Overview
This lecture covers the relationship between molecular mass and boiling point, distinguishes between bond and molecular polarity, and explains how to determine molecular polarity using examples like water and carbon dioxide.
Molecular Mass and Boiling Point
- As molecular mass increases, boiling point generally increases due to stronger intermolecular forces.
- More massive molecules can result from larger atoms, more atoms, or both.
- Intermolecular forces are attractions between molecules, while intramolecular forces are bonds within a molecule.
- Higher molecular mass means molecules are harder to separate, leading to higher boiling points.
- Mass is not the only factor affecting boiling point; molecular properties also play a role.
Bond Polarity vs. Molecular Polarity
- Bond polarity is determined by the difference in electronegativity between two bonded atoms.
- Polar bonds have an electronegativity difference greater than zero but less than 1.7.
- Molecular polarity depends on both bond polarity and the molecule's shape.
- A molecule can have polar bonds but still be non-polar overall if the dipoles cancel (e.g., CO₂).
Examples: Water and Carbon Dioxide
- CO₂ has polar bonds but is a non-polar molecule because its shape causes dipoles to cancel out.
- H₂O has polar bonds and is a polar molecule because its shape causes dipoles to add up instead of cancel.
- The Lewis structure and placement of charges/dipoles are crucial for determining molecular polarity.
Using Dipoles to Determine Polarity
- A dipole is represented by an arrow pointing toward the more electronegative atom (partial negative charge).
- If dipoles within a molecule cancel out, the molecule is non-polar.
- If dipoles do not cancel, the molecule is polar, with one end different from the other.
Steps to Determine Molecular Polarity
- Draw the molecule and its Lewis structure.
- Find electronegativity differences for each bond.
- Assign partial charges (δ+ and δ-) and draw dipoles.
- Analyze the shape to see if dipoles cancel out or add up.
- If they cancel, the molecule is non-polar; if not, it is polar.
Key Terms & Definitions
- Bond Polarity — Difference in electronegativity between two bonded atoms.
- Molecular Polarity — Whether the overall molecule has a positive and negative pole.
- Intermolecular Forces — Attractions between different molecules.
- Intramolecular Forces — Bonds within a single molecule.
- Dipole — Arrow indicating unequal sharing of electrons, points to partial negative charge.
Action Items / Next Steps
- Practice drawing Lewis structures for common molecules.
- Calculate bond electronegativity differences and assign partial charges.
- Apply steps to determine molecular polarity for assigned molecules.