Intermolecular Forces
- Attractive forces between molecules
- Are NOT chemical bonds, rather much weaker
- All molecules contain london dispersion attractions
- Hydrogen bonding is strongest attraction.
- Are NOT chemical bonds, rather much weaker
- All molecules contain london dispersion attractions
- Hydrogen bonding is strongest attraction.
Finding out if a molecule is Polar:
- It must have at least 2 of the 3 requirements below. In this case, CHBr3 and PCl3 are both polar.
- It must have at least 2 of the 3 requirements below. In this case, CHBr3 and PCl3 are both polar.
1.) More than one type of bond
- Carbon is bonding to hydrogen as well as to Boron. ( 2 different elements ) |
2.) Lone pairs of electrons on central atom
- Phosphorus, the central atom, has 1 lone pair of electrons. |
3.) Large difference in electronegativity
- Difference in EN must be greater than 0.3 PCl3: EN of P = 2.2 EN of Cl = 3.0 difference: 3.0 - 2.2 = .8 CHBr3: EN of H = 2.1 EN of C = 2.5 EN of Br = 2.8 difference: 2.8 - 2.5 = 0.3 2.5 - 2.1 = 0.4 |
London Dispersion, Dipole - Dipole, and Hydrogen Bonding:
- Intermolecular forces ( attractions )
- The stronger the attraction, the higher the melting point will be. ( more heat required to break apart attraction )
- Intermolecular forces ( attractions )
- The stronger the attraction, the higher the melting point will be. ( more heat required to break apart attraction )
London Dispersion:
- weakest attraction - Occurs because random motion of electrons create partial+ and partial - ends in atom. - All molecules have london dispersion - Low melting point |
Dipole - Dipole:
- medium attraction - attraction between 2 permanent partial+ and partial- ends ( dipoles ) - found in polar molecules - High melting point |
Hydrogen Bonding:
- strongest attraction - NOT an actual bond, solely an attraction - Occurs when hydrogen bonds with either fluorine, oxygen, or nitrogen. - Highest melting point |
How to know which is the leading attraction in a molecule:
- Does it have hydrogen? If so, is the hydrogen attracted to either fluorine, oxygen, or nitrogen?
----> If so, then hydrogen bonding is the strongest attraction.
- If it does not have hydrogen... is it a polar molecule? ( refer to the 3 requirements of polarity above )
----> If it is a polar molecule, then dipole - dipole is its main attraction.
- If it does not have hydrogen and is not polar molecule, then it has London Dispersion.
Note: All compounds and molecules contain London Dispersion.
If a molecule's strongest attraction is dipole - dipole, then it also contains London Dispersion.
If a molecule's strongest attraction is hydrogen bonding, then it also has dipole - dipole attractions and london dispersion.
- Does it have hydrogen? If so, is the hydrogen attracted to either fluorine, oxygen, or nitrogen?
----> If so, then hydrogen bonding is the strongest attraction.
- If it does not have hydrogen... is it a polar molecule? ( refer to the 3 requirements of polarity above )
----> If it is a polar molecule, then dipole - dipole is its main attraction.
- If it does not have hydrogen and is not polar molecule, then it has London Dispersion.
Note: All compounds and molecules contain London Dispersion.
If a molecule's strongest attraction is dipole - dipole, then it also contains London Dispersion.
If a molecule's strongest attraction is hydrogen bonding, then it also has dipole - dipole attractions and london dispersion.