Around 1827, dust particles were seen to move in a

random, zig-zag pattern

under a microscope,

Chapter 13 “GASES”

Brownian Movement

“I’ve been behind this guy in the hall!”

From the idea of Brownian Movement came the explanation for the behavior of gases and, later, for other particlesof matter.

So, let’s look at some important properties of gases, shall we? Why sure….

1. Gases have mass.

• Gases seem to be weightless, but they are classified as matter, which means they have mass.

• The density of a gas – the mass per unit of volume – is much less than the density of a liquid or solid, however.

GAS DENSITYGAS DENSITYGAS DENSITYGAS DENSITY

Higher Higher densitydensity

Lower Lower densitydensity

22.4 L of ANY gas AT STP = 1 mole

2nd– Gases are Compressible

If you squeeze a gas, its volume can be reduced considerablyThe low density of a gas means there is a lot of empty space between gas molecules.

3rd – Gases fill their containers

Gases spread out to fill containers until the concentration of gases is uniform throughout the entire space.

This is why there is never an absence of air around you!

4th – Gases diffuse

Because of all of the empty space between gas molecules, another gas molecule can pass between them until each gas is spread out evenly throughout the entire container.

This is called diffusion.

5th – Gases exert pressure

The sum of all of the collisions makes up the pressure the gas exerts.

Gas particles exert pressure by colliding with objects in their path.

Imagine a gas in a container as a room of hard rubber balls.

The collisions of the balls bouncing around exert a force on the object that with which they collide.

The definition of Pressure is force per unit area – so the total of all of the tiny collisions makes up the pressure exerted by the gas.

6th – Pressure depends on Temp

The higher the temperature of a gas -the higher the pressure that the gas exertsThe reverse of that is true as well, a the temperature of a gas decreases – the pressure decreases.How are temperature & pressure related?DIRECTLY

Kinetic Molecular Theory: Kinetic Molecular Theory: GasesGases particles in continuous, random, rapid particles in continuous, random, rapid

motion motion (Brownian!)(Brownian!) collisions between particles are collisions between particles are

elasticelastic

(they may transfer energy between (they may transfer energy between them, but they don’t lose any energy)them, but they don’t lose any energy)

volume of the particles is negligiblevolume of the particles is negligible (little effect on their behavior)(little effect on their behavior)

No attractive forces between particles No attractive forces between particles (little(little effect on their behavior)effect on their behavior)

What kind of energy is “energy of What kind of energy is “energy of motion”?motion”?

That’s Right! Kinetic Energy!That’s Right! Kinetic Energy!

What can you say about particles that What can you say about particles that have a lot of kinetic energy?have a lot of kinetic energy?

Right again! They move Fast!Right again! They move Fast!

But the particles in a gas don’t all But the particles in a gas don’t all move at the same speed so we move at the same speed so we measure themeasure the Average Kinetic Energy Average Kinetic Energy of a sample – TEMPERATURE!of a sample – TEMPERATURE!

Let’s think of your average, room temperature oxygen molecule as a car!

It would be traveling at a speed of about 443 m/sec (1000 mil/hr)!

If we compare speed & collisions, our“Oxygen Car” would have a collision every 314 car lengths (or 4.5 x 109 collisionsper second)!

Tough getting insurance!

Gas variablesIn order to describe a gas sample completely and then make predictions about its behavior under changed conditions, it is important to deal with the values of:

4) amount of gas 3) volume 2) temperature 1) pressure

Pressure (Pressure (PP))The pressure of a gas is the force exerted on the wall of the container a gas is trapped in.It is the force of the collisions & the number of collisions with the walls of a container that cause Gas Pressure.No Gas – No Pressure – VACUUM!

Put on your bathing suits (NO SPEEDOS, Please!) and I’ll meet you on a sandy beach in the Bahamas! 1, 2, 3… Let’s go!

You Look Great! Now swim out to that buoy!

When I say go, take a deep breath &dive under water. Go down about 2 feet & look up. Then come back to the surface!

What did you see directly above you?Water! Was it heavy?

Did you even notice?

Now, take a deeper breath & go downAbout 10 feet and look up. Then come up quickly!

What did you notice above you this time?Yes! More water!Did you feel it this time?How about those ears?!

We live under a sea of air.The more air above you, the more pressureon you!

There is less air above a mountain top than There is above a valley, so, less pressureabove the mountain!

Swim to shore! We’ll take the rest of our notes there!

Even though babies know how to drink from a straw, most people, young or old, don't know how it works. Most people think the suction caused by our mouth pulls the liquid up through the straw.

