Chem 122 - Intermolecular Forces

Chem 122 - Fall 2024 - Dr. Rita Hatfield

• Chem 122 Final Exam is Tuesday, December 10th, from 6-8 PM.
• Discussion Worksheet 15 is posted.
• Week 14 homework is due tonight.
• Week 15 homework is due Friday, December 6th.
• Chapter 12, Section 3 covers Intermolecular Forces.
• Chapter 12 Sections (in the lecture):
  • 12.1: Overview of Physical States and Phase Changes
  • 12.2: Quantitative Aspects of Phase Changes
  • 12.3: Types of Intermolecular Forces
  • 12.4: Properties of the Liquid State

Nature of Intermolecular Forces

• Intermolecular forces are attractions between molecules or between ions and molecules.
• Intermolecular forces are weaker than bonding forces. Bonding forces involve smaller charges that are farther apart.
• There are different types of intermolecular forces, each with varying strength and how they arise.

Intermolecular Attractions

• The strength of attractions between particles determines the state of a substance (solid, liquid, or gas) at room temperature.
• Moderate to strong attractive forces result in solids or liquids at room temperature.
• Stronger attractive forces lead to higher boiling and melting points.

Comparison of Bonding Forces (intramolecular)

• Ionic: Cation-anion attraction (400-4000 kJ/mol). Examples include NaCl.
• Covalent: Nuclei-shared electron pair attraction (150-1100 kJ/mol). Examples include H2.
• Metallic: Cation-delocalized electron attraction (75-1000 kJ/mol). Example includes Fe. Bonding forces are much greater than intermolecular forces (IMFs).

Comparison of Non-Bonding Forces (Intermolecular Forces- IMFs):

• Overall strongest: Ion-dipole (40-600 kJ/mol). Examples include Na+ and H2O.
• H-bond (10-40 kJ/mol): Polar bond to a highly electronegative atom (F, O, or N) with lone pairs. Examples include H2O, organic molecules with -OH, NH2, or -FH.
• Dipole-dipole (5-25 kJ/mol): Permanent dipole-dipole attractions.
• Ion-induced dipole (3-15 kJ/mol): Ionic charges distort polarizable electron clouds.
• Dipole-induced dipole (2-10 kJ/mol): Polarizable electron clouds created through dipole interaction with a polar molecule.
• Dispersion (London) (0.05-40 kJ/mol): Temporary fluctuations in electron distribution. All substances have dispersion forces.

Factors Affecting Strength of Induced Dipole

• Polarizability: Larger molar mass = larger electron cloud = increased polarizability = stronger attractions.
• Molecular Shape: More surface-to-surface contact = larger induced dipole = stronger attraction.

Effect of Molecular Size on Dispersion Force

• Increasing molecular size tends to increase dispersion forces.

Effect of Molecular Shape on Dispersion Force

• Linear shape (like n-pentane) allows for more contact points, resulting in stronger dispersion forces than branched shapes (like neopentane).

Dipole-Dipole Forces

• Attractive forces due to permanent polarization in molecules caused by unequal electron distribution and molecular structure.

Hydrogen-Bonding Forces

• A special type of dipole-dipole force; occurs when hydrogen is bonded to a small electronegative atom with lone pairs (N, O, or F).

Vapor Pressure and Boiling Point

• Boiling point is the temperature when vapor pressure equals external pressure. Normal boiling point is at a pressure of 760 torr.
• Higher external pressure increases the boiling point.
• Vapor pressure increases with temperature.
• Weaker intermolecular forces lead to higher vapor pressures.

Summarizing VP Trends in Graph – 3 different molecules

• Vapor pressure decreases with increasing intermolecular forces strength.
• Normal boiling points follow IMF trends (stronger IMF = higher boiling point).

Intermolecular Forces and Solubility

• "Like dissolves like." Polar solvents dissolve polar solutes; nonpolar solvents dissolve nonpolar solutes.
• Substances with similar IMF types will be soluble in each other. A solution forms when solute-solute and solvent-solvent attractions are replaced by solute-solvent attractions.
• Factors affecting solubility include temperature etc.
• A saturated solution is one where the maximum amount of solute has dissolved under given conditions.
• Important intermolecular forces in cases of ionic or polar solutem: Ion-dipole
• The strengths of intermolecular forces affect how well molecules or substances dissolve into each others.

Dual Polarity and Effects on Solubility

• Alcohols have a polar -OH group and a nonpolar hydrocarbon chain.
• The -OH group interacts with water through hydrogen bonding.
• The hydrocarbon chain interacts with nonpolar solvents through dispersion forces.

"Like Dissolves Like": Solubility of Methanol in Water

• Methanol solubility in water is due to hydrogen bonding between the polar methanol molecule and water molecules. Small changes to the structure of the molecule such as an additional CH2 grouping, can dramatically alter the solubility, even with similar molar masses.

Solubility (mol alcohol/1000 g solvent) of a Series of Alcohols in Water and Hexane

• Solubility trend in water increases as the number of CH3 grows.
• Solubility in hexane increases as the number of CH3 groups grows.

Examples of IMFs in Biological Macromolecules

Intermolecular forces are important for the structure and function of biological macromolecules like DNA and proteins. A knowledge of the forces involved provides information on the three dimensional structures of DNA, RNA, proteins, etc.