KINETIC MOLECULAR

THEORY

I. Ideal Gases

 The kinetic-molecular theory accounts for the behavior of atoms and molecules based on the idea that all particles of matter are always in motion.

I. Ideal Gases

 This theory gives us a model of what is called an ideal gas, an imaginary gas that perfectly fits all assumptions of KMT.

II. The 5 Assumptions of KMT

  1) Gases consist of large numbers of tiny particles that are far apart relative to their size.  Particles in the gas state occupy a volume that is,

on average, 1000× greater than the same number of particles in the liquid or solid state.

 Most of the volume of a gas is empty space.

II. The 5 Assumptions of KMT

  2) Collisions between gas particles and other particles or the walls of a container are elastic.  An elastic collision is one in which there is no net

loss of kinetic energy.

II. The 5 Assumptions of KMT

  3) Gas particles are in continuous, rapid, random motion. They possess kinetic energy, which is the energy of motion.

II. The 5 Assumptions of KMT

  4) There are no forces of attraction or repulsion between ideal gas particles.  Like billiard balls, when they hit, they don’t stick

together, they just bounce apart.

II. The 5 Assumptions of KMT

  5) The average kinetic energy of gas particles depends on the temperature of the gas.  As temperature goes up, kinetic energy goes up.

As temperature goes down, kinetic energy goes down.

 KE = ½mv2 where m is mass and v is velocity  Faster = greater kinetic energy = hotter

III. The Nature of Gases

  KMT accounts for many physical properties of actual gases. Here are a few examples.

  1) Expansion – gases have indefinite volume and shape, they expand to fill their containers.  Why? – Gas particles move rapidly in all directions

(3) without significant attractive force (4).   2) Fluidity – gases flow easily.

 Why? – They lack attractive forces (4).   3) Compressibility – A gas can be compressed to a

smaller volume  Why? – Gas particles are far apart (1).

III. The Nature of Gases

  4) Diffusion/Effusion – Gases spontaneously mix and spread out in containers.  Why? – They have random continuous motion

(3).

IV. Real Gases

 A real gas is one that does not fully behave according to the assumptions of kinetic molecular theory.

 This is because real gas molecules take up space and have attractive forces that cause inelastic collisions.

IV. Real Gases

  Small, nonpolar gases behave very much like ideal gases.  Ex: He, Ne, Ar, H2, N2, O2

  Large, polar molecules are “stickier” and don’t behave much like ideal gases.  Ex: NH3, H2O, I2