CHEM 142_Lecture_5_with polleverywhere

Chapter 10: Intermolecular Forces

10.1 Intramolecular Forces vs. Intermolecular Forces

• Intramolecular Forces: These are the forces that hold atoms together within a molecule, primarily through covalent or ionic bonds. They determine the chemical properties of substances.

• Intermolecular Forces: These forces act between molecules and are responsible for determining the physical properties of substances, such as boiling points, melting points, and solubilities. They are weaker than intramolecular forces and play a crucial role in the state of matter (solid, liquid, gas).

10.2 Dispersion Forces

• Dispersion (London) Forces: A type of intermolecular force that is present in nonpolar molecules due to the formation of temporary dipoles. These forces arise when electrons in a molecule are distributed asymmetrically, leading to a brief, uneven charge distribution.

• Temporary Dipole: This is characterized by a brief, localized positive and negative charge resulting from electron movement around the nucleus of an atom.

• Polarizability: This term refers to the ease with which the electron cloud surrounding an atom or molecule can be distorted. Larger atoms or molecules with more electrons tend to have greater polarizability, which enhances the strength of dispersion forces.

10.3 Interactions Involving Polar Molecules

• Ion-Dipole Interaction: This attractive force occurs between an ion (charged particle) and a polar molecule. This interaction is significant in solutions, such as when salt dissolves in water.

• Dipole-Dipole Interaction: These forces occur between molecules that have permanent dipoles due to their polar nature. The positive end of one polar molecule is attracted to the negative end of another.

• Hydrogen Bonding: A particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen. This bond type significantly influences the properties of water and other substances.

• Dipole-Induced Dipole Interaction: This occurs when a polar molecule induces a dipole in a nonpolar molecule by virtue of its electric field, enhancing attraction between the two.

Strength of Dispersion Forces

• Factors Affecting Strength:

  • Size of Molecules: Larger molecules typically have more electrons and greater polarizability, leading to stronger dispersion forces.

  • Shape of Molecules: Molecules with increased surface areas, such as linear molecules, can interact more effectively, thus enhancing dispersion forces compared to branched molecules.

Table 10.2: Boiling Points of Halogens

• This table illustrates how molar mass impacts the boiling points of halogens.

  • F2: 85 K

  • Cl2: 239 K

  • Br2: 332 K

  • I2: 457 K

  • This pattern shows that as the size and molar mass of the halogens increase, their boiling points also increase, largely due to enhanced dispersion forces.

Effect of Shape on Dispersion

• Molecular shape plays a critical role in defining the strength of dispersion forces:

  • Linear molecules generally create greater dispersion forces than their branched counterparts owing to the increased surface contact area, facilitating stronger intermolecular interactions.

Summary of Intermolecular Forces

• Relative Strengths: The ranking of intermolecular forces from weakest to strongest is as follows:

  • Dispersion Forces < Dipole-Dipole Forces < Hydrogen Bonds.

  • Dipole-induced dipole interactions have an intermediate strength, highlighting the varying capabilities of these forces.

• Phenomena Explained: Several phenomena can be explained by understanding intermolecular forces.

  • Example: NaCl dissolves in water primarily due to strong ion-dipole interactions.

  • Example: Ice expands upon freezing, a result of the hydrogen bonding which causes a unique crystalline structure.

Vibrating Molecules

• Atoms within a molecule vibrate due to thermal energy, and these vibrations can be modeled similarly to springs—stretched and compressed depending on energy levels.

Vibrational Wavefunctions

• The energy levels associated with molecular vibrations are quantized. The relationship between vibrational energy, atomic mass, and other variables such as bond strength can be described through mathematical equations.

IR Spectroscopy

• Molecular Vibration: Transitions occur that absorb infrared light, particularly in molecules that exhibit changing dipole moments during vibrations. This is crucial in identifying functional groups in various substances.

• Wavenumbers: Calculated to characterize these vibrational transitions, wavenumbers are an essential aspect of vibrational spectroscopy.

Practical Applications

• Understanding bond energies allows chemists to estimate vibrational frequencies and analyze vibrational transitions effectively.

• For instance, CO2 molecules exhibit various types of vibrations that impact their spectroscopic behavior, making their interpretation important in atmospheric chemistry.