Kinetic Molecular Theory

Collisions of Gas Particles

Kinetic Theory

Kinetic Molecular Theory Postulates of the Kinetic Molecular Theory of Gases 1.Gases consist of tiny particles (atoms or molecules) 2.These particles are so small, compared with the distances between them, that the volume (size) of the individual particles can be assumed to be negligible (zero). 3. The particles are in constant random motion, colliding with the walls of the container. These collisions with the walls cause the pressure exerted by the gas. 4. The particles are assumed not to attract or to repel each other. 5. The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature of the gas

Kinetic Molecular Theory Postulates Evidence 1. Gases are tiny molecules in mostly empty space. The compressibility of gases. 2. There are no attractive forces between molecules. Gases do not clump. 3. The molecules move in constant, rapid, random, straight-line motion. Gases mix rapidly. 4. The molecules collide classically with container walls and one another. Gases exert pressure that does not diminish over time. 5. The average kinetic energy of the molecules is proportional to the Kelvin temperature of the sample. Charles’ Law

Kinetic Molecular Theory (KMT) 1.…are so small that they are assumed to have zero volume 2.…are in constant, straight-line motion 3.…experience elastic collisions in which no energy is lost 4.…have no attractive or repulsive forces toward each other 5.…have an average kinetic energy (KE) that is proportional to the absolute temp. of gas (i.e., Kelvin temp.) AS TEMP., KE  explains why gases behave as they do  deals w /“ideal” gas particles…

Newton’s First Law of Motion (Law of Inertia) Object at rest tends to stay at rest, and object in motion tends to stay in motion at constant velocity unless object is acted upon by an unbalanced, external force.

Elastic vs. Inelastic Collisions 8 3

8 8 v1v1 elastic collision inelastic collision POW v2v2 v3v3 v4v4

8 Elastic Collision 8 v1v1 before v2v2 after

Model Gas Behavior All collisions must be elastic Take one step per beat of the metronome Container –Class stands outside tape box Higher temperature –Faster beats of metronome Decreased volume –Divide box in half More Moles –More students are inside box  Mark area of container with tape on ground.  Add only a few molecules of inert gas  Increase temperature  Decrease volume  Add more gas  Effect of diffusion  Effect of effusion (opening size)

Kinetic Molecular Theory Particles in an ideal gas… –have no volume. –have elastic collisions. –are in constant, random, straight-line motion. –don’t attract or repel each other. –have an avg. KE directly related to Kelvin temperature. Courtesy Christy Johannesson

Molecular Velocities speed Fractions of particles the Maxwell speed distribution molecules sorted by speed many different molecular speeds

Real Gases Particles in a REAL gas… –have their own volume –attract each other Gas behavior is most ideal… –at low pressures –at high temperatures –in nonpolar atoms/molecules Courtesy Christy Johannesson

Characteristics of Gases Gases expand to fill any container. –random motion, no attraction Gases are fluids (like liquids). –no attraction Gases have very low densities. –no volume = lots of empty space Courtesy Christy Johannesson

Characteristics of Gases Gases can be compressed. –n–no volume = lots of empty space Gases undergo diffusion & effusion. –r–random motion Courtesy Christy Johannesson

Properties of Gases V = volume of the gas (liters, L) T = temperature (Kelvin, K) P = pressure (atmospheres, atm) n = amount (moles, mol) Gas properties can be modeled using math. Model depends on:

Pressure - Temperature - Volume Relationship P T V Gay-Lussac’s P T  CharlesV T  P T V Boyle’s P 1V1V  ___

Pressure - Temperature - Volume Relationship P T V Gay-Lussac’s P T  CharlesV T  Boyle’s P 1V1V  ___ P n V

Pressure and Balloons A B = pressure exerted ON balloon A = pressure exerted BY balloon B When balloon is being filled: P A > P B When balloon is filled and tied: P A = P B When balloon deflates: P A < P B

When the balloons are untied, will the large balloon (A) inflate the small balloon (B); will they end up the same size or will the small balloon inflate the large balloon? Why? Balloon Riddle A B C

Behavior of Gases KeysKeys

Kinetic Theory and the Gas Laws Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 323 (newer book) original temperature original pressure original volume increased temperature increased pressure original volume increased temperature original pressure increased volume (a)(b)(c) 10

Kinetic Theory and the Gas Laws Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 323 (newer book) original temperature original pressure original volume increased temperature increased pressure original volume increased temperature original pressure increased volume (a)(c)