chapter 10: states of matter coach kelsoe chemistry pages 328–352

Chapter 10:Chapter 10:States of MatterStates of Matter

Coach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 328–352Pages 328–352

Section 10–1:Section 10–1:The Kinetic-Molecular Theory of The Kinetic-Molecular Theory of

MatterMatterCoach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 329–332Pages 329–332

Section 10–1 ObjectivesSection 10–1 Objectives State the kinetic–molecular theory of State the kinetic–molecular theory of

matter, and describe how it explains matter, and describe how it explains certain properties of matter.certain properties of matter.

List the five assumptions of the kinetic–List the five assumptions of the kinetic–molecular theory of gases.molecular theory of gases.

Define the terms Define the terms ideal gasideal gas and and real gasreal gas..

Section 10–1 ObjectivesSection 10–1 Objectives Describe each of the following Describe each of the following

characteristic properties of gases: characteristic properties of gases: expansion, density, fluidity, expansion, density, fluidity, compressibility, diffusion, and effusion.compressibility, diffusion, and effusion.

Describe the conditions under which a Describe the conditions under which a real gas deviates from “ideal” behavior.real gas deviates from “ideal” behavior.

The Kinetic-Molecular Theory of The Kinetic-Molecular Theory of MatterMatter

In the late 1800s, scientists developed In the late 1800s, scientists developed the kinetic-molecular theory of matter the kinetic-molecular theory of matter to account for the behavior of the atoms to account for the behavior of the atoms and molecules that make up matter.and molecules that make up matter.

The The kinetic-molecular theorykinetic-molecular theory is is based on the idea that particles of based on the idea that particles of matter are matter are alwaysalways in motion. in motion.

The Kinetic-Molecular Theory of The Kinetic-Molecular Theory of GasesGases

The kinetic-molecular theory can help The kinetic-molecular theory can help you understand the behavior of gas you understand the behavior of gas molecules and the physical properties of molecules and the physical properties of gases. The theory provides a model of gases. The theory provides a model of what is called an ideal gas.what is called an ideal gas.

An An ideal gasideal gas is an imaginary gas that is an imaginary gas that perfectly fits all the assumptions of the perfectly fits all the assumptions of the kinetic-molecular theory.kinetic-molecular theory.

The Kinetic-Molecular Theory of The Kinetic-Molecular Theory of GasesGases

The kinetic-molecular theory of gases is The kinetic-molecular theory of gases is based on the following five assumptions:based on the following five assumptions:1.1. Gases consist of large numbers of tiny particles Gases consist of large numbers of tiny particles

that are far apart relative to their size.that are far apart relative to their size.2.2. Collisions between gas particles and between Collisions between gas particles and between

particles and container walls are elastic collisions.particles and container walls are elastic collisions.3.3. Gas particles are in continuous, rapid, random Gas particles are in continuous, rapid, random

motion. They therefore possess kinetic energy, motion. They therefore possess kinetic energy, which is energy of motion.which is energy of motion.

4.4. There are no forces of attraction or repulsion There are no forces of attraction or repulsion between gas particles.between gas particles.

5.5. The average kinetic energy of gas particles The average kinetic energy of gas particles depends on the temperature of the gas.depends on the temperature of the gas.

Assumption #1Assumption #1 ““Gases consist of large numbers of tiny Gases consist of large numbers of tiny

particles that are far apart relative to their particles that are far apart relative to their size.”size.”• These particles, usually molecules or atoms, These particles, usually molecules or atoms,

typically occupy a volume about 1000 times typically occupy a volume about 1000 times greater than the volume occupied by particles in greater than the volume occupied by particles in the liquid or solid state. Thus, molecules of the liquid or solid state. Thus, molecules of gases are much farther apart than those of gases are much farther apart than those of liquids or solids.liquids or solids.

• Most of the volume occupied by a gas is empty Most of the volume occupied by a gas is empty space. This is why their density is much less and space. This is why their density is much less and why they can be compressed so much easier.why they can be compressed so much easier.

Assumption #2Assumption #2 ““Collisions between gas particles and Collisions between gas particles and

between particles and container walls between particles and container walls are elastic.”are elastic.”• An An elastic collisionelastic collision is one in which there is is one in which there is

no net loss of kinetic energy.no net loss of kinetic energy.• Kinetic energy is transferred between two Kinetic energy is transferred between two

particles during collisions. However, the particles during collisions. However, the total kinetic energy of the two particles total kinetic energy of the two particles remains the same as long as the remains the same as long as the temperature is constant.temperature is constant.

