolecular hydrogen bonds with each other. While van der Waals forces also exist, hydrogen bonding is the dominant and much stronger force.
• In liquid PH₃: Phosphorus is significantly less electronegative than nitrogen (electronegativity of P ≈ 2.19, N ≈ 3.04). The P-H bond has very little polarity. Due to this low polarity and the larger size of the phosphorus atom, PH₃ molecules are incapable of forming hydrogen bonds. The primary intermolecular forces in liquid PH₃ are the much weaker van der Waals forces (specifically, dipole-dipole and London dispersion forces).

The statement in the Reason claims the opposite: it incorrectly assigns van der Waals forces to NH₃ and hydrogen bonding to PH₃. Therefore, the statement made in Reason (R) is false.

Step 3: Conclude the relationship between Assertion and Reason.

We have established that Assertion (A) is a true statement, but Reason (R) is a false statement. The actual reason for Assertion (A) being true is that NH₃ has a higher boiling point due to the presence of strong intermolecular hydrogen bonds, which are absent in PH₃.

Final Result:

Based on the analysis, Assertion (A) is a correct statement, but Reason (R) is an incorrect statement.

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