Scientific Tutor
Chem – Intermolecular Forces from Lewis Structures
Author: Andrew
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
Posted in Chemistry