Assumption #3Assumption #3 ““Gas particles are in continuous, rapid, Gas particles are in continuous, rapid,

random motion. They therefore possess random motion. They therefore possess kinetic energy, which is energy of kinetic energy, which is energy of motion.”motion.”• Gas particles move in all directions.Gas particles move in all directions.• The kinetic energy of the particles The kinetic energy of the particles

overcomes the attractive forces between overcomes the attractive forces between them, except near the temperature at them, except near the temperature at which the gas condenses and becomes a which the gas condenses and becomes a liquid.liquid.

Assumption #4Assumption #4 ““There are no forces of attraction or There are no forces of attraction or

repulsion between gas particles.”repulsion between gas particles.”• You can think of ideal gas molecules as You can think of ideal gas molecules as

behaving like small billiard balls. When behaving like small billiard balls. When they collide, they do not stick together but they collide, they do not stick together but immediately bounce apart.immediately bounce apart.

Assumption #5Assumption #5 ““The average kinetic energy of gas The average kinetic energy of gas

particles depends on the temperature of particles depends on the temperature of the gas.”the gas.”• The kinetic energy of any moving object, including The kinetic energy of any moving object, including

a particle, is given by the following equation”a particle, is given by the following equation”

KE = ½mvKE = ½mv22

• In the equation, In the equation, mm is the mass of the particle and is the mass of the particle and vv is its speed. Because all the particles of a specific is its speed. Because all the particles of a specific gas have the same mass, their kinetic energies gas have the same mass, their kinetic energies depend only on their speeds. Average speeds depend only on their speeds. Average speeds increase with an increase in temperature.increase with an increase in temperature.

The Kinetic-Molecular Theory and The Kinetic-Molecular Theory and the Nature of Gasesthe Nature of Gases

The kinetic-molecular theory applies The kinetic-molecular theory applies only to ideal gases.only to ideal gases.

Although ideal gases do not actually Although ideal gases do not actually exist, many gases behave nearly ideally exist, many gases behave nearly ideally if pressure is not very high or if pressure is not very high or temperature is not very low.temperature is not very low.

In the following slides, you will see how In the following slides, you will see how the kinetic-molecular theory accounts the kinetic-molecular theory accounts for the physical properties of gases.for the physical properties of gases.

ExpansionExpansion

Gases do not have a definite shape or definite Gases do not have a definite shape or definite volume. They completely fill any container in volume. They completely fill any container in which they are enclosed, and they take its which they are enclosed, and they take its shape.shape.

A gas transferred from a one-liter vessel to a A gas transferred from a one-liter vessel to a two-liter vessel will quickly expand to fill the two-liter vessel will quickly expand to fill the entire two-liter vessel.entire two-liter vessel.

The kinetic-molecular theory explains these The kinetic-molecular theory explains these facts. According to the theory, gas particles facts. According to the theory, gas particles move rapidly in all directions (assumption 3) move rapidly in all directions (assumption 3) without significant attraction or repulsion without significant attraction or repulsion between them (assumption 4).between them (assumption 4).

FluidityFluidity

Because the attractive forces between Because the attractive forces between gas particles are insignificant gas particles are insignificant (assumption 4), gas particles glide (assumption 4), gas particles glide easily past one another. This ability to easily past one another. This ability to flow causes gases to behave similarly to flow causes gases to behave similarly to liquids.liquids.

Because liquids and gases flow, they are Because liquids and gases flow, they are both referred to as both referred to as fluidsfluids..

Low DensityLow Density

The density of a substance in the The density of a substance in the gaseous state is about 1/1000 the gaseous state is about 1/1000 the density of the same substance in the density of the same substance in the liquid or solid state. That is because the liquid or solid state. That is because the particles are so much farther apart in particles are so much farther apart in the gaseous state (assumption 1).the gaseous state (assumption 1).

CompressibilityCompressibility

During compression, the gas particles, which During compression, the gas particles, which are initially very far apart (assumption 1), are are initially very far apart (assumption 1), are crowded close together.crowded close together.

The volume of a given sample of a gas can be The volume of a given sample of a gas can be greatly decreased.greatly decreased.

Steel cylinders containing gases under Steel cylinders containing gases under pressure are widely used in industry. When pressure are widely used in industry. When they are full, such cylinders may contain 100 they are full, such cylinders may contain 100 times as many particles of gas as would be times as many particles of gas as would be contained in nonpressurized containers of the contained in nonpressurized containers of the same size.same size.

