Chem – Intermolecular Forces from Lewis Structures

What are intermolecular forces?

Intermolecular forces (or bonds) are the forces that hold together two different molecules. For example, the forces that hold together two H2O molecules to each other. Intermolecular forces are the reason why when water comes out of the end of a faucet or a squirt gun it stays together in a stream and does not fly apart in every direction.

 

How do you determine the intermolecular forces of a molecule or chemical?

To determine the intermolecular forces of a molecule or chemical, you must first construct the electron dot structure (Lewis structure). Then you determine the polarity of the molecule. Once you have those done determining the types of intermolecular bonds become much more simple. NON-POLAR molecules have only the strongest intermolecular bonds of LONDON DISPERSION. Polar molecules can either have the intermolecular forces of HYDROGEN BONDING or DIPOLE DIPOLE.

 

What is the difference between Hydrogen Bonding and Dipole Dipole?

The difference between hydrogen bonding and dipole dipole is that hydrogen bonding is a special strong type of dipole dipole. We can tell a HYDROGEN BONDING molecule because it will have within it a bond between H and F or between H and O or between H and N. If it does not have any of those bonds within the molecule, then it will be a Dipole Dipole. I will illustrate this in the table below:

 

(Intermolecular forces table)

 

Examples: Name the strongest intermolecular force between the two identical molecules from each question below. VIDEO Determining Intermolecular Forces from Electron Dot Structure Examples 1.

 

Between SF4 and SF4

Electron dot structure picture with polarity if needed.

ANSWER: London Dispersion

 

Between H2O and H2O

Electron dot structure picture with polarity if needed.

ANSWER: Hydrogen Bonding

 

Between PCl3 and PCl3

Electron dot structure picture with polarity if needed.

ANSWER: Dipole Dipole

 

Between H2CO and H2CO

Electron dot structure picture with polarity if needed.

ANSWER: Dipole Dipole

 

PRACTICE PROBLEMS: Name the strongest intermolecular force between the two identical molecules from each question below.

 

Between SH2 and SH2

Electron dot structure picture with polarity if needed.

ANSWER: Dipole Dipole

 

Between I2 and I2

Electron dot structure picture with polarity if needed.

ANSWER: London Dispersion

 

Between NH3 and NH3

Electron dot structure picture with polarity if needed.

ANSWER: Hydrogen Bonding

 

Between HCN and HCN

Electron dot structure picture with polarity if needed.

ANSWER: Dipole Dipole

 

Between CO2 and CO2

Electron dot structure picture with polarity if needed.

ANSWER: London Dispersion

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