Chapter 12 Lecture Notes - CHEM 1113 Broering

Liquids and solids possess definite volumes due to the attractive forces between their particles.
Intermolecular Forces are temporary attractive forces, applicable to atoms, ions, and molecules; based on electrostatic attractions and are weaker than covalent or ionic bonds.

Ion-Dipole Attractions
- Occur between ions and polar molecules.
Hydrogen Bonding
- A strong type of dipole-dipole interaction.
Dipole-Dipole Attractions
- Occur between two polar molecules.
Dispersion Forces (London Dispersion Forces)
- Present between all molecules but are significant for nonpolar molecules.

The strength and type of intermolecular forces affect trends in melting/boiling points and solubities, explaining unique properties of water.

Polar molecules like H2O exhibit separate partial charges, classifying them as dipoles.
Ionic compounds dissolve in water to create hydrated ions due to ion-dipole attractions, the strongest temporary attractions between particles.
Example: In H2O, the H+ attracts to Cl-, while the O attracts to Na+.
Ion-dipole forces occur between charged particles (ions) and polar molecules.

Permanent dipoles in polar molecules influence their physical properties by allowing interaction with other polar molecules and ions.
Dipole-Dipole Intermolecular Forces involve the attraction of the negative pole of one polar molecule to the positive pole of another.
Molecules with permanent dipoles interact more strongly than nonpolar ones, increasing energy required to vaporize (higher boiling point).

Boiling points of H2O, HF, and NH3 are unusually high due to hydrogen bonds, which are attractions between molecules that have H bonded to highly electronegative elements (N, O, F).
Characteristics of hydrogen bonding include:
1. Requires a hydrogen bond donor with a partially positive H atom.
2. Requires a hydrogen bond acceptor with a partially negative N, O, or F atom that has lone pair electrons.
Hydrogen bonding significantly contributes to the boiling points of these molecules due to the strong attraction forces.

Result from random electron motion creating instantaneous dipoles.
The strength of dispersion forces is influenced by factors such as:
1. Atomic Size: Larger atoms are more easily polarized leading to stronger dispersion forces.
2. Polarizable Electrons: Larger molecules can form multiple instantaneous dipoles.
3. Molecular Shape: Linear shapes enable stronger interactions compared to bulky shapes.

Enthalpy of Vaporization (∆Hvap): Energy change when 1 mole of liquid vaporizes at boiling point; endothermic.
Enthalpy of Fusion (∆Hfus): Energy needed to convert 1 mole of solid to liquid; also endothermic.
Enthalpy of Sublimation (∆Hsub): Energy needed to convert 1 mole of solid directly to gas; calculated as ∆Hfus + ∆Hvap.

Heating a substance (endothermic) increases energy, potentially breaking intermolecular forces.
Cooling a substance (exothermic) reduces energy, allowing intermolecular forces to form.
At phase change points (melting and boiling), temperature remains constant as energy is used to break intermolecular forces.

When comparing substances such as ethanol and dimethyl ether, ethanol exhibits stronger hydrogen bonding and thus a higher boiling point despite equal molar masses due to the presence of -OH groups that facilitate hydrogen bonding.

Defined as the pressure exerted by a gas in equilibrium with its liquid phase at a specific temperature.
Volatile: Substances that vaporize easily, exhibiting higher vapor pressure; e.g. diethyl ether, which cannot form hydrogen bonds and vaporizes readily.
Vapor pressure is affected by:
1. Strength of Intermolecular Forces: Stronger forces correlate to lower vapor pressures.
2. Temperature: Higher temperatures lead to increased kinetic energy, allowing more liquid molecules to escape into the gas phase.

Intermolecular Forces

Liquids and solids have definite volumes due to attractive forces between particles. Intermolecular forces are temporary attractive forces among atoms, ions, and molecules, weaker than covalent or ionic bonds.

Types of Intermolecular Forces:

Ion-Dipole Attractions: Occur between ions and polar molecules, crucial for dissolving ionic compounds in water.
Hydrogen Bonding: A strong dipole-dipole interaction, influencing the high boiling points of H2O, HF, and NH3 due to hydrogen bonding with highly electronegative elements.
Dipole-Dipole Attractions: Occur between two polar molecules.
Dispersion Forces (London Dispersion Forces): Present in all molecules, significant in nonpolar ones, influenced by atomic size, polarizable electrons, and molecular shape.

Phase Changes:

Enthalpy changes occur during phase transitions: Vaporization (∆Hvap), Fusion (∆Hfus), Sublimation (∆Hsub), all endothermic processes.
Heating increases energy, potentially breaking intermolecular forces, while cooling allows their formation.

Vapor Pressure:

The pressure exerted by a gas in equilibrium with its liquid phase, affected by the strength of intermolecular forces and temperature. Stronger forces correlate to lower vapor pressures, while higher temperatures increase kinetic energy and tendency to escape into the gas phase.