Lecture Notes: Kinetic Molecular Theory

Introduction

• Kinetic Molecular Theory explains the behavior of gases.
• It consists of five postulates.
• From these postulates, all ideal gas laws can be derived.

Ideal Gas Laws

• They are summaries of observations:
  • Pressure and volume are inversely proportional.
  • Volume and temperature are directly proportional.
• Laws do not explain why these relationships occur.

Five Postulates of Kinetic Molecular Theory

Postulate 1: Particles in Motion

• Gas is made up of particles (atoms or molecules) always in motion.
• Particles travel in straight lines unless they collide.
• Collisions cause particles to change direction.
• Implies atoms obey laws of motion like macroscopic objects.

Postulate 2: Mostly Empty Space

• Gas is mostly empty space.
• Volume occupied by gas particles is negligible.
• Particles are dimensionless points.
• Different from solids and liquids which are incompressible.

Postulate 3: Pressure from Collisions

• Pressure is due to gas particles colliding with container walls.
• Kinetic energy of particles transferred during collisions.
• Many fast-moving particles = high pressure.
• Few slow-moving particles = low pressure.

Postulate 4: Negligible Interactions

• Gas particles do not exert significant gravitational or electromagnetic forces on each other.
• Interactions like dispersion or dipole-dipole are negligible.
• Collisions are purely elastic (no kinetic energy loss).

Postulate 5: Kinetic Energy and Temperature

• Average kinetic energy of gas particles is proportional to temperature in Kelvin.
• Increasing temperature increases kinetic energy and particle velocity.
• Temperature indicates average molecular velocity.

Application to Ideal Gas Laws

Boyle's Law

• At constant temperature, increasing volume decreases pressure.
• Decreasing volume increases pressure.
• Explains inverse relationship between pressure and volume.

Charles' Law

• Increasing temperature requires volume expansion to keep pressure constant.
• Explains direct relationship between volume and temperature.

Amundsen's Law

• Increasing temperature at constant volume increases pressure.
• Explains direct relationship between pressure and temperature.

Conclusion

• Kinetic Molecular Theory explains observed gas laws.
• Provides a theoretical framework for understanding gas behavior.

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