Intermolecular Forces and States of Matter

• Intermolecular forces (IMF), also known as van der Waals forces, exist between molecules and influence physical properties.
• The physical properties of H2O are different in its three states: ice, water, and steam, even though the bond angles, dipole moment, molecular shape, and the hybridization of oxygen are the same.
• Solid ice (H2O) has hexagonally bonded crystal lattices, possessing a definite shape and volume and is incompressible.
• Liquid water has a definite volume, assumes the shape of its container, and is slightly compressible.
• Steam assumes the shape and volume of its container and is extremely compressible.
• IMF cause real gases to deviate from ideal gas behavior.
• IMF govern the motion of molecules.
• In the gaseous phase, molecules are in random and constant motion.
• In liquids, molecules slide past each other freely.
• In solids, molecules vibrate about fixed positions.

Types of Intermolecular Forces

• The London dispersion force is the weakest IMF and occurs between two nonpolar molecules.
• London dispersion force becomes stronger with larger atoms or molecules.
• Dipole-dipole interaction occurs when two polar molecules get near each other.
• The positively charged portion of one molecule is attracted to the negatively charged portion of another molecule.
• Dipole-dipole interaction example is the interaction between two sulfur dioxide (SO2) molecules.
• Hydrogen bonding is a specific example of dipole-dipole interaction always involving hydrogen.
• Hydrogen bonding refers to the electrostatic interaction between a hydrogen and a lone electronegative atom pair.
• Electronegative atoms in hydrogen bonding are usually fluorine, nitrogen, or oxygen, with a partial negative charge, while hydrogen has a partial positive charge.
• Ion-dipole interaction occurs when an ion encounters a polar molecule, where the charge of the ion either attracts or repels.

Trends in Intermolecular Forces

• Intermolecular forces vary in their strength; given the same molecular size, their magnitudes are as follows: Hydrogen bonding > dipole-dipole > London dispersion.
• Stronger intermolecular forces result in higher boiling points and melting points, assuming all else is equal.
• With stronger intermolecular forces, more energy is required to separate liquid-state molecules and convert them to gas-state molecules.
• The same applies to molecules in the solid-state that get converted to liquid-state molecules.
• At a constant temperature, vapor decreases with increasing IMF strength.
• A substance with stronger IMF has a lower concentration of molecules in the gaseous state and, therefore, lower vapor pressure.

Explore intermolecular forces (IMFs) and their impact on physical properties. Learn how IMFs cause real gases to deviate from ideal behavior. Discover the motion of molecules in gases, liquids, and solids, and understand the different types of IMFs, including London dispersion forces.