Rationalize the difference in boiling points in each pair: (a) HF (20 °C) and HCl (-85 °C) (b) CHCl3 (61 °C) and CHBr3 (150 °C) (c) Br2 (59 °C) and ICl (97 °C)

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Identify the types of intermolecular forces present in each compound. HF has hydrogen bonding, while HCl has dipole-dipole interactions and London dispersion forces. Hydrogen bonding is stronger than dipole-dipole interactions, leading to a higher boiling point for HF.

For CHCl3 and CHBr3, consider the molecular weight and the type of intermolecular forces. Both have dipole-dipole interactions and London dispersion forces, but CHBr3 has a higher molecular weight, leading to stronger London dispersion forces and a higher boiling point.

Examine the molecular structure and polarity of Br2 and ICl. Br2 is nonpolar with only London dispersion forces, while ICl is polar with dipole-dipole interactions in addition to London dispersion forces, resulting in a higher boiling point for ICl.

Discuss how molecular weight affects boiling points. Generally, as molecular weight increases, the strength of London dispersion forces increases, leading to higher boiling points.

Summarize the impact of intermolecular forces on boiling points: stronger intermolecular forces (hydrogen bonding, dipole-dipole interactions) result in higher boiling points compared to weaker forces (London dispersion forces).

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen. In the case of HF, the presence of hydrogen bonds significantly raises its boiling point compared to HCl, which lacks such strong intermolecular forces.

Molecular Weight and Van der Waals Forces

Molecular weight influences boiling points as larger molecules generally have stronger London dispersion forces due to increased surface area and polarizability. In the comparison of CHCl3 and CHBr3, the higher molecular weight of CHBr3 leads to stronger van der Waals forces, resulting in a higher boiling point.

Polarity and Dipole-Dipole Interactions

Polarity arises from differences in electronegativity between atoms, leading to dipole-dipole interactions in polar molecules. ICl is more polar than Br2, which is nonpolar, resulting in stronger intermolecular forces and a higher boiling point for ICl compared to Br2.

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