Intermolecular Forces: Detailed Notes

Intermolecular Forces and Boiling Points

Ethanol vs. Diethyl Ether

• Question: What intermolecular force explains why pure ethanol has a higher boiling point than diethyl ether?

• Answer: Hydrogen bonds.

• Explanation:

  • Ethanol (CH3CH2OH) can form hydrogen bonds due to the presence of a hydrogen atom bonded to a highly electronegative oxygen atom. This allows for strong intermolecular attractions.

  • Diethyl ether (CH3CH2OCH2CH3) lacks a hydrogen atom directly bonded to an oxygen, so it cannot form hydrogen bonds with itself. It primarily exhibits dipole-dipole interactions and London dispersion forces, which are weaker than hydrogen bonds.

• Key Concept: Hydrogen bonds are a particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

Dipole-Dipole Interactions

• Diagram Description: A diagram showing an interaction between two HCl molecules.

• Correct Claim: It shows an example of dipole-dipole interactions.

• Explanation:

  • HCl is a polar molecule due to the difference in electronegativity between hydrogen and chlorine. This creates a partial positive charge (δ+) on the hydrogen atom and a partial negative charge (δ-) on the chlorine atom.

  • Dipole-dipole interactions occur when the positive end of one polar molecule is attracted to the negative end of another polar molecule.

Intermolecular Forces in Hexane

• Question: What is the primary intermolecular force in pure hexane (C6H14)?

• Answer: London dispersion forces.

• Explanation:

  • Hexane is a nonpolar molecule composed of only carbon and hydrogen atoms. All molecules exhibit London dispersion forces.

  • London dispersion forces are temporary, weak attractions that arise from instantaneous fluctuations in electron distribution, creating temporary dipoles.

  • Since hexane is nonpolar, London dispersion forces are the primary intermolecular forces present.

Ion-Dipole Interactions

• Diagram Description: A diagram depicting a phosphate group (P-O-) interacting with magnesium ions (Mg).

• Correct Claim: It shows an example of ion-dipole interactions.

• Explanation:

  • Ion-dipole forces occur between an ion (a charged species, such as Mg2+) and a polar molecule (or a polar region of a molecule).

  • The charged ion attracts either the positive or negative end of the polar molecule.

Hydrogen Bonding Example

• Diagram Description: A diagram showing an interaction between two molecules where an O-H group on one molecule interacts with an oxygen atom on another molecule.

• Correct Claim: The two molecules interact using hydrogen bonds.

• Explanation:

  • Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) is attracted to a lone pair of electrons on another electronegative atom.

Boiling Point Comparison

• Question: Which of the following would have the highest boiling point: H2S, HCl, H2O, H2

• Answer: H2O

• Explanation:

  • H2O has the highest boiling point because it forms hydrogen bonds. Hydrogen bonding is a stronger intermolecular force compared to the dipole-dipole forces present in HCl and H2S, and much stronger than the London dispersion forces present in H2.

  • H2 only has London dispersion forces, being a nonpolar molecule, thus it has the lowest boiling point.

London Dispersion Forces

• Question: What type of intermolecular force is the result of temporary uneven distribution of electrons around molecules that result in momentary dipoles?

• Answer: London dispersion forces.

• Explanation:

  • London dispersion forces (also known as van der Waals forces) arise from the instantaneous polarization of electron clouds in molecules.

  • These temporary, fluctuating dipoles induce dipoles in neighboring molecules, leading to weak attractive forces.