The Nature of The Nature of GasesGases

Kinetic Theory and a Model for GasesKinetic Theory and a Model for Gases

ObjectivesObjectives

Upon completion of this presentation, you Upon completion of this presentation, you will be able to …will be able to … state the Kinetic Theory of Gases.state the Kinetic Theory of Gases.

state the assumptions of the Kinetic Theory state the assumptions of the Kinetic Theory of Gases.of Gases.

apply the assumptions of the Kinetic apply the assumptions of the Kinetic Theory to ideal and real gases.Theory to ideal and real gases.

IntroductionIntroduction

Kinetic means Kinetic means motionmotion.. Kinetic energy, KE, is the energy of motion.Kinetic energy, KE, is the energy of motion.

The The kinetic theorykinetic theory is a model of matter where is a model of matter where ...... ... all matter is composed of ... all matter is composed of tiny particles tiny particles ......

... in ... in constant motionconstant motion..

The kinetic theory applies to The kinetic theory applies to allall states of states of matter.matter.

We will apply it first to gases and then extend We will apply it first to gases and then extend the theory to liquids and solids.the theory to liquids and solids.

IntroductionIntroduction

The kinetic theory describes the behavior The kinetic theory describes the behavior of matter in its various states.of matter in its various states.

There are a series of assumptions we There are a series of assumptions we make about matter in order to use the make about matter in order to use the theory.theory. These assumptions help us to understand These assumptions help us to understand

how to apply the theory to ...how to apply the theory to ... ... gases ...... gases ...

... liquids ...... liquids ...

... and solids.... and solids.

First AssumptionFirst Assumption

The particles in a gas are considered to be The particles in a gas are considered to be very very smallsmall, very , very hardhard spheres with an spheres with an insignificant insignificant volumevolume..

This means that the individual gas particles ...This means that the individual gas particles ... are are atoms atoms or or small moleculessmall molecules

are very are very far apartfar apart in relation to the size of the in relation to the size of the particlesparticles

have have very little attraction or repulsion very little attraction or repulsion towards towards one anotherone another

move move independently independently of each otherof each other

Second AssumptionSecond Assumption

The motion of the particles in a gas is The motion of the particles in a gas is rapidrapid, , constantconstant, and , and randomrandom..

This means that the individual gas This means that the individual gas particles ...particles ... spread out spread out to fill any volume or shape of to fill any volume or shape of

containercontainer

travel in travel in straight lines straight lines until they encounter until they encounter another particle or another objectanother particle or another object

change direction only after a change direction only after a collisioncollision

Second AssumptionSecond Assumption

Experimental measurements of gas Experimental measurements of gas molecules show that they move quite molecules show that they move quite rapidly, even at rapidly, even at room temperatureroom temperature.. OO22 molecules have an molecules have an average speed of average speed of

1,700 km/hr (1,060 mph)1,700 km/hr (1,060 mph)..

However, they only travel about 70 nm However, they only travel about 70 nm until they encounter another particle.until they encounter another particle. This is This is about 500 times their diameterabout 500 times their diameter

Each gas molecule travels in a very Each gas molecule travels in a very crooked path called a crooked path called a random walkrandom walk..

Third AssumptionThird Assumption

All collisions between gas particles are All collisions between gas particles are perfectly elasticperfectly elastic..

This means that the individual gas This means that the individual gas particles ...particles ... transfer kinetic energy during a collisiontransfer kinetic energy during a collision

collide without a loss of kinetic energycollide without a loss of kinetic energy

have a total kinetic energy that remains have a total kinetic energy that remains constantconstant

ApplicationsApplications

We can use these assumptions to We can use these assumptions to understand the behavior of understand the behavior of realreal gases.gases. CompressibilityCompressibility::

Gases are Gases are compressiblecompressible..

This can be explained by the This can be explained by the first first assumptionassumption:: the the small sizesmall size of gas particles of gas particles

the the large distance large distance between gas particlesbetween gas particles

ApplicationsApplications

We can use these assumptions to We can use these assumptions to understand the behavior of understand the behavior of realreal gases.gases. ExpansionExpansion::

Gases Gases expandexpand to fill all available space of a to fill all available space of a container.container.

This can be explained by the This can be explained by the second second assumptionassumption:: gas particles move gas particles move rapidlyrapidly, , constantlyconstantly, and , and

randomlyrandomly

this movement will allow the gas particles to this movement will allow the gas particles to move to the move to the limits of the containerlimits of the container

ApplicationsApplications

We can use these assumptions to We can use these assumptions to understand the behavior of understand the behavior of realreal gases.gases. DensityDensity::

Gases are by far the Gases are by far the least dense least dense of the of the states of matter.states of matter.

This can be explained by the This can be explained by the first first assumptionassumption:: the combination of the the combination of the small size small size of gas of gas

particles and the particles and the large distance between gas large distance between gas particlesparticles leads to a very low density. leads to a very low density.

Note:Note:

The textbook lists only 3 assumptions of The textbook lists only 3 assumptions of the kinetic theory of matter.the kinetic theory of matter.

There are, in fact, many more There are, in fact, many more assumptions in a complete treatment of assumptions in a complete treatment of the kinetic theory.the kinetic theory.

All of these assumptions help us to better All of these assumptions help us to better understand the nature of matter in all of understand the nature of matter in all of its states.its states.

Note:Note:

Some examples of additional assumptions Some examples of additional assumptions include:include: The number of molecules is so large that The number of molecules is so large that

statistical treatment statistical treatment can be applied.can be applied. The The average kinetic energy average kinetic energy of the gas of the gas

particles depends only on the particles depends only on the temperature temperature of of the system.the system.

The time during collision of molecule with the The time during collision of molecule with the container's wall is container's wall is negligiblenegligible as comparable as comparable to the time between successive collisions.to the time between successive collisions.

The equations of motion of the molecules are The equations of motion of the molecules are time-reversibletime-reversible..

SummarySummary

First Assumption:First Assumption: The particles in a gas are considered to be The particles in a gas are considered to be

very small, very hard spheres with an very small, very hard spheres with an insignificant volume.insignificant volume.

Second Assumption:Second Assumption: The motion of the particles in a gas is The motion of the particles in a gas is

rapid, constant, and random.rapid, constant, and random.

Third Assumption:Third Assumption: All collisions between gas particles are All collisions between gas particles are

perfectly elastic.perfectly elastic.