Select the substance in each pair that should have the...

Select the substance in each pair that should have the higher boiling point. In each case identify the principal intermolecular forces involved and account briefly for your choice: (a) $\mathrm{K}_{2} \mathrm{S}$ or $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N},(\mathrm{b}) \mathrm{Br}_{2}$ or $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$.

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Key Concepts

Ionic versus Covalent Interactions

This concept addresses how substances with ionic bonds exhibit very strong electrostatic attractions between oppositely charged ions, typically resulting in very high boiling points compared to covalent compounds where atoms share electrons. The energy required to overcome these ionic attractions in ionic substances far exceeds that needed to overcome the intermolecular forces present in many covalent compounds.

Intermolecular Forces and Boiling Points

The boiling point of a substance is directly related to the strength of the intermolecular forces between its particles. Stronger forces require more energy to separate the particles into the gas phase. This key concept encompasses various forces such as ionic forces, dipole-dipole interactions, hydrogen bonding, and London dispersion forces, and helps explain trends in boiling points across different types of substances.

London Dispersion Forces

London dispersion forces are weak forces present in all molecules, arising from momentary fluctuations in electron distribution that create temporary dipoles. Their strength increases with the number of electrons and the polarizability of the molecule. This concept is crucial for understanding differences in boiling points among nonpolar molecules, where dispersion forces may be the dominant intermolecular force.

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