Figure P 10.3 depicts molecules of XH3 and YH3 (not shown to scale) and the boiling points of XH

step 1 In order to determine which molecule is phosphine, which is pH3, and which one is ammonia, N -H

Figure P 10.3 depicts molecules of XH3 and YH3 (not shown to...

Figure $\mathrm{P} 10.3$ depicts molecules of $\mathrm{XH}_{3}$ and $\mathrm{YH}_{3}$ (not shown to scale) and the boiling points of $\mathrm{XH}_{3}$ and $\mathrm{YH}_{3}$ at 1 atm pressure. One substance is phosphine $\left(\mathrm{PH}_{3}\right)$ and the other substance is ammonia (NH$_{3}$). Which molecule is phosphine? Explain your answer.

Key Concepts

Intermolecular Forces

Molecular boiling points are directly affected by the intermolecular forces present between molecules. A substance with stronger intermolecular attractions (such as hydrogen bonding and dipole-dipole interactions) will have a higher boiling point than one with weaker forces. In the context of this problem, comparing the boiling points provides a way to deduce which molecule exhibits weaker interactions.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. Molecules capable of hydrogen bonding will generally exhibit higher boiling points due to the extra energy required to overcome these strong intermolecular forces. In this problem, NH? can engage in hydrogen bonding whereas PH? cannot, which is key to distinguishing the two.

Molecular Polarity

The polarity of a molecule is determined by the electronegativity differences between the atoms and the molecule's geometry. A polar molecule with a significant dipole moment is more likely to participate in strong intermolecular interactions, such as hydrogen bonding. In the case of ammonia (NH?) compared to phosphine (PH?), the greater electronegativity of nitrogen relative to phosphorus leads to stronger polar bonds and interactions, resulting in a higher boiling point for NH?.

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