*Correction: The bp (boiling point) of "Ethanol" is 173˚ F or 78˚ C. 

All these molecules have similar molecular weights; however they have vastly different boiling points. Why is that?

Intermolecular forces (IMF’s) keep molecules stuck together. Questions about boiling / melting points of molecules are always referring to the strength of IMF’s between molecules. 

1. Hydrogen bonding

Hydrogens attached to small, highly electronegative atoms can hydrogen bond. Namely N, O and F. 

2. Dipole-dipole

We like to call this the net dipole force because it’s only present when the molecule has a net dipole. 

Van der Waals Forces

Also known as London Dispersion Forces or Dipole-Induced Dipole Forces. 

All molecules possess Van der Waals Forces. They increase with the following properties:             

Size (the heavier, the stronger the force)

Shape:Ring > Straight Chains > Branched Chains

This picture is trying to illustrate how rings have greater surface area for interactions than straight or branched chains. 

Practice Questions: Now that we have learned about factors that influence the strength of IMF's let's apply them!