Gases (1)

States of Matter & The Kinetic Molecular Theory

The Kinetic-Molecular Theory of Matter

  • Kinetic pertains to motion.
  • The kinetic-molecular theory of matter is based on the idea that particles of matter are always in motion.
  • This applies to gases, liquids, and solids.

The Kinetic-Molecular Theory of Gases

  • Deals specifically with the behavior of gases.
Five Postulates of the Theory:

  1. Particles are in constant, rapid, straight-line motion.

    • Result:
      • Gases expand to fill their containers (same shape & volume).
      • Gases are fluids—they flow.

  2. Collisions between gas particles, other particles, and the container wall are elastic collisions.

    • Elastic collisions imply particles bounce off with the same speed that they hit with.
    • Result: Pressure is exerted when the particles hit the sides of the container.

  3. Gas pressure is caused by collisions of gas particles with each other and with the walls of the container.

    • As the number of collisions increases, pressure increases.
    • What causes an increase in collisions?
      • Increase in temperature.
      • More particles added.

  4. The temperature of a gas, in Kelvin, is directly proportional to the average kinetic energy of the particles.

    • Particles move faster when it’s hotter!
    • Particles move slower when it’s colder!
    • At the same temperature, lighter gas particles will have a greater speed than heavier gas particles.
    • If temperature doubles, kinetic energy doubles.
    • KE=12mv2KE = \frac{1}{2}mv^2 (Kinetic Energy formula, where m = mass, v = velocity)

  5. The volume of individual gas particles is nearly zero.

    • Result: gases are mostly empty space.

  6. There are no forces of attraction between gas particles.

    • Distance is too great!

Important Considerations

  • The Kinetic-Molecular Theory of Gases applies to ideal gases.
  • Some gases nearly behave as an ideal gas.
  • In reality, there is no ideal gas!
  • Gases deviate the most from the ideal gas when the pressure is high or when the temperature is low.

Properties of Gases

Expansion
  • Gases do not have a definite shape or a definite volume.
  • Gases completely fill the container in which they are enclosed.
  • Gases move rapidly in all directions without any significant attraction between particles.
Fluidity
  • Forces between gas particles are so insignificant that they move about freely.
  • Gases, as well as liquids, flow, and both are fluids.
Density
  • Since gases are spaced so far apart, the density of gases is about 1/1000 the density of the same gas in a liquid or solid state.
Compressibility
  • Gases are easily compressed because a gas is mostly empty space.
  • Compressing a gas increases the pressure.
Diffusion & Effusion
  • Gases spread out and mix with other gases spontaneously, even without stirring!
  • Effusion of gases is directly related to the velocities of the particles.
  • Diffusion: the spontaneous mixture of the gas particles of two or more substances caused by random motion.
  • Effusion: the process of gas particles passing through a tiny opening.
  • Mass also makes a difference. Lighter particles effuse faster than heavier particles.

Diffusion and Effusion

  • Diffusion: Gases mix (e.g., H2 mixing with air).
  • Effusion: Gas particles pass through a tiny opening (e.g., He escaping).

Think About It - Discussion Points

  • Flasks of the same size, at the same temperature, and contain the same number of particles: The gas particles will be moving the slowest in the flask containing the heavier gas because, at the same temperature, lighter gas particles move faster than heavier ones.
  • Equal volume of gases under the same conditions of temperature and pressure have the same number of particles, but they do not have the same mass, because molar mass might be different.
  • Containers of gases post a warning such as: Do not store at a temperature above 120°F (50°C) because increasing the temperature increases the pressure. High temperatures can cause the container to explode due to increased pressure from the gas particles colliding more frequently and forcefully with the container walls.
  • A bottle of ammonia is opened, and the “smell” spreads due to:
    • Particles are in constant, rapid, straight-line motion.
    • Collisions between particles are elastic.
    • Temperature is related to the average kinetic energy.
    • Volume of particles is negligible.
    • No forces of attraction between particles.
    • This is an example of diffusion.