Dipole-dipole interactions, London dispersion forces, hydrogen bonding

Overview of Intermolecular Forces

1. Intermolecular forces are forces of attraction or repulsion between neighboring molecules.
2. They are weaker than intramolecular forces (such as covalent or ionic bonds).
3. These forces determine physical properties like boiling points, melting points, and solubility.
4. The three main types of intermolecular forces are dipole-dipole interactions, London dispersion forces, and hydrogen bonding.
5. Van der Waals forces include dipole-dipole interactions and dispersion forces.

Dipole-Dipole Interactions

1. Dipole-dipole interactions occur between molecules that have a permanent dipole.
2. A permanent dipole arises when there is a significant difference in electronegativity between bonded atoms.
3. The partial positive end of one molecule is attracted to the partial negative end of another.
4. These interactions are stronger than London dispersion forces but weaker than hydrogen bonds.
5. Molecules like HCl and CH₃Cl exhibit dipole-dipole interactions.
6. They are effective over short distances.
7. The strength of dipole-dipole forces increases with the magnitude of the dipole moment.

London Dispersion Forces

1. London dispersion forces, also known as induced dipole forces, exist in all molecules, whether polar or nonpolar.
2. These forces arise due to temporary dipoles created by electron movement.
3. The strength of London forces depends on the polarizability of the molecule.
4. Larger molecules with more electrons exhibit stronger dispersion forces.
5. Dispersion forces are the only type of intermolecular force present in nonpolar molecules, such as CH₄ and O₂.
6. They are weaker than dipole-dipole interactions and hydrogen bonds.
7. These forces increase with molecular size and are responsible for the higher boiling points of larger molecules.

Hydrogen Bonding

1. Hydrogen bonding is a strong type of dipole-dipole interaction.
2. It occurs when hydrogen is covalently bonded to highly electronegative atoms like F, O, or N.
3. The partial positive charge on hydrogen is attracted to the partial negative charge on the electronegative atom of another molecule.
4. Examples include H₂O, NH₃, and HF.
5. Hydrogen bonding significantly increases the boiling point and melting point of substances.
6. It plays a crucial role in the structure of biological molecules like DNA and proteins.
7. There are two types: intermolecular hydrogen bonding (between molecules) and intramolecular hydrogen bonding (within a molecule).
8. Hydrogen bonds are stronger than dipole-dipole interactions and London dispersion forces but weaker than covalent bonds.

Comparison of Intermolecular Forces

1. Dipole-dipole interactions occur in polar molecules, while dispersion forces are universal.
2. Hydrogen bonding is a special case of dipole-dipole interaction with a significant increase in strength.
3. The relative strength of these forces is: Hydrogen bonds > Dipole-dipole > London dispersion forces.
4. Nonpolar substances rely solely on dispersion forces, leading to lower boiling points.
5. Polar substances have higher boiling points due to dipole-dipole interactions and hydrogen bonding.

Key Points

1. Dipole-dipole interactions require polar molecules with a permanent dipole.
2. London dispersion forces are present in all molecules, regardless of polarity.
3. Hydrogen bonding is strongest in molecules with H-F, H-O, or H-N bonds.
4. Hydrogen bonds are responsible for the high boiling point of water.
5. Dispersion forces increase with the molecular size and number of electrons.
6. DNA structure relies on hydrogen bonding between base pairs.
7. Intermolecular forces influence solubility, with polar molecules dissolving in polar solvents.
8. Substances with stronger intermolecular forces have higher boiling and melting points.
9. Understand examples of each type for exams: HCl (dipole-dipole), CH₄ (dispersion), and H₂O (hydrogen bonding).
10. Van der Waals forces include both dipole-dipole and London dispersion forces.