London Dispersion vs. Van der Waals

Attribute	London Dispersion	Van der Waals
Definition	Temporary attractive force between nonpolar molecules	Sum of all intermolecular forces, including London Dispersion, dipole-dipole, and hydrogen bonding
Strength	Weakest intermolecular force	Depends on the specific molecules involved
Origin	Result of temporary fluctuations in electron distribution	Arises from various interactions between molecules
Effect on boiling point	Increases boiling point as molecule size increases	Contributes to overall boiling point along with other intermolecular forces

Introduction

London dispersion forces and Van der Waals forces are two types of intermolecular forces that play a crucial role in determining the physical properties of substances. While they are both weak compared to other types of chemical bonds, they are essential for understanding how molecules interact with each other. In this article, we will compare the attributes of London dispersion and Van der Waals forces to gain a better understanding of their similarities and differences.

London Dispersion Forces

London dispersion forces, also known as dispersion forces or instantaneous dipole-induced dipole forces, are the weakest type of intermolecular force. These forces arise from the temporary fluctuations in electron distribution within molecules, leading to the formation of temporary dipoles. These temporary dipoles induce similar dipoles in neighboring molecules, resulting in a weak attraction between them. London dispersion forces are present in all molecules, regardless of their polarity, but they are more significant in nonpolar molecules due to the absence of other intermolecular forces.

Van der Waals Forces

Van der Waals forces encompass a broader category of intermolecular forces that include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. While London dispersion forces are a subset of Van der Waals forces, the term "Van der Waals forces" is often used to refer to the sum of all intermolecular forces present in a substance. In addition to London dispersion forces, Van der Waals forces also include dipole-dipole interactions, which occur between polar molecules with permanent dipoles, and hydrogen bonding, which is a specific type of dipole-dipole interaction involving hydrogen atoms bonded to highly electronegative atoms.

Strength

One of the key differences between London dispersion forces and Van der Waals forces is their strength. London dispersion forces are the weakest type of intermolecular force, as they arise from temporary fluctuations in electron distribution. In contrast, Van der Waals forces encompass a range of forces, including dipole-dipole interactions and hydrogen bonding, which are stronger than London dispersion forces. The strength of Van der Waals forces varies depending on the nature of the molecules involved, with hydrogen bonding being the strongest type of Van der Waals force.

Effect on Physical Properties

London dispersion forces and Van der Waals forces play a significant role in determining the physical properties of substances. In nonpolar molecules, London dispersion forces are the primary intermolecular force responsible for holding the molecules together. These weak forces contribute to the low boiling points and melting points of nonpolar substances. In contrast, Van der Waals forces, which include dipole-dipole interactions and hydrogen bonding, contribute to the higher boiling points and melting points of polar and hydrogen-bonded substances. The presence of stronger intermolecular forces in these substances leads to a higher degree of cohesion between molecules.

Temperature Dependence

Another important aspect to consider when comparing London dispersion and Van der Waals forces is their temperature dependence. London dispersion forces are temperature-dependent, as they arise from the random fluctuations in electron distribution within molecules. As the temperature increases, the kinetic energy of molecules also increases, leading to a greater magnitude of these fluctuations and, consequently, stronger London dispersion forces. In contrast, Van der Waals forces, including dipole-dipole interactions and hydrogen bonding, are less temperature-dependent, as they involve more permanent interactions between molecules.

Conclusion

In conclusion, London dispersion forces and Van der Waals forces are two types of intermolecular forces that play a crucial role in determining the physical properties of substances. While London dispersion forces are the weakest type of intermolecular force, Van der Waals forces encompass a broader category of forces, including dipole-dipole interactions and hydrogen bonding. The strength, effect on physical properties, and temperature dependence of these forces differ, highlighting the importance of understanding their unique attributes. By comparing London dispersion and Van der Waals forces, we can gain a better understanding of how molecules interact with each other and how these interactions influence the behavior of substances.