Question

Which member in each pair has the greater dispersion forces? (a) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{S},(\mathbf{b}) \mathrm{CO}_{2}\) or \(\mathrm{CO},(\mathbf{c}) \operatorname{siH}_{4}\) or \(\mathrm{GeH}_{4}\) .

Short answer

In summary, the member with greater dispersion forces in each pair are: (a) H2S has greater dispersion forces than H2O, (b) CO2 has greater dispersion forces than CO, and (c) GeH4 has greater dispersion forces than SiH4.

Step-by-step solution

(a) Comparing H2O and H2S

: To determine which molecule has the greater dispersion forces, we need to compare the size of the molecules and the number of electrons. 1. Molecular size: In general, heavier molecules have stronger dispersion forces. H2O has a molecular weight of approximately 18 g/mol, while H2S has a molecular weight of approximately 34 g/mol. Since H2S is heavier, its dispersion forces should be stronger. 2. Number of electrons: The more electrons a molecule has, the stronger the dispersion forces. H2O has 10 electrons (2 from each H atom and 6 from the O atom), while H2S has 18 electrons (2 from each H atom and 16 from the S atom). Since H2S has more electrons, its dispersion forces should be stronger. Based on the molecular size and the number of electrons, H2S has the greater dispersion forces compared to H2O.

(b) Comparing CO2 and CO

: Again, we need to compare the size of the molecules and the number of electrons. 1. Molecular size: CO2 has a molecular weight of approximately 44 g/mol, while CO has a molecular weight of approximately 28 g/mol. Since CO2 is heavier, its dispersion forces should be stronger. 2. Number of electrons: CO2 has 22 electrons (6 from each O atom and 6 from the C atom), while CO has 14 electrons (6 from the O atom and 6 from the C atom). Since CO2 has more electrons, its dispersion forces should be stronger. Based on the molecular size and the number of electrons, CO2 has the greater dispersion forces compared to CO.

(c) Comparing SiH4 and GeH4

: Lastly, we compare the size of the molecules and the number of electrons for SiH4 and GeH4. 1. Molecular size: SiH4 has a molecular weight of approximately 32 g/mol, while GeH4 has a molecular weight of approximately 77 g/mol. Since GeH4 is heavier, its dispersion forces should be stronger. 2. Number of electrons: SiH4 has 20 electrons (4 from the Si atom and 1 from each of the 4 H atoms), while GeH4 has 36 electrons (32 from the Ge atom and 1 from each of the 4 H atoms). Since GeH4 has more electrons, its dispersion forces should be stronger. Based on the molecular size and the number of electrons, GeH4 has the greater dispersion forces compared to SiH4.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles, such as atoms, molecules, or ions. They are weaker than intramolecular forces, which are the forces within molecules that keep them intact. Dispersion forces, also known as London dispersion forces, are one type of intermolecular force.

They arise due to the momentary distribution of electron clouds within molecules or atoms, leading to temporary polarity. These momentary dipoles can induce dipoles in neighboring molecules, resulting in attraction. The strength of dispersion forces is influenced by the size of the molecule and the number of electrons, as more electrons can lead to stronger temporary dipoles. Molecules with larger electron clouds have stronger dispersion forces, contributing to higher boiling and melting points.

Molecular Weight

Molecular weight, or molecular mass, is the sum of the atomic weights of all the atoms in a molecule. It is measured in atomic mass units (amu) or grams per mole (g/mol). In terms of intermolecular forces, a higher molecular weight generally indicates a larger, more complex molecule with more electrons.

As a result, such molecules can exhibit stronger dispersion forces due to their increased polarizability — the ability of the electron cloud to be distorted, creating temporary dipoles. In a pair of molecules, the one with the greater molecular weight often has stronger dispersion forces, assuming their molecular shapes do not differ significantly.

Number of Electrons

The number of electrons in a molecule can greatly influence the strength of its dispersion forces. More electrons mean a larger electron cloud and an increased likelihood of significant momentary dipole formations.

These temporary dipoles can induce similar dipoles in adjacent molecules, intensifying the dispersion forces between them. Thus, when comparing molecules, especially of a similar shape and size, the one with more electrons will typically exhibit stronger dispersion forces. This can affect various physical properties of the substance, such as its volatility, solubility, and boiling point.

Molecular Comparison

When comparing molecules to determine which has the greater dispersion forces, examining both molecular weight and the number of electrons is essential. However, it's not just a matter of comparing these two factors individually; how they influence the overall structure and behavior of the molecule is critical.

This assessment becomes even more pertinent when the shape and size of the molecules diverge significantly, as these characteristics can alter the polarizability and overall dispersion force strength. The shape of the molecule can affect how closely molecules can pack together, influencing the strength of intermolecular interactions. Therefore, a thorough molecular comparison must consider a combination of all these elements to determine the intermolecular forces accurately.