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Unit III: The Physical Properties and Physical Changes of Substances

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)

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Unit III: The Physical Properties and Physical Changes of Substances

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

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