Properties of Gases Kinetic Molecular Theory

Kinetic-Molecular Theory  Based on idea that particles of matter are always in motion.  Provides reasoning for the behavior of gases and their physical properties.

Assumptions of the Kinetic-Molecular Theory  1. Gases consist of a large number of tiny particles that are far apart relative to their size.  Most of the volume of a gas is empty space.  Accounts for lower density of gases in comparison to solids and liquids.  Explains fact that gases are easily compressed.

Assumptions of the Kinetic-Molecular Theory  2. Gas particles are in continuous rapid, random motion  Kinetic energy is the energy of motion.  3. There are no forces of attraction or repulsion between gas particles.  Kinetic energy of the gas particles overcomes the attractive forces between the individual particles.

Assumptions of the Kinetic-Molecular Theory  4. The average kinetic energy (KE) of gas particles depends on the temperature of the gas.  As temperature increases, KE increases.  KE = ½mv 2 (m = mass, v = velocity).  At the same temperature:  Lighter gases have higher average speeds.  Heavier gases have lower average speeds.

Assumptions of the Kinetic-Molecular Theory  5. Collisions between gas particles and between particles and container walls are elastic collisions.  In elastic collisions, there is no loss of kinetic energy.  Kinetic energy is transferred between two gas particles during collisions.

Ideal versus Real Gases  Ideal gases conform to all of the assumptions of the kinetic-molecular theory of gases.  Gases that behave ideally have the following properties:  Low mass.  Low polarity.  Gases behave most ideally under the following conditions:  High temperature.  Low pressure.

Ideal versus Real Gases  Real gases do not obey all of the assumptions of the kinetic-molecular theory.  Gases that behave like real gases have the following properties:  High mass.  High polarity.  Gases behave like real gases under the following conditions:  Low temperature.  High pressure.

Properties of Gases  Expansion and compression:  Gases can expand or compress to fit any size container.  Gases have no definite shape or volume.  Fluidity:  Gas particles easily flow past one another.  Low density:  The gaseous state of a substance is 1/1000 th the density of the same substance as a solid or liquid.

Properties of Gases  Diffusion and effusion:  Diffusion is the mixing of the particles of two gaseous substances caused by their random motion.  Effusion is the process by which gas particles pass through a tiny opening.  Both occur spontaneously.