Intermolecular Forces: London Dispersion, Dipole-Induced Dipole, and Physical Properties

Overview of Intermolecular vs. Intramolecular Forces

• Intermolecular Forces (IMFs): Forces that act between individual molecules, or between molecules and ions.
• Intramolecular Forces: Forces that hold atoms together within a single molecule or compound (e.g., covalent bonds, ionic bonds, metallic bonds).
• Key Differences:
  • Intermolecular forces are significantly weaker than intramolecular forces.
  • Intermolecular forces act over longer distances than intramolecular forces.
  • Intermolecular forces directly govern the macroscopic physical properties of all substances (including melting points, boiling points, phase states, viscosity, and solubility).

Categorization of Intermolecular Attraction Forces

Intermolecular attraction forces can be categorized into several distinct types based on the types of species and dipoles involved:

• London dispersion force
• Dipole–induced dipole interaction
• Dipole–dipole attraction
• Hydrogen bonding
• Ion–dipole attraction
• Ionic attraction

London Dispersion Forces

Mechanism and Definition

• All atoms and molecules can interact via temporary dipoles.
• Fluctuations in electron distribution create instantaneous, temporary dipoles in an atom or molecule.
• This separation of charge is induced in adjacent molecules by electrostatic interaction with a nearby temporary dipole.
• London dispersion forces are the only intermolecular force present between nonpolar molecules.

Polarizability

• Definition: Polarizability refers to the ease with which the electron cloud of an atom or molecule can be distorted.
• An increase in molar mass and electron count leads to greater polarizability.
• Greater polarizability results in stronger temporary dipoles, which in turn leads to stronger London dispersion forces.

Factors Influencing London Dispersion Forces

1. Atomic and Molecular Size (Molar Mass)

• As molar mass and atomic/molecular size increase, electron clouds become larger and held less tightly, increasing polarizability.
• Larger size also increases the total surface area available for contact and dispersion interactions, causing boiling points to increase.
• Trend in Diatomic Halogens:
  • Fluorine ($F_2$): Molar Mass = $38\,\text{g/mol}$, Boiling Point = $85\,\text{K}$
  • Chlorine ($Cl_2$): Molar Mass = $71\,\text{g/mol}$, Boiling Point = $239\,\text{K}$
  • Bromine ($Br_2$): Molar Mass = $160\,\text{g/mol}$, Boiling Point = $332\,\text{K}$
  • Iodine ($I_2$): Molar Mass = $254\,\text{g/mol}$, Boiling Point = $457\,\text{K}$
  • Astatine ($At_2$): Molar Mass = $420\,\text{g/mol}$, Boiling Point = $610\,\text{K}$ (Estimated value)

• Trend in Straight-Chain Alkanes:
  • Methane ($CH_4$): Molar Mass = $16\,\text{g/mol}$, Boiling Point = $111\,\text{K}$
  • Ethane ($C_2H_6$): Molar Mass = $30\,\text{g/mol}$, Boiling Point = $184\,\text{K}$
  • Propane ($C_3H_8$): Molar Mass = $44\,\text{g/mol}$, Boiling Point = $231\,\text{K}$
  • Butane ($C_4H_{10}$): Molar Mass = $58\,\text{g/mol}$, Boiling Point = $272\,\text{K}$
  • Pentane ($C_5H_{12}$): Molar Mass = $72\,\text{g/mol}$, Boiling Point = $309\,\text{K}$
  • Hexane ($C_6H_{14}$): Molar Mass = $86\,\text{g/mol}$, Boiling Point = $342\,\text{K}$
  • Heptane ($C_7H_{16}$): Molar Mass = $100\,\text{g/mol}$, Boiling Point = $371\,\text{K}$
  • Octane ($C_8H_{18}$): Molar Mass = $114\,\text{g/mol}$, Boiling Point = $399\,\text{K}$

