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Chapter 22: Problem 88

Despite the fact that it has the higher molecular mass, XeO \(_{4}\) exists as a gas at \(298 \mathrm{K},\) whereas \(\mathrm{XeO}_{3}\) is a solid. Give a plausible explanation for this observation.

Short Answer

Expert verified
The observed difference in physical states of \(\mathrm{XeO}_{3}\) and \(\mathrm{XeO}_{4}\) at 298 K can be explained by their differences in molecule polarity. Though \(\mathrm{XeO}_{4}\) has a higher molecular mass, it is a non-polar molecule while \(\mathrm{XeO}_{3}\) is polar. Non-polar molecules have weaker intermolecular forces and thus require less energy to break, hence \(\mathrm{XeO}_{4}\) exists as a gas.

Step by step solution

01

Identify the molecular geometry of each compound

For \(\mathrm{XeO}_{4}\), the central atom (Xe) has 8 valence electrons, and each O atom contributes 1 electron for bonding. Hence, the molecule obeys the octet rule with all Oxygen atoms forming double bonds with the Xe atom. This leads to a square planar geometry. Similarly, in \(\mathrm{XeO}_{3}\), Xe has 4 outer shell electrons and each Oxygen shares 1 each. The fourth pair is unbonded (lone pair). This gives a pyramidal shape to the molecule.
02

Determine the Polarity of each compound

The polarity of a molecule is determined by its geometry and electronegativity difference between the atoms. In \(\mathrm{XeO}_{4}\) with either 4 bonding pairs and no lone pairs the molecule is symmetrical and hence it is non-polar. In contrast, \(\mathrm{XeO}_{3}\) is polar because of its pyramidal structure that results in an unequal distribution of electrons.
03

Relate the phase of each compound to their polarity

Polar molecules are generally associated with higher melting and boiling points due to strong intermolecular dipole-dipole interactions, which require a greater amount of energy to break. This is the case with \(\mathrm{XeO}_{3}\), hence, it is a solid at room temperature. On the other hand, non-polar molecules like \(\mathrm{XeO}_{4}\) have weak Van der Waals forces which require less energy to break. Therefore, it exists as a gas at room temperature despite having a larger molecular mass.

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