Solids, Liquids, and Intermolecular Forces: Study Guide

Solids, Liquids, and Intermolecular Forces

Difference Between Intermolecular and Intramolecular Forces

Understanding the distinction between intermolecular and intramolecular forces is fundamental in chemistry. Intramolecular forces are the chemical bonds (such as covalent, ionic, or metallic bonds) that hold atoms together within a molecule. Intermolecular forces are the forces of attraction or repulsion between neighboring molecules, affecting physical properties like boiling and melting points.

• Intramolecular forces: Strong, responsible for molecule formation (e.g., O–H bond in water).

• Intermolecular forces: Weaker, responsible for interactions between molecules (e.g., hydrogen bonding between water molecules).

• Example: Water's O–H bond is intramolecular; hydrogen bonds between water molecules are intermolecular.

Types of Intermolecular Forces

London Dispersion Forces (LDFs)

London Dispersion Forces are present in all molecules and atoms, arising from temporary fluctuations in electron distribution. They are the only intermolecular force in nonpolar molecules and increase with molecular weight.

• Present in: All molecules and atoms.

• Strength: Weak in small molecules, significant in large molecules.

• Example: I2 and BF3 exhibit only LDFs.

Dipole-Dipole Forces

Dipole-dipole forces occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another.

• Present in: Polar molecules.

• Strength: Stronger than LDFs, but weaker than hydrogen bonds.

• Example: CH3F and HCl exhibit dipole-dipole forces.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction, occurring when hydrogen is bonded to highly electronegative atoms (F, O, or N). It is much stronger than other intermolecular forces and has significant effects on physical properties.

• Atoms involved: H, F, O, N only.

• Energy: Average energy is about 40 kJ/mol.

• Examples: HF, H2O, NH3, ethanol, hydrogen peroxide, acetic acid, methylamine.

• Effect: Substances with hydrogen bonding have higher melting and boiling points.

Example: Hydrogen bonding in ethanol and water.

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 Examples: Hydrogen bonding in ammonia, hydrogen peroxide, methanol, acetic acid, methylamine.

Effect of Hydrogen Bonding on Physical Properties

• Hydrogen bonding is stronger than London dispersion and dipole-dipole forces.

• Compounds capable of hydrogen bonding have higher melting and boiling points.

Example: Boiling point trends for hydrides of groups 4A and 6A.

Example: Comparison of formaldehyde, fluoromethane, and hydrogen peroxide.

• Similar molar masses and dipole moments, but only hydrogen peroxide has hydrogen bonding, making it liquid at room temperature.

Hydrogen Bonding in DNA

Hydrogen bonds are crucial in the structure of DNA, holding the base pairs together.

Important: Hydrogen bonding is an intermolecular force, not a chemical bond. It should not be confused with covalent or ionic bonds within molecules.

Ion-Dipole Forces

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

• Present in: Solutions of ionic compounds and polar solvents.

• Strength: Very strong, crucial for dissolving ionic compounds in water.

• Example: Sodium chloride solution.

The strength of ion-dipole forces depends on ion size, charge, and dipole moment. Cations interact more strongly than anions of the same charge due to their smaller size.

Summary Table: Types of Intermolecular Forces

Type	Present In	Molecular Perspective	Strength
Dispersion	All molecules and atoms	Temporary dipoles	0.05–20 kJ/mol
Dipole-dipole	Polar molecules	Permanent dipoles	3–20 kJ/mol
Hydrogen bonding	H bonded to F, O, or N	Special dipole-dipole	10–40 kJ/mol
Ion-dipole	Ions and polar molecules	Ion and dipole	30–100+ kJ/mol

Comparison of Intermolecular Forces

• For substances with similar molar mass and shape, LDFs are similar; differences arise from dipole-dipole or hydrogen bonding.

• Hydrogen bonding leads to stronger intermolecular attractions.

• For substances with very different molar mass and similar shape, LDFs dominate if no hydrogen bonding is present.

Example: Formaldehyde vs. ethane boiling and melting points.

Practice: Identifying Intermolecular Forces

Compound	LDF	D-D	HB
PCl3	yes	yes	no
CH3F	yes	yes	no
I2	yes	no	no
HF	yes	yes	yes
HCl	yes	yes	no
CH2O	yes	yes	no
CH3OH	yes	yes	yes
BF3	yes	no	no

Intermolecular Forces and Physical Properties

Surface Tension

Surface tension is the net inward pull experienced by molecules at the surface of a liquid, causing the surface to tighten like an elastic film. It is a measure of the energy required to increase the surface area of a liquid.

• Higher surface tension: Indicates stronger intermolecular forces.

• Example: Water has much higher surface tension than ethanol due to extensive hydrogen bonding.

Substance	Surface Tension (J/m2)
Water, H2O	7.3×10‒2
Ethanol, C2H5OH	2.3×10‒2

• Surface tension decreases with increasing temperature due to increased molecular motion.

Viscosity

Viscosity is the resistance to flow exhibited by liquids and gases. It is affected by intermolecular forces and temperature.

• Strong IMFs: Lead to higher viscosity.

• Viscosity increases: With increasing molar mass.

• Viscosity decreases: With increasing temperature.

Capillary Action

Capillary action is the rising of a liquid through a narrow tube against gravity, resulting from the competition between cohesive and adhesive forces.

• Cohesive forces: Attraction between molecules of the same substance.

• Adhesive forces: Attraction between molecules and a surface.

• Example: Water rises in glass tubes (concave meniscus) due to strong adhesive forces; mercury does not (convex meniscus) due to strong cohesive forces.

Additional info: This study guide covers Chapter 12 topics relevant to general chemistry, including intermolecular forces, their types, effects on physical properties, and examples. All images included directly reinforce the explanations provided.