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Van der Waals Forces

Van der Waals forces

Intermolecular forces are forces of attraction or repulsion that act between neighboring particles (molecules, atoms, or ions). Van der Waals forces are a fundamental aspect of intermolecular interactions, affecting the physical properties of substances, their phase behavior, and many biological and material processes. While individually weak, these forces are important in understanding the behavior of molecules in various states of matter.

1.0What are Van der waals forces

While all Van der Waals forces are intermolecular forces, not all intermolecular forces are Van der Waals forces. Specifically:

  • Van der Waals forces include only London Dispersion Forces and Dipole-Dipole Interactions.
  • Intermolecular forces encompass a broader range, including Van der Waals forces, Hydrogen Bonds, Ion-Dipole Forces, Dipole-Induced Dipole Forces, and Ion-Induced Dipole Forces.

Van der Waals forces are a type of weak intermolecular force that plays a significant role in the behavior of molecules, particularly in the condensed phases (liquids and solids). These forces are named after Dutch scientist Johannes Diderik van der Waals, who first described them in the context of real gas behavior. 

Image showing van der waals forces

There are three main types of van der Waals forces: London dispersion forces, dipole-dipole interactions, and dipole-induced dipole interactions.

2.0Types of Van der waals forces

1. London Dispersion Forces:

  • These are weak forces that arise from temporary fluctuations in the electron distribution within atoms or molecules, leading to the formation of instantaneous dipoles. These dipoles induce similar dipoles in neighboring atoms or molecules, resulting in a weak attraction.
  • London dispersion forces are universal and present in all molecules, whether polar or nonpolar. They are particularly important in nonpolar molecules where no permanent dipole exists.
  • Examples- In atoms of noble gases.

2. Keesom Interactions:

  • Keesom interactions arise from the electrostatic interactions between permanent dipoles in polar molecules. They can occur due to:
    • The interaction between charges in ionic molecules.
    • The interaction between dipoles in polar molecules.
    • Quadrupole interactions in molecules with lower-than-cubic symmetry.
    • The interaction between permanent multipoles.
  • These forces are named after the Dutch physicist Willem Hendrik Keesom. Keesom interactions only occur between two permanent dipoles and are temperature-dependent.
  • Examples- HCl, H2O, NH3 etc.

3. Debye Forces:

  • Debye forces, also known as induction forces, are caused by interactions between permanent dipoles and neighboring atoms or molecules, which induce temporary dipoles in them. For example, a permanent dipole can induce a dipole in a nearby nonpolar molecule due to repulsive forces between their electrons.
  • Unlike Keesom interactions, Debye forces are not temperature-dependent. These forces are named after the Dutch-American physical chemist Peter Debye.
  • Examples- When a non-polar substance like benzene comes in contact with polar molecules like NH3, an induced dipole moment in benzene appears (Induction effect).

4. Dipole-Dipole Interactions:

  • These forces occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. Though still a non-valence interaction, these forces are stronger than London dispersion forces.
  • Dipole-dipole interactions are significant in polar substances, contributing to their higher boiling and melting points compared to nonpolar substances of similar molecular weight.

5. Dipole-Induced Dipole Interactions:

  • This force occurs when a polar molecule with a permanent dipole induces a dipole in a neighboring nonpolar molecule. The interaction between the permanent dipole and the induced dipole results in attraction.
  • These interactions are important in mixtures of polar and nonpolar substances and contribute to the solubility of nonpolar molecules in polar solvents.

6. Hydrogen Bonding (Special Case of Dipole-Dipole Interaction):

  • Although a type of dipole-dipole interaction, hydrogen bonding is a relatively strong form of attraction that occurs when hydrogen is covalently bonded to a highly electronegative atom (like nitrogen, oxygen, or fluorine) and interacts with a lone pair of electrons on another electronegative atom.
  • Hydrogen bonds in biological molecules like DNA and proteins and significantly affect the properties of water and alcohols.

3.0Characteristics of Van der Waals Forces

  • Nature: These forces are weaker than chemical bonds but crucial for understanding the physical properties of substances, such as boiling points, melting points, viscosity, and solubility.
  • Relevance: They explain the existence of condensed phases (liquids and solids) and the behavior of gases under non-ideal conditions.
  • Universal Presence: Non-valence forces are present in all matter, regardless of whether the species are polar or nonpolar.

4.0 Factors affecting Van der Waals forces

Factors

Description

Example 1

Example 2

Comparison

Molecular Size and Surface Area

Larger molecules with more electrons have stronger Van der Waals forces due to increased polarizability.

Methane (CH₄)

Octane (C₈H₁₈)

Octane has a larger molecular size and surface area than methane, leading to stronger Van der Waals forces and making octane a liquid at room temperature.

Molecular Shape

Linear molecules can pack closer, leading to stronger Van der Waals forces compared to branched or compact shapes.

n-Pentane (C₅H₁₂)

Neopentane (C₅H₁₂)

n-Pentane, with its linear shape, has stronger Van der Waals forces than the more compact neopentane, resulting in a higher boiling point for n-pentane.

Polarizability

Molecules with more easily distorted electron clouds (higher polarizability) exhibit stronger Van der Waals forces.

Helium (He)

Xenon (Xe)

Xenon, with its larger and more polarizable electron cloud, has stronger Van der Waals forces compared to helium, allowing xenon to be liquefied more easily.

Presence of Permanent Dipoles

Molecules with permanent dipoles experience stronger interactions through dipole-dipole forces.

Carbon Dioxide (CO₂)

Water (H₂O)

Water, with its permanent dipole and hydrogen bonding, has much stronger intermolecular forces compared to nonpolar carbon dioxide, affecting their physical states.

Temperature

Higher temperatures weaken Van der Waals forces due to increased molecular motion, while lower temperatures enhance them.

Water at 100°C (Boiling)

Water at 0°C (Freezing)

At 100°C, the kinetic energy in water molecules overcomes the Van der Waals forces (including hydrogen bonds), causing water to boil, while at 0°C, water freezes.

Distance Between Molecules

Van der Waals forces are strongest at short distances and weaken rapidly as the distance increases.

Liquid Nitrogen (N₂)

Gaseous Nitrogen (N₂)

In liquid nitrogen, molecules are closer together, leading to stronger Van der Waals forces, whereas in gaseous nitrogen, the distance weakens these forces.

Frequently Asked Questions

Van der Waals forces are weak intermolecular forces that arise from temporary or permanent dipoles in molecules. They include London dispersion forces, dipole-dipole interactions, and dipole-induced dipole interactions.

Van der Waals forces are much weaker than covalent and ionic bonds. While covalent and ionic bonds involve the sharing or transfer of electrons, Van der Waals forces are based on temporary or permanent dipoles and do not involve electron transfer or sharing.

The main types of Van der Waals forces are: London Dispersion Forces: Arise from temporary dipoles in atoms or molecules. Dipole-Dipole Interactions: Occur between permanent dipoles in polar molecules. Dipole-Induced Dipole Interactions: Occur when a permanent dipole induces a dipole in a neighboring nonpolar molecule.

The strength of Van der Waals forces is influenced by molecular size, surface area, shape, polarizability, presence of permanent dipoles, temperature, distance between molecules, and molecular symmetry.

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