Intermolecular Forces

Agenda:

  1. Types of Intermolecular Forces

  2. Properties of the Liquid State

  3. The Uniqueness of Water

Intermolecular vs Intramolecular Forces

  • Intermolecular Forces:

    • Definition: Nonbonding forces present between molecules.

    • Characteristics:

    • Physical behavior varies across different phases of matter due to differences in intermolecular force strength.

  • Intramolecular Forces:

    • Definition: Bonding forces found within a molecule.

    • Characteristics:

    • Chemical behavior remains the same across phases due to the same basic particle being present.

Nature of Intermolecular Forces

  • Definition: Arise from attractions between molecules with partial charges or between ions and molecules.

  • Strength Comparison:

    • Intermolecular forces are weaker than bonding forces because they involve smaller charges that are farther apart.

Attraction Mechanism

  • Why are molecules attracted to each other?

    • Intermolecular attractions occur due to:

    • Attractive forces between oppositely charged ions (e.g., ion (+) to ion (−)).

    • Attraction between the (+) end of a polar molecule and the (−) end of another polar molecule.

    • Even in non-polar molecules, temporary charges can induce attraction.

  • Effect of Charge and Distance:

    • Larger charges result in stronger attractions.

    • Longer distances yield weaker attractions.

  • Example: Water

Strength of Intermolecular Attraction Forces

  • Greater attraction results in more energy required to separate the molecules.

  • Example: Boiling Point

    • The amount of energy required to overcome all attractive forces between particles determines boiling point.

    • Higher normal boiling points correlate with stronger intermolecular forces.

Comparison of Bonding and Nonbonding Forces

Bonding Forces

Type

Model

Basis of Attraction

Energy (kJ/mol)

Example

Ionic

+

Cation-anion

400-4000

NaCl

Covalent

+

Nuclei-shared e-pair

150-1100

H−H

Metallic

+

Cations-delocalized electrons

75-1000

Fe

Nonbonding (Intermolecular) Forces

Type

Model

Basis of Attraction

Energy (kJ/mol)

Example

Ion-Dipole

+

Ion charge-dipole charge

40-600

Na⁺…H₂O

Hydrogen Bond

-

Dipole charge

6-20+

H−H…B−

Dipole-Dipole

-

Dipole charge

10-40

H−Cl⋅⋅⋅H−Br

Ion-Induced Dipole

-

Ion charge-polarizable electrons

3-15

Fe²⁺…02+

Dipole-Induced Dipole

-

Dipole charge-polarizable electrons

2-10

H−Cl…Cl−I

Dispersion (London)

-

Nonpolar temporary dipoles

0.05-40

F−…F−

Dispersion Forces

  • Definition: Result from fluctuations in the electron distribution within molecules or atoms.

  • Mechanism:

    • For nonpolar molecules, dispersion forces induce a temporary dipole moment.

    • For polar molecules, the field enhances the existing dipole moment.

  • Polarizability:

    • Defined as the ease with which an electron cloud can be distorted.

    • Stronger dispersion forces correlate with higher polarizability; larger particles show stronger dispersion forces than smaller particles.

Dispersion Forces: Molar Mass and Boiling Points

  • General Trend:

    • Dispersion forces increase with increasing molar mass.

  • Data Examples:

    • Noble Gases:

    • He: Molar Mass 4.00 g/mol, Boiling Point 4.2 K

    • Ne: Molar Mass 20.18 g/mol, Boiling Point 27 K

    • Ar: Molar Mass 39.95 g/mol, Boiling Point 87 K

    • Kr: Molar Mass 83.80 g/mol, Boiling Point 120 K

    • Xe: Molar Mass 131.30 g/mol, Boiling Point 165 K

    • Halogens:

    • F₂: Molar Mass 38.00 g/mol, Boiling Point 85.0 K

    • Cl₂: Molar Mass 70.91 g/mol, Boiling Point 239 K

    • Br₂: Molar Mass 159.8 g/mol, Boiling Point 333 K

    • I₂: Molar Mass 253.8 g/mol, Boiling Point 458 K

Dispersion Forces and Molecular Shape

  • Larger surface area correlates with increased intermolecular contact and stronger dispersion interactions.

  • Comparative Example:

    • n-Pentane (C₅H₁₂): Molar mass = 72.15 g/mol, Boiling point = 36.1 °C

    • Neopentane (C₅H₁₂): Molar mass = 72.15 g/mol, Boiling point = 9.5 °C

Dipole-Dipole Forces

  • Defined as attractive forces between adjacent polar molecules due to their dipole moments.

  • Boiling Point Comparison:

    • Dimethyl ether (C₂H₆O): Molar mass = 46.07 g/mol, boiling point = -22.0 °C

    • Acetaldehyde (C₂H₄O): Molar mass = 44.05 g/mol, boiling point = 20 °C

Hydrogen Bonding

  • Defined as a strong type of dipole-dipole attraction, occurring when H is directly bonded to highly electronegative atoms (F, O, or N).

  • Mechanism:

    • Intermolecular hydrogen bond occurs between the H atom of one molecule and the lone pair of the electronegative atom from another molecule.

  • Compare boiling points of various substances to underline the effect of hydrogen bonding.

Biological Implications of Hydrogen Bonding

  • Example: In DNA structure, hydrogen bonds play a critical role, connecting base pairs (adenine with thymine, guanine with cytosine).

  • Antimalarial Drug Discovery: Numerous studies have highlighted the role of hydrogen bonding in drug design effectiveness (O’Neill et al. publications).

Ion-Dipole Forces

  • Defined as the attractive forces between an ion and a polar molecule. Important in solutions of ionic compounds.

Summary of Intermolecular Attraction Forces

  • Types:

    1. Dispersion (London) Forces

    2. Dipole-Dipole Forces

    3. Hydrogen Bonding

    4. Ion-Dipole Forces

  • Correlating properties:

    • Surface Tension

    • Viscosity

    • Capillary Action

Properties of Liquids

  • Surface Tension:

    • Definition: The tendency of liquids to minimize surface area.

    • Surface molecules interact with fewer neighbors compared to interior molecules.

    • Factors influencing surface tension include intermolecular attractive forces and temperature.

Viscosity

  • Defined as the resistance of a liquid to flow, measured in units of poise (1 P = 1 g/cm·s).

  • Example: Water has a viscosity of 1 cP at room temperature.

  • Factors affecting viscosity include intermolecular forces, molecular shape, and temperature.

Capillary Action

  • Defined as the ability of a liquid to flow upward in a thin tube against gravity due to cohesive and adhesive forces.

  • Meniscus Shape:

    • Water forms a concave meniscus; mercury forms a convex meniscus.

The Uniqueness of Water

  • Water (H₂O): Notable properties include a high boiling point, specific heat capacity, and unique density characteristics.

  • Boiling Point Comparison:

    • Water’s boiling point is significantly elevated compared to other molecules of similar mass due to hydrogen bonding.

Specific Heat Capacity of Water

  • High heat capacity: 4.184 J/mol·K, regulating Earth’s temperature. Applications include use as a coolant and protection for crops.

Density of Water

  • Density values at different temperatures:

    • At 0 °C: 0.99987 g/cm³

    • At 4 °C: 1.00000 g/cm³

  • Implications of water's unique density behavior (e.g., why ice floats on water).

Interactive Discussion Topics

  • Identify compounds that are liquid at room temperature and rationalize why.

  • Determine which substances exhibit hydrogen bonding through molecular structure analysis.