CEM141 Recitation #12

Intermolecular Forces: CEM 141 Recitation 12 KEY

1. Ethane (CH3CH3)

1a. Liquid State Representation

  • Draw 3 molecules of ethane (CH3CH3) in the liquid state.

  • Show the strongest interactions present between the molecules.

1b. Type of Interaction

  • Type of Interaction: London Dispersion Forces (LDFs)

  • Description:

    • London dispersion forces are temporary interactions that occur due to the formation of an instantaneous dipole in one molecule inducing a dipole in a neighboring molecule.

    • Important Features:

    • Dipoles are labeled as instantaneous and induced dipoles.

    • Partial positive ($ ext{δ}^+$) and partial negative ($ ext{δ}^-$) charges are indicated on the electron clouds of the molecules.

    • The arrangement should depict the interaction of the partial positive end of one molecule with the partial negative end of another.

1c. Other Intermolecular Forces

  • None

2. Dimethyl Ether (CH3OCH3)

2a. Liquid State Representation

  • Draw 3 molecules of dimethyl ether (CH3OCH3) in the liquid state to show the strongest interactions present.

2b. Type of Interaction

  • Type of Interaction: Dipole-Dipole Interactions

  • Description:

    • Dipole-dipole interactions arise from the attractive force between the molecular dipoles of two polar molecules.

    • Important Features:

    • A molecular dipole arrow is used to indicate overall polarity.

    • Molecules are organized such that the interaction occurs between the partial positive end ($ ext{δ}^+$) of one molecule and the partial negative end ($ ext{δ}^-$) of another.

2c. Other Intermolecular Forces

  • London Dispersion Forces (LDFs)

3. Methanol (CH3OH)

3a. Liquid State Representation

  • Draw 3 molecules of methanol (CH3OH) in the liquid state showing the strongest interactions present.

3b. Type of Interaction

  • Type of Interaction: Hydrogen Bonding

  • Description:

    • Hydrogen bonding occurs when a hydrogen atom covalently bonded to a highly electronegative atom (oxygen in this case) interacts with a lone pair of electrons on another electronegative atom.

    • Important Features:

    • The dotted line representing the hydrogen bond starts and ends at the involved atoms (the hydrogen and the lone pair oxygen).

3c. Other Intermolecular Forces

  • Dipole-Dipole Interactions

  • London Dispersion Forces (LDFs)

4. Comparison of Boiling Points

Relative Boiling Points

  • Predicted Order of Boiling Points:

    1. Methanol (CH3OH)

    2. Dimethyl Ether (CH3OCH3)

    3. Ethane (CH3CH3)

Explanation

  • The boiling point of a substance correlates with the strength of the intermolecular forces that must be overcome to transition from the liquid to gas phase.

    • Methanol:

    • Strongest interactions (hydrogen bonding, dipole-dipole, and LDFs) require the most energy to overcome.

    • Results in the highest boiling point.

    • Dimethyl Ether:

    • Contains dipole-dipole interactions and LDFs which are weaker than hydrogen bonds but stronger than only LDFs.

    • Hence, it has the next highest boiling point.

    • Ethane:

    • Exhibits only London dispersion forces which are the weakest.

    • Consequently, it has the lowest boiling point.

Note on Electron Clouds

  • All molecules have similarly sized electron clouds, hence the LDFs are comparable in strength across these molecules.