Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Amongst \(\mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{~S}, \mathrm{H}_{2} \mathrm{Se}\) and \(\mathrm{H}_{2} \mathrm{Te}\), the one with the highest boiling point is [2000S] (a) \(\mathrm{H}_{2} \mathrm{O}\) because of hydrogen bonding (b) \(\mathrm{H}_{2}\) Te because of higher molecular weight (c) \(\mathrm{H}_{2} \mathrm{~S}\) because of hydrogen bonding (d) \(\mathrm{H}_{2} \mathrm{Se}\) because of lower molecular weight

Step-by-Step Solution

Answer

(a) \(\mathrm{H_2O}\) due to hydrogen bonding.

Step 1: Understanding Boiling Point Trends

Molecules with stronger intermolecular forces generally have higher boiling points. In the case of hydrides of Group 16 elements like \(\mathrm{H_2O}\), \(\mathrm{H_2S}\), \(\mathrm{H_2Se}\), and \(\mathrm{H_2Te}\), the boiling point is influenced by hydrogen bonding, dipole-dipole interactions, and molecular weight.

Step 2: Analyzing Influence of Hydrogen Bonding

Hydrogen bonding significantly increases the boiling point of a compound. \(\mathrm{H_2O}\) is the only molecule among the given options that forms hydrogen bonds, due to the high electronegativity of oxygen. This results in a much higher boiling point for \(\mathrm{H_2O}\) compared to the other molecules, which involve weaker van der Waals forces or dipole-dipole interactions without the presence of hydrogen bonds.

Step 3: Considering Molecular Weight

Although molecular weight can affect boiling points, it is less significant than hydrogen bonding. \(\mathrm{H_2Te}\) has the heaviest molecular weight among the options listed, but the absence of hydrogen bonding makes its boiling point lower than \(\mathrm{H_2O}\). Therefore, while heavier molecules often have higher boiling points, hydrogen bonding in \(\mathrm{H_2O}\) is the dominant factor.

Step 4: Conclusion

Considering both hydrogen bonding and molecular weight, \(\mathrm{H_2O}\) has the highest boiling point. The significant hydrogen bonding in water outweighs the effect of increased molecular weight found in \(\mathrm{H_2Te}\). Thus, the correct choice is (a) \(\mathrm{H_2O}\) because of hydrogen bonding.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that plays a significant role in determining the boiling points of substances. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom. This bond is particularly strong compared to other dipole-dipole interactions, which contributes to higher boiling points.

In the list of compounds,

• \(\mathrm{H}_{2} \mathrm{O}\) can form extensive hydrogen bonds because of the high electronegativity of oxygen. This gives water its relatively high boiling point compared to its molecular weight.
• The other molecules like \(\mathrm{H}_{2} \mathrm{S}\), \(\mathrm{H}_{2} \mathrm{Se}\), and \(\mathrm{H}_{2} \mathrm{Te}\) do not exhibit hydrogen bonding because sulfur, selenium, and tellurium are not electronegative enough.

Therefore, hydrogen bonding significantly increases water's boiling point, making it higher than the other molecules under consideration.

Molecular Weight

Molecular weight refers to the mass of a molecule and can affect physical properties like boiling points. In general, heavier molecules tend to have higher boiling points due to stronger van der Waals forces (induced dipole-induced dipole interactions) that increase with increased molecular size.

Among the options,

• \(\mathrm{H}_{2} \mathrm{Te}\) has the highest molecular weight, as tellurium is heavier than sulfur or selenium.
• Typically, one might expect \(\mathrm{H}_{2} \mathrm{Te}\) to have a higher boiling point based on molecular weight alone. However, the absence of hydrogen bonding in \(\mathrm{H}_{2} \mathrm{Te}\) diminishes its effect.

While molecular weight is indeed a factor in boiling points, it's less significant here because it fails to overcome the strong hydrogen bonding present in water.

Intermolecular Forces

Intermolecular forces are attractions between molecules that determine many physical properties, including boiling points. The main types are:

• **Van der Waals Forces**: These are weaker forces that include attractions called dispersion forces and dipole-dipole interactions.
• **Dipole-Dipole Interactions**: Stronger than van der Waals forces, these occur between molecules with permanent dipoles.
• **Hydrogen Bonding**: The strongest type, significant in molecules like water where hydrogen is bonded to highly electronegative atoms.

Among the molecules in question, water's hydrogen bonds are the strongest intermolecular forces present, significantly raising its boiling point. Meanwhile, the other molecules primarily exhibit weaker intermolecular forces. Due to water's capacity to form hydrogen bonds, its boiling point is much higher, despite having a lower molecular weight than \(\mathrm{H}_{2} \mathrm{Te}\). This example emphasizes the dominance of hydrogen bonding over other intermolecular forces when determining boiling points.