Molecular Mass, Polarity, and Boiling Point

Overview

This lecture explains how molecular mass and polarity affect boiling point, highlights the differences between bond and molecular polarity, and details how to determine if a molecule is polar.

Relationship Between Molecular Mass and Boiling Point

• As molecular mass increases, boiling point generally increases due to stronger intermolecular (between molecules) forces.
• Mass can increase by having more or bigger atoms in the molecule.
• Stronger intermolecular forces make it harder to separate molecules, raising the boiling point.
• Example: methane (mass 16, boiling point 109 K), ethane (30, 145 K), propane (44, higher).

Exceptions: Role of Molecular Polarity

• Mass is not the only factor; polarity also affects boiling point.
• Water (mass 18) has a higher boiling point than carbon dioxide (mass 44) despite being lighter, due to molecular polarity.

Bond Polarity vs. Molecular Polarity

• Bond polarity depends on difference in electronegativity between atoms; polar bonds have unequal electron sharing.
• Molecular polarity is determined by both the polarity of bonds and the shape of the molecule.
• Carbon dioxide has polar bonds but is a non-polar molecule; water has polar bonds and is also a polar molecule.

Determining Molecular Polarity

• Draw the Lewis structure of the molecule.
• Identify polar bonds using differences in electronegativity (ΔEN).
• Assign partial positive (δ+) and negative (δ−) charges based on electronegativity.
• Add dipole arrows pointing toward the more electronegative atom.
• Analyze if dipoles cancel out (non-polar molecule) or create distinct poles (polar molecule).

Dipoles and Molecular Shape

• Dipoles are represented as arrows pointing towards the atom with the partial negative charge.
• If dipoles are equal and opposite (e.g., CO₂ linear), they cancel, and the molecule is non-polar.
• If dipoles do not cancel (e.g., H₂O angular), the molecule is polar with distinct positive and negative ends.

Steps to Identify Molecular Polarity

• Draw the molecule and identify bonds.
• Calculate ΔEN for each bond and determine bond polarity.
• Label partial charges and draw dipoles.
• Assess the overall shape and check if dipoles cancel.
• If dipoles cancel, molecule is non-polar; if not, it is polar.

Key Terms & Definitions

• Intermolecular Force — Attraction between separate molecules.
• Intramolecular Force — Bonding within a molecule.
• Bond Polarity — Unequal sharing of electrons between two atoms due to electronegativity difference.
• Molecular Polarity — Overall polarity of the molecule, determined by bond polarity and molecular shape.
• Electronegativity (EN) — Atom’s ability to attract electrons in a bond.
• Dipole — An arrow representing partial charges due to unequal electron sharing.

Action Items / Next Steps

• Practice drawing Lewis structures for water and carbon dioxide.
• Calculate electronegativity differences for bonds in sample molecules.
• Determine molecular polarity following outlined steps.
• Review VSEPR theory for more on molecular shapes.