Overview
This lecture explains the difference between intramolecular and intermolecular forces, their types, and how these forces influence the physical properties of substances, especially water.
Intramolecular Forces
- Intramolecular forces are forces within a molecule that hold atoms together.
- Types of intramolecular bonds include ionic, covalent, and metallic bonds.
- The type of intramolecular force depends on the electronegativity difference between atoms.
- Covalent bonds involve sharing of electrons, ionic bonds involve electron transfer, and metallic bonds involve a "sea" of shared electrons.
Intermolecular Forces
- Intermolecular forces act between different molecules.
- London dispersion forces occur between all molecules due to attractions between temporary dipoles.
- The strength of London dispersion forces increases with the size (number of protons and electrons) of the molecule.
- Dipole-dipole forces occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another.
- Hydrogen bonds are a special, strong type of dipole-dipole force, present when hydrogen is bonded to nitrogen, oxygen, or fluorine.
Properties Influenced by Intermolecular Forces
- Nonpolar molecules are held together mainly by weak London dispersion forces, making them easy to vaporize.
- Polar molecules have stronger dipole-dipole attractions, requiring more energy to separate.
- Both molecular size and polarity affect the strength of intermolecular forces and physical properties.
- Hydrogen bonding leads to high surface tension, high solubility for ionic and polar substances, and high heat capacity in water.
- Water’s strong hydrogen bonds explain phenomena like surface tension (insects walking on water) and its role as a universal solvent.
- Oil and water do not mix because water molecules prefer to bond with each other over oil molecules due to strong hydrogen bonding.
Key Terms & Definitions
- Intramolecular force — force that binds atoms within a single molecule.
- Intermolecular force — force of attraction between separate molecules.
- London dispersion force — weak attraction from temporary dipoles in all atoms and molecules.
- Dipole-dipole force — attraction between the positive and negative ends of polar molecules.
- Hydrogen bond — strong intermolecular force between hydrogen bonded to N, O, or F and another electronegative atom.
- Heat capacity — amount of energy needed to increase the temperature of a substance.
Action Items / Next Steps
- Review how to determine molecular polarity and identify types of intermolecular forces in sample molecules.
- Practice drawing molecules to indicate possible hydrogen bonds and other intermolecular forces.
- Prepare for discussion on the impact of intermolecular forces on boiling and melting points.