Kinetic Theory Solids, Liquids and Gases. The Nature of Gases Objectives: Objectives: Describe the motion of gas particles according the kinetic theory

Kinetic TheoryKinetic Theory

Solids, Liquids and GasesSolids, Liquids and Gases

The Nature of GasesThe Nature of Gases

Objectives:Objectives:Describe the motion Describe the motion

of gas particles of gas particles according the according the kinetic theorykinetic theory

Interpret gas Interpret gas pressure in terms of pressure in terms of kinetic theorykinetic theory


Most compounds exist in only one state at Most compounds exist in only one state at a time –a time –Water can beWater can be

Ice (solid)Ice (solid)Water (liquid)Water (liquid)Steam (gas)Steam (gas)


However, the state of existence is easily However, the state of existence is easily changed throughchanged throughChanges in temperatureChanges in temperature

Cool water down and it freezes into iceCool water down and it freezes into iceHeat water up and it boils into steamHeat water up and it boils into steam


The word kinetic meansThe word kinetic meansMotionMotion

Kinetic energy isKinetic energy isThe energy held by an object because of its The energy held by an object because of its

motionmotion


Kinetic theory isKinetic theory isThe concept that the particles in all matter are The concept that the particles in all matter are

in constant motion.in constant motion.


For gases, the following assumptions For gases, the following assumptions have been applied:have been applied:

1.1. The particles of a gas are small, hard The particles of a gas are small, hard spheres with insignificant volume and large spheres with insignificant volume and large distances between one another. These distances between one another. These particles are not attracted to one another; particles are not attracted to one another; nor are they repulsed from one another.nor are they repulsed from one another.



2.2. Each particle in a sample of a gas is in Each particle in a sample of a gas is in constant, straight-line motion. They will only constant, straight-line motion. They will only change direction when they run into change direction when they run into something, like another particle of gas or the something, like another particle of gas or the container holding them. This straight-line container holding them. This straight-line motion is called a “random walk.”motion is called a “random walk.”



3.3. Energy is not lost when the particles hit each Energy is not lost when the particles hit each other. This means all of the collisions other. This means all of the collisions between the particles are called perfectly between the particles are called perfectly elastic. Overall, this mean the gas does not elastic. Overall, this mean the gas does not lose the energy it has because it is always lose the energy it has because it is always running into itself… it doesn’t wear itself out.running into itself… it doesn’t wear itself out.

Gas PressureGas Pressure

Everything that is moving willEverything that is moving willExert a force when it collides with another Exert a force when it collides with another

objectobject Imagine getting hit by a car, a bicycle and a Imagine getting hit by a car, a bicycle and a

rubber ball that are all moving at the same rubber ball that are all moving at the same speedspeedWhich one hits you the hardest?Which one hits you the hardest?Which one hits you the softest?Which one hits you the softest?Why?Why?

carrubber ball

More mass = harder hit


Well, gas particles exert force too.Well, gas particles exert force too.The force applied by one gas particle is The force applied by one gas particle is

extremelyextremely tiny tinyA container of gas will have A container of gas will have lotslots of particles, of particles,

so billions of gas particles will run into the so billions of gas particles will run into the container simultaneously. container simultaneously.

All of these forces hitting at the same time All of these forces hitting at the same time add up to a measurable amount of force.add up to a measurable amount of force.


Gas pressure isGas pressure is The sum of the forces applied by all the gas particles The sum of the forces applied by all the gas particles

divided by the area over which the forces are applied.divided by the area over which the forces are applied.

This is a measurement of how “hard” the gas is hitting This is a measurement of how “hard” the gas is hitting the container.the container.

How could we increase the gas pressure?How could we increase the gas pressure? More particles, make them move faster, smaller area, etc.More particles, make them move faster, smaller area, etc.

2m area

N forcePa pressure


What if there were no gas particles What if there were no gas particles present to run into anything, like out in present to run into anything, like out in space? What would the pressure of that space? What would the pressure of that sort of system be?sort of system be?The pressure would be zero.The pressure would be zero.This is called a vacuum.This is called a vacuum.


In this class, we will express pressure In this class, we will express pressure using the units of the SI system, Pascals using the units of the SI system, Pascals (Pa); and an older unit called the (Pa); and an older unit called the atmosphere.atmosphere.We will sometimes see other units also; just We will sometimes see other units also; just

convert them to one that is easy to use.convert them to one that is easy to use.101325 Pa = 1 atm = 14.7 psi = 760 mmHg101325 Pa = 1 atm = 14.7 psi = 760 mmHg


As a review, what does STP stand for?As a review, what does STP stand for?Standard temperature and pressureStandard temperature and pressure

What is STPWhat is STP1 atm and 01 atm and 0ooCC

What is that temperature in Kelvin?What is that temperature in Kelvin?273.15 K273.15 K

