Unit III: The Physical Properties and Physical Changes of Substances 3.6 – Role of Kinetic Energy in Physical Changes (pg. 62-64, Hebden)

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

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

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

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

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

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

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

The Role of Kinetic Energy in Phase Changes

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

Homework Pg. 64 #64, 66-71