Intermolecular Forces

Now that we understand how bonds keep atoms together, the next question is, what force keeps molecules together? Do they bond to each other as well? Let’s take a look. 

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. 

Strength of IMF's

  1. Hydrogen bonding

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

  1. Dipole-dipole

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

  1. 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:             

  1. Size (the heavier, the stronger the force)
  2. 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. 

Problem: PRACTICE: Which of the following pairs of molecules would have the highest boiling point?

2m

Problem: PRACTICE: Which of the following pairs of molecules would have the highest boiling point?

2m

Problem: PRACTICE: Which of the following pairs of molecules would have the highest boiling point?

2m

Problem: PRACTICE: Which of the following pairs of molecules would have the highest boiling point?

2m