Diffusion and EffusionDiffusion and Effusion

Gases spread out and mix with one another, Gases spread out and mix with one another, even without being stirred. If the stopper is even without being stirred. If the stopper is removed from a container of ammonia in a removed from a container of ammonia in a room, ammonia gas will mix uniformly room, ammonia gas will mix uniformly through the air and spread throughout the through the air and spread throughout the room.room.

The random and continuous motion of the The random and continuous motion of the ammonia molecules (assumption 3) carries ammonia molecules (assumption 3) carries them throughout the available space.them throughout the available space.

Such spontaneous mixing of the particles of Such spontaneous mixing of the particles of two substances caused by their random two substances caused by their random motion is called motion is called diffusiondiffusion..


The rate of diffusion of one gas through The rate of diffusion of one gas through another depends on three properties of the another depends on three properties of the gas particles:gas particles:• Their speedsTheir speeds• Their diametersTheir diameters• The attractive forces between themThe attractive forces between them

Hydrogen gas diffuses rapidly into other gases Hydrogen gas diffuses rapidly into other gases at the same temperature because its at the same temperature because its molecules are lighter and move faster than molecules are lighter and move faster than the molecules of the other gases.the molecules of the other gases.


Diffusion is a process by which particles of a Diffusion is a process by which particles of a gas spread out spontaneously and mix with gas spread out spontaneously and mix with other gases.other gases.

In contrast, In contrast, effusioneffusion is a process by which is a process by which gas particles pass through a tiny opening.gas particles pass through a tiny opening.

The rates of effusion of different gases are The rates of effusion of different gases are directly proportional to the velocities of their directly proportional to the velocities of their particles. Because of this proportionality, particles. Because of this proportionality, molecules of low mass effuse faster than molecules of low mass effuse faster than molecules of high mass.molecules of high mass.

Deviations of Real Gases from Ideal Deviations of Real Gases from Ideal BehaviorBehavior

When their particles are far enough When their particles are far enough apart and have enough kinetic energy, apart and have enough kinetic energy, most gases behave ideally. However, most gases behave ideally. However, all real gases deviate to some degree all real gases deviate to some degree from ideal-gas behavior.from ideal-gas behavior.

A A real gasreal gas is a gas that does not is a gas that does not behave completely according to the behave completely according to the assumptions of the kinetic-molecular assumptions of the kinetic-molecular theory.theory.


In 1873, Johannes van der Waals accounted In 1873, Johannes van der Waals accounted for this deviation from ideal behavior by for this deviation from ideal behavior by pointing out that particles of real gases pointing out that particles of real gases occupy space and exert attractive forces on occupy space and exert attractive forces on each other.each other.

At very high pressures and low temperatures, At very high pressures and low temperatures, the deviation may be considerable. Under the deviation may be considerable. Under such conditions, the particles will be closer such conditions, the particles will be closer together and their kinetic energy will be together and their kinetic energy will be insufficient to completely overcome the insufficient to completely overcome the attractive forces.attractive forces.


The kinetic-molecular theory is more The kinetic-molecular theory is more likely to hold true for gases whose likely to hold true for gases whose particles have little attraction for each particles have little attraction for each other.other.

The noble gases, such as helium and The noble gases, such as helium and neon, show essentially ideal gas neon, show essentially ideal gas behavior over a wide range of behavior over a wide range of temperatures and pressures. The temperatures and pressures. The particles of these gases are monatomic particles of these gases are monatomic and thus nonpolar.and thus nonpolar.


The particles of gases, such as nitrogen, NThe particles of gases, such as nitrogen, N22, , and hydrogen, Hand hydrogen, H22, are nonpolar diatomic , are nonpolar diatomic molecules. The behavior of these gases most molecules. The behavior of these gases most closely approximates that of the ideal gas closely approximates that of the ideal gas under certain conditions.under certain conditions.

The more polar a gas’s molecules are, the The more polar a gas’s molecules are, the greater the attractive forces between them greater the attractive forces between them and the more the gas will deviate from ideal and the more the gas will deviate from ideal gas behavior.gas behavior.

For example, highly polar gases, like For example, highly polar gases, like ammonia, NHammonia, NH33, and water vapor, deviate from , and water vapor, deviate from ideal behavior to a larger degree than ideal behavior to a larger degree than nonpolar gases.nonpolar gases.

VocabularyVocabulary

DiffusionDiffusion EffusionEffusion Elastic collisionElastic collision FluidsFluids Ideal gasIdeal gas Kinetic-molecular theoryKinetic-molecular theory Real gasReal gas