1. Kinetic Molecular Theory & Intermolecular Forces OR KMT & IMF’s
Chapter 13 – 2010 – Mr Nelson

2. States of Matter
The fundamental difference between states of matter is the distance between particles.

3. The States of Matter
The state a substance is in at a particular temperature and pressure depends on two variables:
the kinetic energy of the particles;
the strength of the attractions between the particles.

4. Solid Liquid Gas
Container
Compress
Flow
Diffusion

6. Kinetic-Molecular Theory
A model that aids in our understanding of how gas particles behave and respond to changes.

7. Theory of KMT
Gases consist of large numbers of molecules that are in continuous, random motion.
Most molecules
Nobles - atoms

8. Theory of KMT
Total volume of all the molecules of the gas is negligible relative to the total volume in which the gas is contained.

9. Theory of KMT
Attractive and repulsive forces between gas molecules are negligible.

10. Theory of KMT
Energy can be transferred between molecules during collisions
Elastic collisions
the average kinetic energy of the molecules does not change with time

11. Theory of KMT
The average kinetic energy of the molecules is proportional to the absolute temperature.

12. Effusion (diffusion through a hole)
Effusion is the escape of gas molecules through a tiny hole into an evacuated space.

13. Effusion Let’s compare effusion for helium and nitrogen
Helium: 4 g/mol
N2: 28 g/mol
With a lower mass the helium balloon would deflate faster.

14. Graham's Law KE1 KE2 = 1/2 m1v12 1/2 m2v22 = = m1 m2 v22 v12 m1

15. Intermolecular Forces (IMFs)
IMF’s are found between molecules
Not between the atoms that compose the molecule
IMF’s are weaker that single, double or triple bonds

16. IMF’s IMF’s are strong enough to control physical properties such as:
boiling and melting points
vapor pressure
viscosity

17. IMF’s
IMF’s as a group are referred to as van der Waals forces.

18. van der Waals Forces Dipole-dipole interactions Hydrogen bonding
London dispersion forces

19. Dipole-Dipole Interactions
Molecules that have permanent dipoles are attracted to each other.

20. Dipole-Dipole Interactions
The more polar the molecule, the higher is its boiling point.

21. London Dispersion Forces
Helium has two electrons in the 1s orbital that repel each other.
However, it does happen that they occasionally wind up on the same side of the atom.

22. London Dispersion Forces
At that instant, the helium atom is polar, with an excess of electrons on the left side

23. London Dispersion Forces
Another helium nearby, then, would have a dipole induced in it:
The electrons on the left side of helium become attracted to the dipole in He atom 1

24. London Dispersion Forces
London dispersion forces are attractions between an instantaneous dipole and an induced dipole.

25. London Dispersion Forces
These forces are present in all molecules, whether they are polar or nonpolar.
Only IMF’s present in Noble Gases
The tendency of an electron cloud to distort in this way is called polarizability.

26. Two Main Factors Affecting London Forces
Molecular weight
London forces increase with increased MW.
Number of electrons
larger electron clouds are easier to polarize.

27. Which Have a Greater Effect
Which Have a Greater Effect? Dipole-Dipole Interactions or Dispersion Forces
If two molecules are of comparable size and shape, dipole-dipole interactions will likely the dominating force.
If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

28. Boiling Points
The nonpolar series (SnH4 to CH4) follow the expected trend.
The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.

29. Hydrogen Bonding
The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.
We call these interactions hydrogen bonds.

30. Hydrogen Bonding
Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine.
Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.