The Nature of Gases: The Nature of Gases: Part 1Part 1Kinetic Theory and a Model for GasesKinetic Theory and a Model for Gases

IntroductionIntroductionKineticKinetic means motion. means motion.

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 is a model of matter where ...where ...

... all matter is composed of ... all matter is composed of tiny particlestiny 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.

IntroductionIntroductionThe kinetic theory describes the behavior of The kinetic theory describes the behavior of matter in its various states.matter in its various states.There are a series of assumptions we make There are a series of assumptions we make about matter in order to use the theory.about matter in order to use the 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 AssumptionThe particles in a gas are considered to be very The particles in a gas are considered to be very small, very hard spheres with an insignificant small, very hard spheres with an insignificant volume.volume.This means that the individual gas particles ...This means that the individual gas particles ...

are atoms or small moleculesare atoms or small moleculesare very far apart in relation to the size of are very far apart in relation to the size of the particlesthe particleshave very little attraction or repulsion have very little attraction or repulsion towards one anothertowards one anothermove independently of each othermove independently of each other

Second AssumptionSecond AssumptionThe motion of the particles in a gas is rapid, The motion of the particles in a gas is rapid, constant, and random.constant, and random.This means that the individual gas particles ...This means that the individual gas particles ...

spread out to fill any volume or shape of spread out to fill any volume or shape of containercontainertravel in straight lines until they encounter travel in straight lines until they encounter another particle or another objectanother particle or another objectchange direction only after a collisionchange direction only after a collision

Second AssumptionSecond AssumptionExperimental measurements of gas molecules Experimental measurements of gas molecules show that they move quite rapidly, even at show that they move quite rapidly, even at room temperature.room temperature.

OO22 molecules have an average speed of molecules have an average speed of 1,700 km/hr (1,060 mph).1,700 km/hr (1,060 mph).However, they only travel about 70 nm until However, they only travel about 70 nm until they encounter another particle.they encounter another particle.

This is about 500 times their diameterThis is about 500 times their diameterEach gas molecule travels in a very crooked Each gas molecule travels in a very crooked path called a path called a random walkrandom walk..

Third AssumptionThird AssumptionAll collisions between gas particles are All collisions between gas particles are perfectly elastic.perfectly elastic.This means that the individual gas particles ...This means that the individual gas particles ...

transfer kinetic energy during a collisiontransfer kinetic energy during a collisioncollide without a loss of kinetic energycollide without a loss of kinetic energyhave a total kinetic energy that remains have a total kinetic energy that remains constantconstant

ApplicationsApplicationsWe can use these assumptions to understand We can use these assumptions to understand the behavior of gases.the behavior of gases.Compressibility:Compressibility:

Gases are compressible.Gases are compressible.This can be explained by the first This can be explained by the first assumption:assumption:

the small size of gas particlesthe small size of gas particlesthe large distance between gas particlesthe large distance between gas particles

ApplicationsApplicationsExpansion:Expansion:

Gases expand to fill all available space of a Gases expand to fill all available space of a container.container.This can be explained by the second This can be explained by the second assumption:assumption:

gas particles move rapidly, constantly, and gas particles move rapidly, constantly, and randomlyrandomlythis movement will allow the gas particles this movement will allow the gas particles to move to the limits of the containerto move to the limits of the container

ApplicationsApplicationsDensity:Density:

Gases are by far the least dense of the states Gases are by far the least dense of the states of matter.of matter.This can be explained by the first This can be explained by the first assumption:assumption:

the combination of the small size of gas the combination of the small size of gas particles and the large distance between particles and the large distance between gas particles leads to a very low density.gas particles leads to a very low density.

Note:Note:The textbook lists only 3 assumptions of the The textbook lists only 3 assumptions of the kinetic theory of matter.kinetic theory of matter.There are, in fact, many more assumptions in There are, in fact, many more assumptions in a complete treatment of the kinetic theory.a complete treatment of 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 its understand the nature of matter in all of its states.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 can be applied.statistical treatment can be applied.The average The average kinetic energykinetic energy of the gas particles of the gas particles depends only on the depends only on the temperaturetemperature of the of the system.system.The time during collision of molecule with the The time during collision of molecule with the container's wall is negligible as comparable to container's wall is negligible as comparable to the time between successive collisions.the time between successive collisions.The equations of motion of the molecules are The equations of motion of the molecules are time-reversible.time-reversible.

SummarySummaryFirst 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 rapid, The motion of the particles in a gas is rapid, constant, and random.constant, and random.

Third Assumption:Third Assumption:All collisions between gas particles are All collisions between gas particles are perfectly elastic.perfectly elastic.