The molecular weights of SiH4 and PH3 are nearly the same. Account for the fact that the melting

The molecular weights of SiH4 and PH3 are nearly the same....

The molecular weights of $\mathrm{SiH}_{4}$ and $\mathrm{PH}_{3}$ are nearly the same. Account for the fact that the melting and boiling points of $\mathrm{PH}_{3}\left(-133^{\circ} \mathrm{C}\right.$ and $\left.-88^{\circ} \mathrm{C}\right)$ are higher than those of $\mathrm{SiH}_{4}\left(-185^{\circ} \mathrm{C}\right.$ and $\left.-112^{\circ} \mathrm{C}\right)$.

Key Concepts

Molecular Geometry and Polarity

The shape of a molecule strongly influences whether it has an overall dipole moment. Even though two molecules might have nearly identical molecular weights, a tetrahedral molecule like SiH4 has a symmetrical arrangement of atoms that cancels out any bond dipoles, resulting in a non?polar molecule. In contrast, a molecule like PH3 adopts a trigonal pyramidal shape due to a lone pair on the phosphorus, which can give rise to a net dipole moment. This polarity enhances intermolecular dipole–dipole interactions, contributing to higher boiling and melting points.

Intermolecular Forces

Intermolecular forces, including dipole–dipole interactions and London dispersion forces, play a crucial role in determining the physical properties of a substance. While both SiH4 and PH3 experience dispersion forces, the additional dipole–dipole interactions in PH3, even if relatively weak compared to those in strongly polar molecules, lead to stronger overall intermolecular attractions. This results in higher observed melting and boiling points for PH3 than for the nonpolar SiH4.

Polarizability

Polarizability refers to the ease with which the electron cloud of a molecule can be distorted, inducing temporary dipoles that enhance dispersion forces. The size and electronic structure of the central atom affect polarizability; in PH3, the larger, more diffused electron cloud of phosphorus compared to silicon can lead to increased polarizability. This results in stronger London dispersion forces in PH3, further contributing to its higher boiling and melting points.

Transcript

00:01 This question is asking why these molecules have different melting and boiling points.

00:05 So remember, when we are melting, say we've got molecules.

00:17 So when we are melting, what we need to appreciate is that when we are melting or boiling a substance, what we are breaking or overcoming with using that heat energy is the intermolecular forces.

00:33 So when we are melting, we are trying to overcome these intermolecular forces such that, these molecules move apart.

00:43 Remember when we are melting we are changing state from solid from solid to liquid and when we are boiling we are now moving from liquid to gas.

01:00 So through this process we are bringing the molecules further and further apart from each other.

01:07 So when we are doing that what we are doing is overcoming the intermolecular forces that are holding these particles together so what will determine whether a substance is got a high melting point is the strength of these intermolecular forces so for us to then explain why c -i -h -4 and ph 3 have different molecular if different melting and boiling points while they have almost similar molecular molecules we have to look at the forces or the intermolecular forces that are at play within each of these molecules.

01:48 So if we are looking at c -i -h -4, we've got silicon.

01:54 Silicon is in group four.

01:56 So it has got four electrons in its outermost shell.

01:59 So it is bonded to four hydrogen atoms and when we look at this molecule they are non -polar...

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