Rationalize the difference in boiling points between the...

Rationalize the difference in boiling points between the members of the following pairs of substances: (a) HF $\left(20^{\circ} \mathrm{C}\right)$ and $\mathrm{HCl}\left(-85{ }^{\circ} \mathrm{C}\right)$, (b) $\mathrm{CHCl}_{3}\left(61{ }^{\circ} \mathrm{C}\right)$ and $\mathrm{CHBr}_{3}$ $\left(150^{\circ} \mathrm{C}\right)$ (c) $\mathrm{Br}_{2}\left(59^{\circ} \mathrm{C}\right)$ and $\mathrm{ICl}\left(97{ }^{\circ} \mathrm{C}\right)$

Rationalize the difference in boiling points between the members of the following pairs of substances: (a) HF $\left(20^{\circ} \mathrm{C}\right)$ and $\mathrm{HCl}\left(-85{ }^{\circ} \mathrm{C}\right)$, (b) $\mathrm{CHCl}_{3}\left(61{ }^{\circ} \mathrm{C}\right)$ and $\mathrm{CHBr}_{3}$
$\left(150^{\circ} \mathrm{C}\right)$
(c) $\mathrm{Br}_{2}\left(59^{\circ} \mathrm{C}\right)$ and $\mathrm{ICl}\left(97{ }^{\circ} \mathrm{C}\right)$

Key Concepts

Molecular Size and Polarizability

Molecular size and polarizability are key factors in determining the strength of London dispersion forces. Larger and more polarizable molecules have electron clouds that can more easily distort, leading to stronger instantaneous dipoles and greater dispersion forces. This results in higher boiling points, as more energy is required to overcome these interactions.

London Dispersion Forces

London dispersion forces are weak intermolecular forces that arise from temporary fluctuations in electron density, which create transient dipoles even in non-polar molecules. The strength of these forces becomes more significant as the size and electron cloud polarizability of the molecule increases, contributing to higher boiling points in larger or heavier molecules.

Hydrogen Bonding

Hydrogen bonding is a strong type of intermolecular attraction that occurs when hydrogen is bonded to a highly electronegative atom, such as fluorine, oxygen, or nitrogen, and interacts with a lone pair on a neighboring molecule. This force significantly increases the boiling points of compounds due to the additional energy needed to break these bonds.

Intermolecular Forces

Intermolecular forces are the attractive interactions between molecules that influence physical properties such as boiling points. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Their relative strength and presence determine how much energy is required to separate molecules, thereby affecting the boiling point of a substance.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that possess permanent dipoles. These forces arise from the electrostatic attraction between the positive end of one polar molecule and the negative end of another. They contribute to an increase in boiling points when compared to non-polar molecules that do not have these interactions.

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