Kinetic Energy and Kelvin Kinetic Energy and Kelvin TemperatureTemperature

When you heat something up, a little of the When you heat something up, a little of the energy is stored as potential energy while energy is stored as potential energy while the rest speeds the particles up.the rest speeds the particles up.That is, it increases the kinetic energy of the That is, it increases the kinetic energy of the

particles in the matter.particles in the matter.The particles will NOT all have the same The particles will NOT all have the same

kinetic energykinetic energySome (about half) are moving faster than averageSome (about half) are moving faster than averageSome (the other half) are moving slower than Some (the other half) are moving slower than

averageaverage


Theoretically, if you cooled something Theoretically, if you cooled something down enough all molecular motion would down enough all molecular motion would stop. This would happen atstop. This would happen atAbsolute zero (0 Kelvin)Absolute zero (0 Kelvin)We’ve never been able to cool anything down We’ve never been able to cool anything down

to absolute zero in a labto absolute zero in a lab


In the Kelvin scale, temperature and kinetic In the Kelvin scale, temperature and kinetic energy are directly related to each other. What energy are directly related to each other. What does this mean?does this mean?

55

22

(K) etemperatur

energy kinetic

12

12

1212

TT

KEKE

TTKEKE

T

KE

The Nature of LiquidsThe Nature of Liquids

Objectives:Objectives:Describe the nature Describe the nature

of a liquid in terms of of a liquid in terms of the attractive forces the attractive forces between the particlesbetween the particles

Differentiate between Differentiate between evaporation and evaporation and boiling of a liquid, boiling of a liquid, using kinetic theoryusing kinetic theory

The Nature of LiquidsThe Nature of Liquids

What makes a liquid different from a solid?What makes a liquid different from a solid?

A Model for LiquidsA Model for Liquids

Does the kinetic theory apply to liquids?Does the kinetic theory apply to liquids?Yes – the particles in a liquid are in motion.Yes – the particles in a liquid are in motion.The particles move around more slowly than The particles move around more slowly than

gas particles. gas particles. The particles are attracted to each other.The particles are attracted to each other.

These attractions are called intermolecular forces.These attractions are called intermolecular forces.These forces are what cause liquids to have a These forces are what cause liquids to have a

defined volume.defined volume.The forces are too weak to lock the particles into The forces are too weak to lock the particles into

place, which is why liquids flow.place, which is why liquids flow.

EvaporationEvaporation

What happens to a What happens to a saucer of water left saucer of water left sitting out for a long sitting out for a long time?time?The water turns The water turns

from a liquid into a from a liquid into a gas – it evaporatesgas – it evaporates


Vaporization:Vaporization:The conversion of a liquid to a gas or vapor.The conversion of a liquid to a gas or vapor.

Evaporation:Evaporation:Vaporization that occurs at the surface of a Vaporization that occurs at the surface of a

liquid that is not boilingliquid that is not boilingFaster than average liquid particles at the Faster than average liquid particles at the

surface of the liquid can “escape” into the gas surface of the liquid can “escape” into the gas statestate

Cools a liquid down (the hot particles leave)Cools a liquid down (the hot particles leave)


Vapor pressure:Vapor pressure:The pressure produced The pressure produced

when vapor particles above when vapor particles above a liquid in a closed a liquid in a closed container collide with the container collide with the container wallscontainer walls

A A dynamic equilibrium dynamic equilibrium exists between the vapor exists between the vapor and the liquid.and the liquid.


What does dynamic mean?What does dynamic mean? In motionIn motion

What is the opposite of dynamic?What is the opposite of dynamic?Static (still)Static (still)

What does equilibrium mean?What does equilibrium mean?A state of balance or equality between A state of balance or equality between

opposing forcesopposing forces


How can an equilibrium be dynamic?How can an equilibrium be dynamic?Everything going one direction has an Everything going one direction has an

opposite that goes the other wayopposite that goes the other way In a dynamic equilibrium, the two opposing In a dynamic equilibrium, the two opposing

sides are equal – essentially canceling each sides are equal – essentially canceling each other outother out

For evaporation, a dynamic equilibrium For evaporation, a dynamic equilibrium means the rate of particles going from liquid means the rate of particles going from liquid to gas is equal to the rate of particles going to gas is equal to the rate of particles going from gas to liquidfrom gas to liquid

Boiling PointBoiling Point

How do you make How do you make a liquid evaporate a liquid evaporate faster?faster?Heat it up – hotter Heat it up – hotter

particles move particles move faster and are more faster and are more likely to escape into likely to escape into the gas statethe gas state


Boiling point:Boiling point:The temperature at The temperature at

which the vapor which the vapor pressure of a liquid pressure of a liquid is just equal to the is just equal to the external pressure on external pressure on the liquidthe liquid


Boiling:Boiling:Vaporization throughout a liquidVaporization throughout a liquidBubbles of vapor form all over the sampleBubbles of vapor form all over the sample