2. Molecular Shape and Branching

• Molecules with identical chemical formulas and molar masses (isomers) can display different boiling points based on their geometric structure.
• Increased branching produces a more compact, spherical structure with a smaller surface area, decreasing the contact area between adjacent molecules and lowering London dispersion forces.
• Comparison of Structural Isomers of Pentane ($C_5H_{12}$):
  • Pentane ($CH_3-CH_2-CH_2-CH_2-CH_3$): Extended linear shape, maximum surface area contact, Boiling Point = $309\,\text{K}$.
  • 2-Methylbutane ($CH_3-CH_2-CH(CH_3)-CH_3$): Single branch, reduced surface area contact, Boiling Point = $301\,\text{K}$.
  • 2,2-Dimethylpropane ($CH_3-C(CH_3)_2-CH_3$): Highly branched spherical shape, minimal surface area contact, Boiling Point = $282\,\text{K}$.

Physical Properties Influenced by Intermolecular Forces: Viscosity

• Viscosity: The measure of a liquid's resistance to flow.
• Mechanism: As intermolecular forces strengthen, molecules cannot slip past one another as easily, resulting in higher resistance to flow.
• Alkane Trend: The viscosity of liquid alkanes increases systematically as the number of carbon atoms and molar mass increase due to the accumulation of stronger London dispersion forces.

Dipole–Induced Dipole Interactions

• Definition: An intermolecular attraction that occurs between a polar molecule (having a permanent dipole) and a nonpolar molecule.
• Mechanism: The proximity of a permanent dipole in a polar molecule (such as water, $H_2O$) distorts the electron cloud of a neighboring nonpolar molecule (such as molecular oxygen, $O_2$), inducing a temporary dipole in the nonpolar species.
• Solubility Significance: Dipole–induced dipole interactions explain the solubility of nonpolar gases, such as $O_2$, in polar liquids like water ($H_2O$).

Questions & Discussion

• Question 1: Which molecule do you expect to have the smallest attractions from dispersion forces?

  • Option a: $CH_4$
  • Option b: $CH_3CH_3$
  • Option c: $CH_2Cl_2$
  • Option d: $CCl_4$
  • Answer: Option a ($CH_4$). $CH_4$ has the lowest molar mass and the smallest number of electrons among the options, making its electron cloud the least polarizable and producing the weakest London dispersion forces.

• Question 2: Polarizability refers to:

  • Option a: the ease with which the electron cloud of an atom or molecule can be distorted.
  • Option b: the magnitude of the dipole moment of a molecule.
  • Option c: the ease with which a hydrogen bond can form.
  • Option d: perturbation in electron density because of hydrogen bonding.
  • Option e: the ability to transmit polarized light.
  • Answer: Option a (the ease with which the electron cloud of an atom or molecule can be distorted).

• Question 3: Explain why, at room temperature, $CF_4$ is a gas but $CCl_4$ is a liquid?

  • Answer: Both $CF_4$ and $CCl_4$ are nonpolar tetrahedral molecules whose primary intermolecular forces are London dispersion forces. Chlorine atoms are larger than fluorine atoms, giving $CCl_4$ a higher molar mass ($\approx 153.8\,\text{g/mol}$) and a larger, more polarizable electron cloud than $CF_4$ ($\approx 88.0\,\text{g/mol}$). The significantly stronger London dispersion forces in $CCl_4$ hold its molecules together in the liquid phase at room temperature, whereas $CF_4$ has weaker dispersion forces and remains a gas.

• Question 4: Predict which compound has the stronger London dispersion force in each pair:

  • Pair (a): $C_2Cl_6$ or $C_2F_6$
  • Answer: $C_2Cl_6$. Chlorine has a larger atomic radius and molar mass than fluorine, making the electron cloud of $C_2Cl_6$ larger and more polarizable.
  • Pair (b): $CH_4$ or $C_3H_8$
  • Answer: $C_3H_8$. Propane ($C_3H_8$) has a higher molar mass ($44\,\text{g/mol}$ vs. $16\,\text{g/mol}$) and a larger molecular surface area than methane ($CH_4$).
  • Pair (c): $CS_2$ or $CO_2$
  • Answer: $CS_2$. Sulfur atoms are larger and have more electron shells than oxygen atoms, making $CS_2$ larger and more polarizable than $CO_2$.