Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is arranged in the increasing order of enthalpy of vaporization? (a) \(\mathrm{PH}_{3}, \mathrm{AsH}_{3}, \mathrm{NH}_{3}\) (b) \(\mathrm{AsH}_{3}, \mathrm{PH}_{3}, \mathrm{NH}_{3}\) (c) \(\mathrm{NH}_{3}, \mathrm{AsH}_{3}, \mathrm{PH}_{3}^{3}\) (d) \(\mathrm{NH}_{3}, \mathrm{PH}_{3}, \mathrm{AsH}_{3}^{3}\)

Step-by-Step Solution

Answer

The correct order is (a) \(\text{PH}_3, \text{AsH}_3, \text{NH}_3\).

Step 1: Understanding Enthalpy of Vaporization

Enthalpy of vaporization is the energy required to convert a liquid into a vapor at constant temperature and pressure. It is influenced by intermolecular forces: stronger intermolecular forces result in a higher enthalpy of vaporization.

Step 2: Analyze Molecular Structure and Forces

To predict enthalpy of vaporization, consider molecular structure and hydrogen bonding potential. - \( ext{NH}_3\) forms strong hydrogen bonds, leading to higher vaporization enthalpy.- \( ext{PH}_3\) and \( ext{AsH}_3\) have weaker van der Waals forces compared to hydrogen bonds.

Step 3: Compare Hydrogen Bonding

- \(\text{NH}_3\) can form hydrogen bonds, significantly increasing its enthalpy of vaporization.- \(\text{PH}_3\) and \(\text{AsH}_3\) don't form hydrogen bonds and primarily exhibit weaker forces like van der Waals, with \(\text{AsH}_3\) being heavier and having slightly stronger intermolecular forces than \(\text{PH}_3\) due to more electrons.

Step 4: Arrange by Enthalpy of Vaporization

Given the above analysis, arrange these substances in increasing order of enthalpy of vaporization. \(\text{PH}_3\) has the lowest due to weakest forces, followed by \(\text{AsH}_3\), and finally \(\text{NH}_3\) with the highest due to hydrogen bonding.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. These forces play a critical role in determining the physical properties of substances, such as boiling points and enthalpy of vaporization. There are several types of intermolecular forces:

• Van der Waals forces: These are the weakest type of intermolecular forces. They arise from temporary dipoles that occur when electrons are unevenly distributed in a molecule. This type of force is present in all molecules, but is more pronounced in larger or polarizable molecules.
• Dipole-dipole interactions: These forces occur between polar molecules with permanent dipoles. The positive end of one molecule attracts the negative end of another, leading to a moderate force of attraction.
• Hydrogen bonding: This is a special, stronger type of dipole-dipole interaction. It occurs when hydrogen atoms are bonded to electronegative atoms like nitrogen, oxygen, or fluorine. Hydrogen bonding significantly impacts the enthalpy of vaporization, as it requires more energy to break these bonds during phase changes from liquid to gas.

Understanding these forces helps predict why some substances, like ammonia (NH₃), have higher enthalpy of vaporization compared to others like phosphine (PH₃) and arsine (AsH₃).

Hydrogen Bonding

Hydrogen bonding is a dominant factor in determining enthalpy of vaporization for some substances. It occurs when a hydrogen atom, covalently bonded to an electronegative atom (like oxygen, nitrogen, or fluorine), experiences attraction to another electronegative atom with a lone electron pair. This interaction creates a significantly strong force that is challenging to overcome.
In the comparison of ammonia (NH₃), phosphine (PH₃), and arsine (AsH₃), ammonia stands out because it can form hydrogen bonds due to nitrogen's high electronegativity. This causes NH₃ to have a notably higher enthalpy of vaporization than PH₃ and AsH₃.
Why Hydrogen Bonds Matter:

• They significantly raise the boiling points of substances.
• They increase the energy required to convert from liquid to gas phases.
• Substances with hydrogen bonds are generally more stable in their liquid form.

Thus, NH₃ requires more energy to vaporize compared to PH₃ and AsH₃ which rely on weaker van der Waals intermolecular forces.

Molecular Structure

The molecular structure of a substance directly affects its intermolecular forces and consequently its enthalpy of vaporization. The structure can determine how molecules interact, align, and bond with each other.
NH₃ (Ammonia):

• Ammonia has a trigonal pyramidal structure.
• This structure allows nitrogen to form hydrogen bonds due to its high electronegativity.
• The presence of these bonds greatly increases its enthalpy of vaporization.

PH₃ and AsH₃ (Phosphine and Arsine):

• These molecules have similar molecular structures but form no hydrogen bonds.
• Instead, they exhibit weaker van der Waals forces.
• The molecular weight of AsH₃ is higher than PH₃, enhancing its van der Waals forces slightly more engagedly.

The differences in molecular structure and the presence or absence of hydrogen bonding dictate the energy needed for vaporization, resulting in NH₃ having the highest enthalpy of vaporization among them.