chemistry

INTRAMOLECULAR FORCES VS INTERMOLECULAR FORCES

Introduction

• Intramolecular forces: Forces that hold atoms together within a molecule.

• Intermolecular forces: Attractive forces between molecules.

Objectives

• Differentiate between intramolecular and intermolecular forces based on:

  • Relative strengths

  • Roles in determining chemical behavior and physical properties.

• Identify specific types of intermolecular forces present in a chemical system by analyzing molecular polarity and structure.

• Appreciate the fundamental role of intermolecular forces in biological and physical systems, such as hydrogen bonding in water.

Kinetic Molecular Theory (KMT) Premises

1. All matter consists of particles.

2. Particles are in constant random motion.

3. There are spaces between particles.

4. There are forces of attraction and repulsion between the particles.

Characteristics of States of Matter

• Gases:

  • Assumes the volume and shape of its container.

  • Low density and very compressible.

  • Molecules move very freely.

• Liquids:

  • Has a definite volume but assumes the shape of its container.

  • High density, only slightly compressible.

  • Molecules slide past one another freely.

• Solids:

  • Has a definite volume and shape.

  • High density and virtually incompressible.

  • Molecules vibrate about fixed positions.

Fundamental Difference

• The fundamental difference between states of matter is the distance between particles, influenced by:

  • Kinetic energy of the particles.

  • Strength of the attraction between the particles.

Types of Forces in Molecules

Intramolecular Forces

• Strong forces that hold atoms within a molecule together and determine the chemical behavior of a substance.

• Types of intramolecular forces:

  1. Ionic Bond

  2. Covalent Bond

  3. Metallic Bond

Intermolecular Forces

• Weaker forces compared to intramolecular forces but influential, especially in condensed phases (solids and liquids).

• Types of intermolecular forces, ranked from weakest to strongest:

  1. London Dispersion Forces (LDF)

  2. Dipole-Dipole Interactions

  3. Ion-Dipole Interactions

  4. Hydrogen Bond

Identifying Molecular Polarity

Steps to Identify if a Molecule is Polar or Nonpolar

1. Determine the electronegativity of the atoms in the molecule:

   • Electronegativity: The tendency of an atom to attract a bonding pair of electrons.

2. Determine the shape or geometry of the molecule using:

   • Lewis structure

   • Count 'things' (bonded atoms or lone pairs) on the central atom.

   • Maximize the distance between electron domains.

   • Lone pairs occupy more space than bonds.

Example Analysis

1. For HCl:

   • Electronegativity Difference (AEN): 0.9

   • Bond Type: Polar Covalent

2. For CO2:

   • AEN: 1.0

   • Bond Type: Polar Covalent

3. For CH4:

   • AEN: 0.4

   • Bond Type: Nonpolar Covalent

Important Notes

• If the bond is polar, the molecule can either be polar or nonpolar.

• If the bond is nonpolar, the molecule is automatically nonpolar.

Characterization of Intermolecular Forces

1. London Dispersion Forces (LDF)

• Weakest intermolecular force; exists in all types of substances: ionic, covalent, polar, or nonpolar.

• Occurs via instantaneous dipoles.

• Strength increases with the number of electrons in a molecule.

2. Dipole-Dipole Interactions

• Attracts molecules with permanent dipoles (separation of charge due to electronegativity differences).

• Only significant when molecules are close together.

• Example: H₂O, where H is partially positive and O is partially negative.

3. Dipole-Induced Dipole Interaction

• Exists between polar and nonpolar molecules; a polar molecule induces a dipole in a nonpolar molecule.

4. Ion-Dipole Interaction

• Attraction between an ion and a polar molecule, significant in solutions of ions.

5. Ion-Induced Dipole Interaction

• Occurs when an ion approaches a nonpolar molecule, inducing a temporary dipole.

6. Hydrogen Bonding

• A special type of dipole-dipole interaction.

• Exists in polar molecules containing an H atom bonded to highly electronegative atoms (F, O, or N).

• Significantly influences properties in water, DNA, and proteins.

Real-life Applications of Intermolecular Forces

1. DNA & Proteins:

   • Hydrogen bonds maintain the DNA double helix structure.

   • Intermolecular forces stabilize protein folding.

2. Water Transport in Plants:

   • Cohesion (water-water hydrogen bonds) and adhesion (water-xylem) facilitate water movement against gravity.

3. Electronic Devices:

   • Interactions influence LCD screens; material choices for conductivity depend on intermolecular forces.

4. Construction:

   • Polymers in gears and composites rely on specific intermolecular forces for desired traits like strength and flexibility.

Conclusion

Summary of Distinctions

Knowledge Assessment Questions

1. Which premise of the Kinetic Molecular Theory explains why matter can be compressed?
   A. All matter consists of particles.
   B. Particles are in constant random motion.
   C. There are spaces between particles.
   D. There are forces of attraction and repulsion between particles.

2. Which of the following is a characteristic of intramolecular forces?
   A. They are forces that hold atoms together within a molecule.
   B. They are responsible for the physical properties of a substance.
   C. They are generally weaker than intermolecular forces.
   D. They act only between different molecules in a condensed phase.

3. Which type of force determines the chemical behavior of a substance?
   A. Intermolecular Forces
   B. Intramolecular Forces
   C. London Dispersion Forces
   D. Hydrogen Bonding.

Assignment Instructions

• Determine whether various substances are Polar, Nonpolar, or Ions.

• Identify the primary intermolecular force existing between substances in the system.

End of Notes