List the intermolecular forces present in the following molecules:
b. NF3 c. C2H6
Solution: Intermolecular forces are forces between molecules (we can think of them as forces of attraction between molecules).
Let's go through different intermolecular forces as they are listed from the strongest to the weakest:
Ionic = Ionic compounds (metal+nonmetal OR metal +polyatomic OR polyatomic ion+polyatomic ion OR an ion) Example: NaCl
Hydrogen bonding (strongest type of dipole dipole) = when the molecule has either OH, NH or FH. Example: H2O
Dipole-Dipole = polar molecules. Example: NH3
London dispersion = all molecules have it but it is the only force for non polar molecules. The higher the mass (molecular size), the stronger the force. Example: H2
Mayya's trick to identify a non polar molecule
A molecule is non polar if:
It has no polar bonds ( 2 same elements bonded (O2) or only carbons and hydrogens (CH4) or just 1 element (Ne)) OR
The central atom is connected to all the same elements AND has no lone pairs.
If the molecule is not non polar, it will be POLAR.
Now, lets look at the answer choices.
H2O has hydrogen bonding because it has OH. It also has dipole-dipole (if a molecule has H bonding, it automatically has dipole-dipole force) and London dispersion (all molecules have London dispersion).
NF3 The lewis dot structure of NF3 has the central atom nitrogen. N is surrounded by the same atoms (fluorines) and has a lone pair of electrons. Therefore, NF3 is a polar molecule. It will have dipole-dipole and London dispersion forces.
C2H6 is non polar since it consists of only carbons and hydrogens. It should have only London dispersion forces.
C2H5OH has hydrogen bonding (OH group) as well as dipole-dipole and London Dispersion.