Kinetic-Molecular TheoryKinetic-Molecular Theory

ModelsModels

•Scientists use models Scientists use models because simpler than real lifebecause simpler than real life

•Kinetic-molecular theory can Kinetic-molecular theory can be extended to all phases of be extended to all phases of mattermatter

Kinetic-Molecular Theory Kinetic-Molecular Theory (KMT)(KMT)

Lets discuss how pertains to gases:

There are 4 assumptions to the KMT

1) Gases consist of spherical 1) Gases consist of spherical molecules moving in constant, molecules moving in constant, random, straight-line motionrandom, straight-line motion

Random, straight-line?Random, straight-line?

- move in straight lines until hit - move in straight lines until hit something, then can bounce off at something, then can bounce off at any angleany angle

Kinetic-Molecular TheoryKinetic-Molecular Theory

2) Gas particles undergo “elastic” 2) Gas particles undergo “elastic” collisions with each other and with collisions with each other and with walls of containerwalls of container

“ “Elastic” means total KE is conservedElastic” means total KE is conserved• KE does KE does notnot get converted to heat or get converted to heat or

sound or any other kind of energysound or any other kind of energy• KE KE cancan be transferred between particles be transferred between particles

Kinetic-Molecular TheoryKinetic-Molecular Theory

3) Gas molecules separated by great 3) Gas molecules separated by great distances that volume of molecule is distances that volume of molecule is negligiblenegligible

• distance between molecules is more distance between molecules is more important than actual size of moleculeimportant than actual size of molecule

Kinetic-Molecular TheoryKinetic-Molecular Theory

4) Gas molecules do not attract or 4) Gas molecules do not attract or repel each otherrepel each other

• Assume there are no interactions Assume there are no interactions between gas moleculesbetween gas molecules

Real vs. ideal gasReal vs. ideal gas

• Ideal gasIdeal gas obeys assumptions of KMT obeys assumptions of KMT all the time all the time • It’s It’s a modela model

• It It doesn’t existdoesn’t exist

• Real gasesReal gases follow KMT most of time, follow KMT most of time, but but notnot at at high pressure & low high pressure & low temperaturetemperature• KMT breaks down when molecules are KMT breaks down when molecules are

close togetherclose together

Review of KMTReview of KMT

• What features of the Kinetic-What features of the Kinetic-Molecular model are close to real Molecular model are close to real gases?gases?

• What features of the Kinetic-What features of the Kinetic-Molecular model break down?Molecular model break down?