1. Intermolecular Forces
Chemistry Notes
Intermolecular Forces

2. Intermolecular Forces
What is an Intermolecular Force?
Force between molecules (weak force)
Differs from an intramolecular force (strong force)

3. Relative Magnitude of Forces
Intermolecular Forces
Covalent Bonds
H-Bonds
Dipole-dipole
London Dispersion
400 kcal
12-16 kcal
2-0.5 kcal
Less than 1 kcal
Notice: covalent bonds are almost 40 times the strength

4. Intermolecular Forces
What creates an Intermolecular force?
Unequal distribution of electrons
Created as a result of differences in:
Electronegativity
Hydrogen and bromide bond with an unequal charge distribution.
Bonded hydrogen atoms showing equal charge distribution

5. What creates an Intermolecular force?
The unequal distribution of electrons causes HBr to be attracted to another HBr Br Br

6. Intermolecular Forces
Main types of IMF we will discuss:
London Dispersion Forces
Dipole-Dipole
Hydrogen Bonds

7. London Dispersion Forces
Is the result of temporary charge imbalance
Exists due to the random movement of electrons
When imbalance is present, nucleus of another atom is attracted to partially charged part of the atom or molecule
Found in ALL molecules
polar and nonpolar
Weakest IMF

8. London Dispersion Forces

9. Dipole-Dipole Exists between polar molecules
Polar molecules have partial positive charges at the end of the molecules
The stronger the polarity the stronger the dipole force

10. Dipole-Dipole

11. Dipole-Dipole
Dipole-Dipole forces exist between neutral polar molecules.

12. Hydrogen Bonds
Exist between a hydrogen that is bonded to a highly electronegative atom and unshared pair of electrons on another highly electronegative atom
H-bonds are possible only with N, O, and F.
Seen in Water,
Ammonia, and DNA
Strongest IMF

13. Hydrogen Bonds
The molecules which have this extra bonding are N,O, and F
NOTICE where the H-bond is occurring. See previous slide.

14. Hydrogen Bonds Hydrogen Oxygen
Notice: H’s electrons are virtually pulled out of it’s 1s sublevel

15. Hydrogen Bonds
Water
Notice: the virtually empty 1s orbitals on H are attracted to the unshared pair of electrons on the O from another H2O.

16. The evidence for hydrogen bonding
Many elements form compounds with hydrogen - referred to as "hydrides". If you plot the boiling points of the hydrides of the Group 4 elements, you find that the boiling points increase as you go down the group.
                   
                                                                                                               
The increase in boiling point happens because the molecules are getting larger with more electrons, and so dispersion forces become greater.

17. The evidence for hydrogen bonding
If you repeat this exercise with the hydrides of elements in Groups 5, 6 and 7, something odd happens.
Although for the most part the trend is exactly the same as in group 4 (for the same reason), the boiling point of the hydride of the first element in each group is abnormally high.
In the cases of NH3, H2O and HF there must be some additional IM forces of attraction, requiring significantly more heat energy to break.
These relatively powerful IM forces are described as hydrogen bonds.

18. Hydrogen Bonds

19. Overview of IMF

20. Other IMF Types

21. Summary
Be able to…
Explain what an IMF is and talk about their relative strengths
Explain what causes IMF
Describe the different types of IMF
Go in depth about H-bonds
Draw diagrams that represent the IMF types