Question

What are dispersion forces? Which has greater dispersion forces, \(\mathrm{C}_{3} \mathrm{H}_{8}\) or \(\mathrm{C}_{9} \mathrm{H}_{20}\) ? Justify your answer.

Short answer

...

Step-by-step solution

Understanding Dispersion Forces

Dispersion forces, also known as London dispersion forces, are a type of van der Waals force. They are weak interactions that arise from temporary fluctuations in electron density in atoms and molecules, creating instantaneous dipoles. These forces are present in all molecules, whether they are polar or nonpolar.

Factors Affecting Dispersion Forces

Dispersion forces increase with the molecular size and mass. Larger and heavier molecules have more electrons, which lead to greater temporary dipoles and stronger dispersion forces.

Key concepts

London dispersion forces

Dispersion forces, also known as London dispersion forces, are a type of weak intermolecular force. These forces arise from temporary shifts in electron density within molecules. When electrons in an atom or molecule randomly end up on one side more than the other, they create temporary dipoles. This temporary dipole can induce a dipole in a neighboring atom or molecule, leading to an attraction between them.

This interaction is fleeting but happens continuously, making sure that dispersion forces are always present. They are the only type of intermolecular force in noble gases and nonpolar molecules, ensuring that all matter experiences some form of attraction.

These forces were first described by Fritz London, which is why they carry his name. Yet, despite their weak nature, they play a significant role in the behavior of molecules, especially large ones.

molecular interactions

Molecular interactions are forces that act between molecules. They can be categorized into several types, including ionic bonds, covalent bonds, hydrogen bonds, and van der Waals forces. London dispersion forces fall under the van der Waals forces category.

Here’s a quick breakdown:

• Ionic Bonds: Strong interactions between ions with opposite charges.
• Covalent Bonds: Bonds formed by the sharing of electrons between atoms.
• Hydrogen Bonds: Special dipole-dipole interactions involving hydrogen atoms and electronegative atoms like oxygen or nitrogen.
• Van der Waals Forces: Includes London dispersion forces and dipole-dipole interactions.

The nature of these interactions can greatly affect properties like boiling point, melting point, and solubility. In nonpolar molecules, dispersion forces are the most significant molecular interactions. Their cumulative effect can influence the physical properties of substances.

van der Waals forces

Van der Waals forces are a group of intermolecular forces that include London dispersion forces, dipole-dipole forces, and induced dipole interactions. Named after the scientist Johannes Diderik van der Waals, these forces are critical for understanding molecular interactions in both gases and liquids.

In the context of nonpolar molecules, London dispersion forces are the most prominent type of van der Waals force. However, in polar molecules, dipole-dipole interactions also play an essential role.

• Dipole-Dipole Forces: These occur when the positive end of a polar molecule is attracted to the negative end of another polar molecule.
• Induced Dipole Interactions: These involve a temporary dipole in a nonpolar molecule induced by a nearby charged particle.

Van der Waals forces, although weaker than covalent or ionic bonds, are crucial for the physical properties of materials, such as viscosity, boiling points, and the aggregation of substances.

molecular size

Molecular size significantly affects the strength of London dispersion forces. The larger the molecule, the more electrons it has, which increases the likelihood of temporary dipoles forming. Larger molecules have more surface area for interactions, leading to stronger dispersion forces.

To address the original question: Which has greater dispersion forces, \(\text{C}_3\text{H}_8\) or \(\text{C}_9\text{H}_20\)?\?

Let's consider their sizes:

• \(\text{C}_3\text{H}_8\): A smaller molecule with fewer electrons and shorter chains.


• \(\text{C}_9\text{H}_20\): A larger molecule with more electrons and longer chains, leading to more significant dispersion forces.

Thus, \(\text{C}_9\text{H}_20\) would exhibit stronger dispersion forces than \(\text{C}_3\text{H}_8\).

In conclusion, as molecular size increases, so does the strength of the London dispersion forces, affecting the overall behavior and properties of the substance.