Describe the intermolecular forces that are present in each...

Describe the intermolecular forces that are present in each of the following compounds. Which kind of force would have the greatest influence on the properties of each compound? (a) ethanol, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$ (contains one $\mathrm{C}-\mathrm{O}$ single bond $)$ (b) phosphine, $\mathrm{PH}_{3}$; (c) sulfur hexafluoride, $\mathrm{SF}_{6}$.

Key Concepts

Dipole–Dipole Interactions

Dipole–dipole interactions occur between molecules that have permanent dipole moments; the positive end of one polar molecule attracts the negative end of another. These forces are of intermediate strength—stronger than dispersion forces but generally weaker than hydrogen bonds—and they play a key role in determining the physical properties of polar substances.

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are weak intermolecular attractions that occur in all molecules, whether polar or nonpolar. They arise from temporary fluctuations in electron density that create momentary dipoles. These forces become particularly significant in nonpolar molecules or larger atoms where the electron cloud is more polarizable.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole–dipole attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine, and this hydrogen is attracted to a lone pair of electrons on another electronegative atom in a nearby molecule. It significantly raises boiling and melting points, and is a dominant force in many compounds that have these structural features.

Molecular Polarity

Molecular polarity is determined by the electronegativity differences between atoms and the geometry of the molecule, which together influence the distribution of electrical charge. This polarity dictates which types of intermolecular forces—such as dipole–dipole interactions or hydrogen bonding—can occur and thus greatly influences the physical properties of the substance.

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