Molecular Polarity and Boiling Points

This post contains a PHET model of molecular bond polarities including dipole, charge density and electronegativity. WE discuss diploes, dipole moments, hydrogen bonds, and the effect of dipole moment on boiling point.

PHET MODEL

Molecule Polarity

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What are the different types of molecular polarity? pg 372-374

VOCABULARY

polar
non-polar
dipoles
dipole moment
dipole – dipole attraction (example: hydrogen bonds in water)
dipole – ion attraction (example: how salt NaCl dissolves in water)
induced dipole interaction

Predict molecular polarity from molecular shape and bond polarities

 

Hydrogen Bonding:
Hydrogen bonding is the unusually strong dipole-dipole interaction that occurs when a highly electronegative atom (N, O, or F) is bonded to a hydrogen atom. This bond nearly strips the hydrogen atom of its electrons leaving, essentially, a naked proton. This proton is highly attracted to the electron pairs on nearby molecules.  Hydrogen bonding is significantly stronger than the dipole-dipole interactions which are in turn stronger than London dispersion forces. Hydrogen bonding exists only in molecules with an N-H, O-H, or F-H bond.
Dipole-Dipole:
Dipole-dipole interaction is the attraction between a partially negative portion of one molecule and a partially positive portion of a nearby molecule. Dipole-dipole interaction occurs in any polar molecule as determined by molecular geometry.

 

dipole-bonding

 

 

 

 

Conclusion:  The Hydrogen Bond is a common form of Dipole-Dipole Interaction.

So why are dipole interactions and Hydrogen Bonds important?

Charge-Dipole Forces:
An uncharged molecule can still have an electric dipole moment. Electric Dipoles arise from opposite but equal charges separated by a distance. Molecules that possess a dipole moment are called Polar molecules (remember the polar covalent bond?). Water is polar and has a dipole moment of 1.85 Debye. The Debye is a unit of dipole moment and has a value of 3.336 x 10^-30 Coulomb meter.
When salt is dissolved in water, the ions of the salt dissociate from each other and associate with the dipole of the water molecules. This results in a solution called an Electrolyte.
dipole-nacl-water
Salt is NaCl.
Which ions are Na? Which are Cl?
Explain how water molecules can be attracted to and surround both Na and Cl ions.
Dipole-Dipole Forces
Dipole-Dipole forces exist between neutral polar molecules. Think of a dipole as an arrow with its tail at the – (negative) charged part of the molecule and the arrows tip is at the + (positive) part of the molecule. The “dipole moment” is a measure of the electrical strength of the dipole. The unit of dipole moment is a Debye (named after the scientist who discovered the effect).
The following diagram shows dipoles and dipole attraction of polar molecules in gas, liquid and solid phase.
dipoles-2
Dipoles and Boiling Points
The dipole-dipole interaction causes polar molecules to stick together a little more strongly than they would if they were non-polar molecules having no dipoles. As a result, it takes more energy for polar  molecules to evaporate than non-polar molecules of the same molecular weight.  This results in a higher boiling point for the polar molecules.  Molecules with a larger dipole moment have even higher boiling points than molecules with a smaller dipole moment.
dipole-moment-and-boiling-point
Why does Acetaldehyde have a higher boiling point than Propane?

London Dispersion Forces: The weakest and most temporary of all the intermolecular forces:

For detailed explanation of the factors effecting intermolecular forces and boiling points please see this reference paper.
Reference – see pages 4-12, especially charts on page 10.