Ch.11 – Liquids, Solids, and Intermolecular Forces: Molecular Polarity and Intermolecular Forces

Liquids, Solids, and Intermolecular Forces

Molecular Polarity

Molecular polarity is a key concept in understanding the behavior of molecules in liquids and solids. It arises from differences in electronegativity and the geometry of the molecule, leading to dipole moments.

• Dipole Moment: A measure of the separation of positive and negative charges in a molecule. A molecule has a net dipole moment if the vector sum of its bond dipoles does not cancel out.

• Electronegativity: The tendency of an atom to attract electrons in a chemical bond. Differences in electronegativity between atoms can create polar bonds.

• Molecular Geometry: The three-dimensional arrangement of atoms in a molecule determines whether dipole moments cancel or reinforce each other.

Examples:

• AsCl5: Trigonal bipyramidal geometry; dipole moments cancel, nonpolar.

• SeCl4: Se is less electronegative than Cl; seesaw geometry; net dipole moment, polar.

• Fluoromethane (CH3F): Tetrahedral geometry; F is more electronegative than C and H; net dipole moment, polar.

• SF6: Octahedral geometry; dipole moments cancel, nonpolar.

Polarity Classification:

• Polar Molecules: H2Se, CH3F, NFCI2

• Nonpolar Molecules: BI3

Why Water is Polar

Water's polarity is due to its molecular structure and the electronegativity difference between hydrogen and oxygen.

• Oxygen is covalently bonded to two hydrogen atoms.

• Oxygen is more electronegative than hydrogen, pulling electrons closer.

• Electrons are not distributed uniformly, resulting in partial charges.

Result: Water has a net dipole moment, making it a polar molecule.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. They are responsible for many physical properties of substances.

• Hydrogen Bonding: Occurs when a hydrogen atom is bonded to a highly electronegative atom (N, O, F) and is attracted to another electronegative atom in a different molecule.

• Dipole-Dipole Forces: Attraction between polar molecules due to their dipole moments.

• London Dispersion Forces: Weak forces present in all molecules, especially nonpolar ones, due to temporary dipoles.

Example: In H2O, the partial negative charge of oxygen attracts the partial positive charge of hydrogen in another molecule, resulting in hydrogen bonding.

Strength of Intermolecular Forces

The strength of intermolecular forces affects various physical properties of substances.

• Order of Increasing Strength: I2 < Br2 < NH(CH3)2

• Boiling Point: Stronger intermolecular forces lead to higher boiling points.

• Vapor Pressure: Stronger intermolecular forces lead to lower vapor pressure.

Types of Forces: Intramolecular vs. Intermolecular

It is important to distinguish between intramolecular and intermolecular forces:

• Intramolecular Forces: Forces within a molecule (e.g., covalent bonds).

• Intermolecular Forces: Forces between molecules (e.g., hydrogen bonds, dipole-dipole, London dispersion).

Examples:

• Boiling of water: Intermolecular

• Formation of H2O in a fuel cell: Intramolecular

• Vulcanization of rubber: Intramolecular

• Coffee dissolves in water: Intermolecular

• Liquid flowing in a narrow space (capillary action): Intermolecular

Intermolecular Forces and Physical Properties

Stronger intermolecular forces affect physical properties in the following ways:

• Decrease vapor pressure

• Increase boiling point

Classification of Intermolecular Forces in Compounds

Different compounds exhibit different types of intermolecular forces:

Compound	Strongest Intermolecular Force
CF4	London Dispersion
CH2Br2	Dipole-Dipole
HOCH2OH	Hydrogen Bonding

Properties:

Property	Compound with Highest Value
Molar heat of vaporization	HOCH2OH
Melting point	HOCH2OH
Vapor pressure	CF4
Viscosity	HOCH2OH

Properties of Liquids and Intermolecular Forces

The strength of intermolecular forces in liquids affects their physical properties:

• Low surface tension: Weak intermolecular forces

• High viscosity: Strong intermolecular forces

• High boiling point: Strong intermolecular forces

• Low vapor pressure: Strong intermolecular forces