Kinetic Molecular Theory (KMT)

• This module introduces the Kinetic Molecular Theory (KMT) in physics and chemistry.
• At the end of this module, you should be able to:
  • Discuss the postulates of the Kinetic Molecular Theory of Matter. (C10.A.1)
  • Demonstrate how KMT explains the observed behavior of gases. (C10.A.2)

Kinetic Molecular Theory

• Matter comprises atoms, molecules, or ions.
• Atoms are the smallest particle of an element with the same chemical properties.
• Molecules are the smallest particle of a compound.
• Ions are charged atoms due to lost or gained electrons.
• These particles are in constant motion, vibrating, moving, and rotating as they collide.
• Kinetic energy is the energy a substance has due to its motion, is related to position and motion.
• Translational kinetic energy arises from translational (straight-line) motion.
• Vibrational and rotational kinetic energy also contribute.
• Potential energy is gained as particles get closer due to attractions or repulsions.
• Thermal energy (heat energy) is the sum of all forms of energy in a substance.
• Physicists often refer to thermal energy as internal energy.

Kinetic Molecular Theory of Gases

• The KMT of Matter was developed around 100 years ago.
• Rudolf Clausius (1822-1888) developed a complete form of the theory.
• It explains forces and energy between molecules and can be summarized into three points:
  • Matter comprises small particles (atoms, ions, or molecules).
  • The volume of molecules depends on the space between them.
  • The molecules are in constant random motion, differing across states of matter (gases, liquids, solids).
• Molecules collide with each other and container walls without significant energy loss.
• Absolute zero (0 Kelvin or -273.15 °C) defines when the motion is very slow to none.
• KMT defines both temperature and pressure at the molecular level.
• Gas pressure results from molecules colliding with container walls.
• Temperature measures the hotness or coldness of a substance and is the total kinetic energy of particles.
• Water molecules at 0°C have less kinetic energy than at 100°C.
• A gas' absolute temperature measures its molecules' average kinetic energy, equal for gases at the same temperature.
• Doubling a gas' absolute temperature doubles its molecules' average kinetic energy.
• Greater temperature corresponds to greater kinetic energy.

Application of Gas Laws

• Empirical findings of gas properties are interpreted by the principle of kinetic molecular theory.
• At constant temperature, increased volume causes pressure to drop:
  • Constant temperature means the average kinetic energy and molecular speed remain constant.
  • Increased volume means molecules travel farther between collisions.
  • Fewer collisions occur with container walls per time unit, decreasing pressure.
• At constant volume, increased temperature causes pressure to rise:
  • Increased temperature increases molecules' average kinetic energy and root-mean-square speed.
  • More collisions occur with the walls per unit of time.
  • Increased collision momentum leads to higher pressure.

Summary

• KMT explains the energy and forces between molecules.
• Kinetic energy is energy from a substance's movement.
• Temperature describes a substance's coldness or hotness.
• At constant temperature, increased volume decreases pressure.
• At constant volume, increased temperature increases pressure.

Explore the Kinetic Molecular Theory (KMT) of Matter. Learn about the postulates of KMT and how it explains gas behavior. Understand the different types of energy including kinetic, potential, translational, vibrational, and rotational energy.