1. Unit III: The Physical Properties and Physical Changes of Substances
3.6 – Role of Kinetic Energy in Physical Changes (pg , Hebden)

2. Today’s Objectives Describe the characteristics of matter, including:
State the kinetic molecular theory (KMT)
Describe the simple molecular motions and arrangements of particles in solids, liquids, and gases

3. All molecules are constantly in motion
Kinetic Energy (KE):
The energy that molecules possess as a result of their motion
Molecules have 3 main types of KE:
1) Rotational energy (ER) or (EROT): causes a molecule to rotate around one of its axes; bond lengths and bond angle don’t change
Example: water

4. Types of Kinetic Energy
2) Vibrational Energy (EV) or (EVIB): changes the bond lengths and/or angles between atoms in a molecule
Example: water

5. Types of Kinetic Energy
3) Translational Energy (ET) or (ETRANS): causes the molecule to travel in a straight line from place to place, but has no effect on bond length and angles
Example: water

6. Practical Applications of Kinetic Energy Changes
1) Microwave Ovens:
Supply energy which causes water molecules in food to vibrate
As molecules absorb energy and bump into each other, the food “heats up”
2) Infrared (IR) Spectroscopy: forensics
Molecules absorb infrared (heat) energy
IR energy is just light energy with less energy than red light; if white light is passed through a prism it will look like the example below

7. Practical Applications of Kinetic Energy Changes
The frequency (energy of light) of the infrared light aimed at the sample is slowly changed and the amount of energy absorbed at each different frequency is automatically recorded
Different molecules absorb different amounts of light at different frequencies, thus giving a “fingerprint” for that molecule
Can be used to identify unknown molecules

8. Practical Applications of Kinetic Energy Changes
Each bond in a molecule absorbs IR energy in a certain way producing a unique spectra such as the one shown above
3) Thermometers:
Temperature rises when KE of liquid in thermometer increases

9. The Role of Kinetic Energy in Phase Changes

10. The Role of Kinetic Energy in Phase Changes
A) molecules possess increasing amounts of vibrational and translational energy
B) translational energy reaches a critical value, allowing molecules to break out of the solid phase
C) translational, vibrational and rotational energy steadily increase, but greatest increase is in translational
D) translational energy reaches a critical value, allowing molecules to break out of the liquid phase
E) translational, vibrational and rotational energy steadily increase, but greatest increase is in translational

11. Homework
Pg. 64 #64, 66-71