Intermolecular Forces of Attraction

Intermolecular Forces of Attraction

• Known as "Van der Waals Forces", arising from weak electrostatic attractions between molecules.
• Different from intramolecular forces (covalent bonds) which exist within a molecule.

Forces Between Polar Molecules

• Dipole-Dipole Attractions: Occur between polar molecules that have permanent dipoles, characterized by asymmetrical shapes and polar bonds.
• Hydrogen Bonding: A strong type of dipole-dipole interaction that occurs when hydrogen atoms bond with highly electronegative atoms (F, O, N).
  • Results in very polar bonds and unusually high melting and boiling points (MP/BP).
  • Water and ethanol are examples where hydrogen bonding occurs, affecting miscibility and physical properties.
  • Hydrogen bonds allow ice to float due to its less dense, ordered structure compared to liquid water.

Properties of Water

• Ice’s open structure optimizes hydrogen bonding, contributing to its lower density compared to liquid water.
• Hydrogen bonding plays a key role in water's surface tension and beading properties.

Forces Between Nonpolar Molecules

• Dispersion Forces: The only attractive forces among nonpolar molecules and noble gas atoms; recognized as the weakest intermolecular forces.
  • The strength of dispersion forces correlates with the total number of electrons; more electrons strengthen the forces.
  • Example: Cl2 (34 electrons, gas), Br2 (70 electrons, liquid), I2 (106 electrons, solid).

Molecular Interactions Activity

• Identify the type of intermolecular forces in listed molecules (NH3, HBr, I2, PBr3, CO2).
• Arrange compounds (CCl4, H2O, NaCl, ICl) by decreasing intermolecular forces of attraction (IMFA).

Solubility of Ionic Compounds

• Ionic compounds dissolve in water due to molecule-ion attractions, where polar solvent molecules interact with ions, creating hydrated ions.
• Polar solvents like water effectively pull ions from ionic crystals into solution.

Effects of Intermolecular Forces

• Melting and Boiling Points: Stronger intermolecular forces increase the difficulty of separating molecules, resulting in higher MP/BP values.
  • Hydrogen bonding yields the highest MP/BP, followed by dipole-dipole interactions, and lowest for London dispersion forces.

Solubility Principles

• Solutes must form attractions with solvents to dissolve; polar solutes dissolve in polar solvents, while nonpolar solutes dissolve in nonpolar environments.
• Polar solvents also attract and dissolve ionic compounds.

Applications of Intermolecular Forces

• Important in the design of medical devices, construction materials, sports equipment, and electrical devices.
• Medical implants require materials with high melting points to endure sterilization, preventing infection and allergic reactions.

Explore the various intermolecular forces, including Van der Waals forces, that hold molecules together. This quiz delves into the differences between intermolecular and intramolecular forces, focusing on the nature of dipole-dipole attractions between polar molecules. Test your understanding of these fundamental concepts in molecular chemistry.