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

step 1 On this problem, we have selain, which is S -I -H -4 and Phosphine pH -3. So they have molecular

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

Q 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 Polarizability

Molecular polarizability refers to how easily a molecule's electron cloud can be distorted. PH3, with its larger central atom and more diffuse electron cloud compared to SiH4, exhibits higher polarizability. This increase in polarizability enhances its London dispersion forces, thereby raising its melting and boiling points despite a similar molecular weight.

Molecular Geometry

Molecular geometry influences the distribution of electron density and can affect how molecules interact with each other. SiH4 is tetrahedral and highly symmetric, which often leads to lower polarizability and weaker dispersion interactions, whereas PH3 has a trigonal pyramidal shape that, alongside its lone pair, can contribute to a greater polarizability and slightly different intermolecular interactions. This structural difference is a key factor in the observed differences in their physical properties.

Intermolecular Forces

The boiling and melting points of a substance depend heavily on the strength of its intermolecular forces rather than merely on its molecular weight. In nonpolar molecules like SiH4 and PH3, these intermolecular attractions are typically weak London dispersion forces. However, slight differences in the nature of these forces can lead to notable differences in physical properties such as phase transition temperatures.

London Dispersion Forces

London dispersion forces are a type of intermolecular force that arise due to temporary fluctuations in electron density, inducing instantaneous dipoles. Even though both SiH4 and PH3 are largely nonpolar, the strength of these dispersion forces can vary due to differences in electron cloud extensibility and size, affecting how strongly the molecules attract each other in the condensed phase.

Transcript

Please give Ace some feedback