Learning Outcomes
By the end of this lesson, students will be able to:
i. Define London dispersion forces, also known as dispersion forces, a type of van der Waals force that arises from temporary dipole moments induced in all molecules.
ii. Explain the concept of instantaneous dipoles, arising from fluctuations in electron distribution, and their role in London dispersion forces.
iii. Describe how London dispersion forces contribute to intermolecular attractions in all liquids, regardless of their polarity, influencing properties like boiling point, viscosity, and surface tension.
iv. Recognize London dispersion forces as the weakest type of van der Waals force but still significant in shaping the behavior of nonpolar molecules, such as noble gases and hydrocarbons.
v. Appreciate the significance of understanding London dispersion forces in various fields, from explaining the condensation of gases to designing new materials with desired properties.
Introduction
In the realm of intermolecular forces, London dispersion forces, also known as dispersion forces, stand as a ubiquitous and fundamental attraction, present in all molecules, regardless of their polarity. These forces, though the weakest among van der Waals forces, play a crucial role in shaping the behavior of nonpolar liquids and gases.
i. Instantaneous Dipoles: A Symphony of Fluctuating Electrons
The foundation of London dispersion forces lies in the concept of instantaneous dipoles. These temporary dipoles arise from the constant, random motion of electrons within molecules. As electrons fluctuate around the nucleus, they can create momentary imbalances in charge distribution, resulting in a partial positive end and a partial negative end.
ii. Induction: The Art of Creating Temporary Dipoles
The presence of an instantaneous dipole in one molecule can induce a dipole in neighboring molecules. This induction occurs due to the electrostatic attraction between the opposite charges of the dipoles. The induced dipole, though temporary, contributes to the overall intermolecular attraction.
iii. London Dispersion Forces: A Universal Presence
London dispersion forces are present in all molecules, even those lacking permanent dipole moments. This is because the constant motion of electrons ensures that instantaneous dipoles can arise in any molecule. As a result, London dispersion forces contribute to the intermolecular attractions in all liquids, regardless of their polarity.
iv. Impact on Liquid Properties: Boiling Point, Viscosity, and Surface Tension
London dispersion forces, though weaker than dipole-dipole interactions and hydrogen bonding, still influence the physical properties of liquids. Stronger London dispersion forces lead to higher boiling points, increased viscosity, and greater surface tension. These properties are essential for various phenomena, from the evaporation of liquids to the cohesion of droplets and the adhesion of liquids to surfaces.
Examples of London Dispersion Forces in Action: From Noble Gases to Hydrocarbons
Noble gases, like helium and neon, with their symmetrical electron distribution lacking permanent dipole moments, rely solely on London dispersion forces for intermolecular attractions. These forces are responsible for the condensation of noble gases at extremely low temperatures.
Hydrocarbons, such as methane (CH4) and ethane (C2H6), also exhibit London dispersion forces as their primary intermolecular attraction. These forces contribute to their relatively low boiling points compared to polar substances.
v. The Significance of London Dispersion Forces: From Everyday Phenomena to New Materials
Understanding London dispersion forces extends beyond the realm of liquids and gases, impacting various fields. In everyday life, these forces play a role in phenomena like the condensation of water vapor on cold surfaces or the clinging of dust particles to surfaces.In materials science, manipulating London dispersion forces allows for the design of new materials with desired properties, such as low thermal conductivity, high surface area, and specific adhesion characteristics.
London dispersion forces, though often overlooked, stand as a testament to the ubiquitous nature of intermolecular attractions, shaping the behavior of nonpolar molecules and influencing various phenomena in our world. By understanding these forces, we gain a deeper appreciation for the intricate interplay between molecules and their interactions, shaping the properties of matter and guiding our understanding of the physical world around us.
