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Types of Intermolecular Forces

SECTION 10 - Liquids Solids and Intermolecular Forces


Inter
molecular forces occur between molecules.  Intramolecular forces (bonds) occur within molecules.

Types of Intermolecular Forces

There are two types of intermolecular forces:

1. Dipole-Dipole Forces = only occur between polar molecules.

   ➞ Note: Hydrogen Bonds are a special type, or "subset" of Dipole-Dipole Forces.

2. London Dispersion Forces = also called "van der Waals forces," these occur between all types of molecules. But these are the only forces holding nonpolar molecules and noble gas atoms together.

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Dipole-Dipole Forces

Dipole-dipole forces are only 1% as strong as covalent bonds or ionic bonds.

ex:  polar molecule, HCl

Dipole-Dipole Forces

As you can see in the image above, a dipole-dipole force occurs between two molecules of HCl, and of course the sample goes on and on like this in three dimensions.

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A special type of dipole-dipole force is called Hydrogen Bonding...

Hydrogen Bonds

- occur in polar molecules having O-H , N-H , or F-H bonds.
stronger than a regular dipole-dipole interaction. 

WHY ? ...

Hydrogen Bonds are stronger than a regular dipole-dipole force because H is small, allowing for dipoles to be in close proximity to each other. 

And also because N , O , and F are very electronegative which means that the dipole-dipole forces are highly polarized.

ex:  H2O intermolecular forces...

Hydrogen Bonding in Water

We learned in SECTION 8 - Chemical Bonding that water is a polar molecule with a bent or v-shaped geometry. This is critical to its polarity.

In the image above, you can see that hydrogen bonds are not really "bonds" at all, but instead they are a special type of intermolecular force (IMF).

Each water molecule makes 4 hydrogen bonds to neighboring H2O molecules.

Let's do another example below...

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ex:  H2O has a melting point and boiling point of 0°C and 100°C respectively, while H2S has a melting point and boiling point of -82°C and -60°C respectively. Why the enormous difference?

Well,...

➞ H2O molecules are aggregated "tightly" to each other by very strong hydrogen bonding. H2S molecules are not.

The stronger the intermolecular forces, the more difficult it is to separate the individual molecules from each other.

➞ this results in a higher melting point (m.p.) and a higher boiling point (b.p) for H2O.

➞ H2S molecules are held together by "regular" dipole-dipole forces.

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The second type of Intermolecular Force has several names but is most commonly called London Dispersion Forces, abbreviated LDF.

London Dispersion Forces

London Dispersion Forces = also called "Dispersion Forces" or "van der Waals Forces."

In nonpolar molecules and noble gas atoms, London Dispersion Forces are especially important because they are the only intermolecular forces.

LDF Forces are caused by an instantaneous dipole (polarization) that can occur randomly at any given instant within a nonpolar molecule or atom:

Induced Dipoles

ex:  Instantaneous Dipoles and Induced Dipoles.

Induced Dipoles and London Dispersion Forces

Read through the above example and you'll see that three things are happening:

1.  A nonpolar H2 molecule is behaving normally. Then, an instantaneous dipole occurs randomly and H2 is polarized as 2 electrons no longer spread out equally.

2. This polarization will, in turn, induce a dipole in a neighboring H2 molecule, and...

3. This process gathers momentum and continues on and on.

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In the above example, H2 has only two electrons. Imagine if we were talking about the nonpolar halogen, I2

I2 has 106 electrons!  The larger the molecule, the greater the London Dispersion Forces.

Why?

➞ because large molecules have many electrons and it's easier for an uneven electron-distribution to occur.

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Next up in SECTION 10 - Liquids, Solids, and Intermolecular Forces, we'll talk about: 

Liquids - Surface Tension, Capillary Action, and Viscosity  :-)