Normal boiling point:Normal boiling point:Boiling point at a pressure of 1 atm (air Boiling point at a pressure of 1 atm (air

pressure at sea level)pressure at sea level)While boiling the temperature will While boiling the temperature will remain remain

constantconstantThe addition of more heat will vaporize more liquid The addition of more heat will vaporize more liquid

rather than raise the temperaturerather than raise the temperature

The Nature of SolidsThe Nature of Solids

Objectives:Objectives:Describe how the degree of organization of Describe how the degree of organization of

particles distinguishes solids from liquids and particles distinguishes solids from liquids and gasesgases

Distinguish between a crystal lattice and a Distinguish between a crystal lattice and a unit cellunit cell

ReviewReview

Name three molecular compoundsName three molecular compoundsCarbon dioxide, water, sulfur trioxideCarbon dioxide, water, sulfur trioxide

Name three ionic compoundsName three ionic compoundsSodium chloride, magnesium hydroxide, Sodium chloride, magnesium hydroxide,

calcium carbonatecalcium carbonateHow are molecular and ionic compounds How are molecular and ionic compounds

different?different?Molecular compounds exist as molecules. Molecular compounds exist as molecules.

Ionic compounds are huge collections of ions.Ionic compounds are huge collections of ions.

A Model for SolidsA Model for Solids

Do the particles in solids Do the particles in solids move?move?Yes, but with much less freedom Yes, but with much less freedom

than the particles in liquids.than the particles in liquids.They vibrate around a fixed They vibrate around a fixed

point.point.These particles are locked into These particles are locked into

position, so they can’t flow – position, so they can’t flow – solids have both a definite solids have both a definite volume and a definite shape.volume and a definite shape.


Melting point:Melting point:The temperature at The temperature at

which a solid turns which a solid turns into a liquid.into a liquid.

Also the Also the temperature at temperature at which a liquid turns which a liquid turns into a solid into a solid

Melting point = Melting point = freezing pointfreezing point


Melting is a result of adding heat to an Melting is a result of adding heat to an object.object.More heat = higher temperature = increased More heat = higher temperature = increased

kinetic energy of the particles = faster kinetic energy of the particles = faster movement of the particlesmovement of the particles

At the melting point, the particles are moving At the melting point, the particles are moving so fast that they are able to break free of the so fast that they are able to break free of the intermolecular forces holding them together.intermolecular forces holding them together.They begin to slide past one another as in a liquidThey begin to slide past one another as in a liquid


Why are the melting points of ionic Why are the melting points of ionic compounds higher than molecular compounds higher than molecular compounds?compounds?The ionic compounds have stronger forces The ionic compounds have stronger forces

holding them together.holding them together.Stronger forces = higher melting pointStronger forces = higher melting point

Can all compounds be melted?Can all compounds be melted?No – some, like wood and various plastics, will No – some, like wood and various plastics, will

decompose when heated. They never melt.decompose when heated. They never melt.

Crystal Structure and Unit CellsCrystal Structure and Unit Cells

Many solids are crystalline Many solids are crystalline in nature.in nature.The atoms, ions or molecules The atoms, ions or molecules

of the solid are arranged in of the solid are arranged in an orderly, repeating, 3D an orderly, repeating, 3D pattern called the crystal pattern called the crystal lattice.lattice.

Crystals are classified into Crystals are classified into seven systems.seven systems.You don’t have to learn their You don’t have to learn their

names.names.


The smallest group of particles within a The smallest group of particles within a crystal that retains the shape of the crystal that retains the shape of the crystal is calledcrystal is called

The unit cellThe unit cell


Three types of Three types of unit cellsunit cells1.1. Simple cubicSimple cubic

2.2. Body-centered Body-centered cubiccubic

3.3. Face-centered Face-centered cubiccubic


An allotrope isAn allotrope isA different form of A different form of

an element.an element.For example, For example,

carbon has 3 carbon has 3 allotropes:allotropes:GraphiteGraphiteCoalCoalDiamondDiamond


Not all solids are crystalline –Not all solids are crystalline –Some lack the organized structure found in Some lack the organized structure found in

crystalline solidscrystalline solidsThese unorganized solids are called These unorganized solids are called

amorphous solidsamorphous solidsAmorphous means “without shape”Amorphous means “without shape”


There are two key difference between There are two key difference between crystalline and amorphous solids:crystalline and amorphous solids:

1.1. Crystalline solids have a sharply defined Crystalline solids have a sharply defined melting point. Amorphous solids don’t – melting point. Amorphous solids don’t – they will have temperature range over which they will have temperature range over which they soften and then melt.they soften and then melt.

2.2. When struck, crystals will fragment into When struck, crystals will fragment into pieces with the same shape as the original. pieces with the same shape as the original. Amorphous solids will break into pieces with Amorphous solids will break into pieces with sharp edges (like glass) that are shaped sharp edges (like glass) that are shaped randomly.randomly.

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Kinetic Theory Solids, Liquids and Gases. The Nature of Gases Objectives: Objectives: Describe the motion of gas particles according the kinetic theory