Intermolecular and Intramolecular Forces in GOB Chemistry

2 Major Attractive (Electrostatic) Forces

Intramolecular vs. Intermolecular Forces

Attractive forces in chemistry are classified as either intramolecular or intermolecular forces. Understanding the distinction between these is essential for predicting chemical behavior and properties.

• Intramolecular Forces: Exist within a molecule, holding atoms together and influencing chemical properties.

  • Examples: Covalent bonds, ionic bonds

  • Stronger than intermolecular forces

• Intermolecular Forces: Exist between molecules and influence physical properties.

  • Examples: Hydrogen bonding, dipole-dipole, ion-dipole, London dispersion

  • Hold liquid and solid molecules together

Example: Identify the type of force involved in the following situations:

• Condensation of water vapor: intermolecular

• Formation of NH3 through the combination of N and H: intramolecular

• Sugar dissolves in water: intermolecular

• Water flowing up the veins of a plant due to capillary action: intermolecular

Types of Intermolecular Forces

Overview of Intermolecular Forces

Intermolecular forces are the forces that hold molecules together in liquids and solids. The polarity of compounds plays a major role in determining the type and strength of these forces.

• There are four main types of intermolecular forces:

Type of Force	Exists Between	Strength	Example
Ion-Dipole	Ions and polar compounds	Strongest	Na+ (aq) & H2O
Hydrogen Bonding	Compounds containing H directly bonded to F, O, or N	2nd strongest	H2O & NH3
Dipole-Dipole	Two polar covalent compounds	3rd strongest	HCl & SO2
London Dispersion (van der Waals)	Dominant between two nonpolar covalent compounds	Weakest (increases with size/mass)	CH4 & CCl4

• London dispersion forces are present between all types of compounds, but are the only forces in nonpolar molecules.

Practice: Identifying Intermolecular Forces

• Between molecules and weaker than a chemical bond

• Types of intermolecular forces found in all molecules: London dispersion forces

• Species with the largest dispersion forces: Larger, heavier molecules (e.g., CH3CH3 vs. CH4)

Examples of Intermolecular Forces

• N2: London dispersion

• CH3OH: Hydrogen bonding

• CH2Cl2: Dipole-dipole

• KCl & CH3OH: Ion-dipole

Comparing Intermolecular Forces

Relative Strengths and Examples

The strength of intermolecular forces affects boiling points, melting points, and solubility.

• Ion-dipole: Strongest, found in solutions of ions and polar molecules

• Hydrogen bonding: Strong, found in molecules with H bonded to F, O, or N

• Dipole-dipole: Moderate, found in polar molecules

• London dispersion: Weakest, present in all molecules, dominant in nonpolar molecules

Practice Questions and Applications

• Highest number of different intermolecular forces: Molecules with both polar and nonpolar regions (e.g., CH3CH2CH2CH3)

• Primary force in Epsom salts and water: Dipole-dipole forces

Key Terms and Definitions

• Intramolecular force: Force holding atoms together within a molecule (e.g., covalent bond)

• Intermolecular force: Force between molecules affecting physical properties

• Polarity: Distribution of electrical charge over atoms in a molecule

• Hydrogen bond: Special dipole-dipole interaction involving H and F, O, or N

• London dispersion force: Temporary attractive force due to momentary electron distribution

Formulas and Equations

• Electrostatic force (Coulomb's Law):

$F = k \frac{q_1 q_2}{r^2}$

• Dipole moment:

$\mu = q \times d$

Summary Table: Types of Intermolecular Forces

Force Type	Exists Between	Relative Strength	Example
Ion-Dipole	Ions & polar molecules	Strongest	Na+ in H2O
Hydrogen Bonding	H bonded to F, O, or N	2nd strongest	H2O, NH3
Dipole-Dipole	Polar molecules	3rd strongest	HCl, SO2
London Dispersion	All molecules (dominant in nonpolar)	Weakest	CH4, CCl4

Additional info: London dispersion forces increase with molecular size and mass. Hydrogen bonding is a special case of dipole-dipole interaction, but much stronger due to the high electronegativity of F, O, and N.