Chap 12 lecture 2 study guide chem 136

Solids, Liquids, and Intermolecular Forces

Dipole-Dipole Intermolecular Forces

Dipole-dipole forces are a type of intermolecular force that occur between polar molecules. These forces arise due to the presence of permanent dipoles within molecules, which result from uneven distribution of electrons between atoms of differing electronegativities.

• Definition: Dipole-dipole forces are electrostatic attractions between the positive end of one polar molecule and the negative end of another.

• Polar Molecules: Molecules with regions of partial positive and partial negative charge due to differences in electronegativity.

• Dipole Moment (\( \mu \)): A quantitative measure of the polarity of a molecule, given by \( \mu = q \times r \), where \( q \) is the charge and \( r \) is the distance between charges. Dipole moments are reported in Debyes (D), where 1 D = 3.34 × 10-30 C·m.

• Vector Nature: The overall dipole moment of a molecule is the vector sum of individual bond dipoles.

• Example: Hydrogen fluoride (HF) is polar because fluorine is highly electronegative, pulling electron density toward itself and creating a partial negative charge on F and a partial positive charge on H.

Polarity and Molecular Geometry

The presence of polar bonds does not guarantee that a molecule is polar. The molecular geometry, determined by VSEPR theory, must be considered to determine the net dipole moment.

• Linear Molecule Example: CO2 has polar bonds, but the dipoles cancel due to its linear geometry, making it nonpolar.

• Tetrahedral Molecule Example: CH4 is nonpolar because the dipoles cancel out in its tetrahedral geometry.

• CH2Cl2 Example: The dipoles do not cancel, so the molecule is polar.

Identifying Dipole-Dipole Attraction

To determine if a molecule exhibits dipole-dipole attraction, assess both the presence of polar bonds and the molecular geometry.

• Examples: HCl, CH3Cl, and NH3 are polar and exhibit dipole-dipole forces. BF3 and CCl4 are nonpolar due to their geometry.

Physical Properties Influenced by Dipole-Dipole Forces

Polar molecules with similar shape and mass have higher melting and boiling points than nonpolar molecules due to stronger dipole-dipole attractions.

• Boiling Point Trends: As polarity increases, so does the strength of dipole-dipole forces and boiling point.

Miscibility and Polarity

The polarity of a molecule determines its miscibility in liquids. "Like dissolves like" is a guiding principle: polar liquids mix with polar liquids, and nonpolar liquids mix with nonpolar liquids.

• Examples: Water (polar) and n-hexane (nonpolar) are immiscible. Methanol and water (both polar) are miscible. Carbon tetrachloride and n-hexane (both nonpolar) are miscible.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms (F, O, or N). This results in a strong attraction between the hydrogen atom of one molecule and the electronegative atom of another.

• Definition: Hydrogen bonding is an attractive force between a hydrogen atom bonded to F, O, or N and the F, O, or N atom of another molecule.

• Atoms Involved: H, F, O, N only.

• Examples: HF, H2O, NH3 exhibit hydrogen bonding.

• Energy: Average energy of a hydrogen bond is about 40 kJ/mol.

Unique Properties of Water

Water expands upon freezing due to hydrogen bonding, making ice less dense than liquid water. Each water molecule can form four hydrogen bonds, resulting in a high boiling point for its low molar mass.

Other Molecules with Hydrogen Bonding

• Examples: Ammonia (NH3), hydrogen fluoride (HF), hydrogen peroxide (H2O2), methanol (CH3OH), acetic acid (CH3COOH), methylamine (CH3NH2).

Effect of Hydrogen Bonding on Physical Properties

Hydrogen bonding is stronger than London dispersion and dipole-dipole forces. Substances capable of hydrogen bonding have higher melting and boiling points.

• Boiling Point Trends: Hydrides of group 6A (e.g., H2O) have higher boiling points than expected due to hydrogen bonding.

Hydrogen Bonding and Room Temperature Liquids

Among compounds with similar molar masses and dipole moments, only those with hydrogen bonding are liquid at room temperature.

Hydrogen Bonding in DNA

Hydrogen bonds are crucial for the structure of DNA, holding the individual bases together.

Distinguishing Hydrogen Bonding from Chemical Bonds

Hydrogen bonding is an intermolecular force, not a chemical bond. Chemical bonds occur within molecules, while hydrogen bonding occurs between molecules.

Ion-Dipole Intermolecular Forces

Ion-dipole forces are Coulombic attractions between ions and polar molecules, especially important in aqueous solutions.

• Definition: Ion-dipole interactions occur between ions (cations or anions) and polar molecules.

• Strength: Depends on the size and charge of the ion and the magnitude of the dipole moment.

• Cations vs. Anions: Cations interact more strongly with dipoles than anions of the same charge due to their smaller size.

• Example: In aqueous sodium chloride, water molecules orient their positive ends toward Cl- and negative ends toward Na+.

Summary Table: Types of Intermolecular Forces

Type	Occurs Between	Relative Strength	Example
London Dispersion	All molecules	Weakest	CH4
Dipole-Dipole	Polar molecules	Intermediate	HCl, CH3Cl
Hydrogen Bonding	H bonded to F, O, or N	Strong	H2O, NH3, HF
Ion-Dipole	Ions and polar molecules	Strongest	NaCl in water