Normally when your mouth is closed, there's not much air (blue spheres) inside your mouth. They bounce around causing 15 psi pressure in the mouth. However, when you drop your jaw and keep your lips closed, there's more room for the air to spread out . so the air pressure inside the mouth is less.

Outside air, at a higher pressure, is trying to get into the mouth. It pushes on the cheeks causing them to be sunken.

If you have a straw in your mouth, then air pressure pushing on your drink has more force than the force from the air in your mouth. The outside air pressure pushes onto the surface of the drink. This pressure pushes liquid up through the straw to your mouth.

Normal Air Normal Air (atmospheric) (atmospheric) Pressure Pressure is the is the

average average pressure of the pressure of the air at sea level air at sea level under normal under normal conditions.conditions.

It will support a column of mercury 760 mm high

There are several units for pressure depending on the instrument used to measure it including:1) atmospheres (atm)2) Millimeters of Mercury (mmHg)3) Kilopascal (kPa)

Air pressure is measured with a Barometer

Remember STP? 0° C & 1 atm1atm = 760 mm Hg = 101.3 kPa

Pressure Pressure ConversionsConversions

A. What is 475 mm Hg expressed in A. What is 475 mm Hg expressed in atm?atm?

1 atm1 atm

760 mm Hg760 mm Hg

B. The pressure of a tire is measured as B. The pressure of a tire is measured as 233 kPa.233 kPa.

What is this pressure in mm Hg?What is this pressure in mm Hg?

760 mm Hg 760 mm Hg

101.3 kPa101.3 kPa

= 1.75 x 103 mm Hg

= 0.625 atm475 mm Hg x

233 kPa x

Temperature (T)

The temperature of a gas is generally measured with a thermometer in Celsius. All calculations involving gases should be made after converting the Celsius to Kelvin temperature.

Kelvin = C° + 273

Volume (V)

The volume of the gas is simply the volume of the container it is contained in.

The metric unit of volume is the liter (L)

1 L = 1 dm3 = 1000 ml = 1000 cm3

Amount (n)

The quantity of gas in a given sample is expressed in terms of moles of gas (n).This of course is in terms of 6.02 x 1023 molecules of the gas. Don’t forget to convert mass to moles you just divide by the molar mass of the gas.

Gas LawsGas Laws

Studies of the behavior of gases played a major role in the development of physical sciences in the 17th and 18th centuries.The Kinetic Molecular theory marked a significant achievement in understanding the behavior of gases.Observations have become mathematical laws which we can use to predict quantitative outcomes.

Avogadro’sAvogadro’s HypothesisHypothesis

Equal volumes of gases at the Equal volumes of gases at the same T and P have the same same T and P have the same number of moleculesnumber of molecules..

V and n are directly relatedV and n are directly related..

(twice as many (twice as many molecules)molecules)

1 mole of gas6.02 x 1023 atoms22.4 L at STP

2 moles2 x avogadro’s #44.8 L at STP

Avogadro’s Hypothesis Avogadro’s Hypothesis and Kinetic Molecular and Kinetic Molecular

TheoryTheory

Avogadro’s Hypothesis Avogadro’s Hypothesis and Kinetic Molecular and Kinetic Molecular

TheoryTheory

P is P is alsoalso directly proportional to number directly proportional to number of particles (n).of particles (n).

The gases in this The gases in this experiment are all experiment are all measured at the measured at the same T and V.same T and V.

Dalton’s Dalton’s LawLaw

John DaltonJohn Dalton1766-18441766-1844

Dalton’s Law of Partial Dalton’s Law of Partial PressuresPressures

At constant temperature, the At constant temperature, the pressure of a mixture of gases pressure of a mixture of gases that do not react equals the sum that do not react equals the sum of the partial pressures of the of the partial pressures of the gases in the mixture.gases in the mixture.

PPtotaltotal (in gas mixture)(in gas mixture) = P = P11 + P + P22 + P + P33......

Dalton’s Law of Partial Dalton’s Law of Partial PressurePressure

Dalton’s Law of Partial Dalton’s Law of Partial PressuresPressures

What is the total pressure in the What is the total pressure in the flask?flask?

PPtotaltotal = P = PHH22OO + P + POO22 = 0.48 atm = 0.48 atm

2 H2 H22OO2 2 (l) ---> 2 H(l) ---> 2 H22O (g) + OO (g) + O2 2 (g)(g)

0.32 atm 0.32 atm 0.16 0.16 atmatm

Collecting a gas “over Collecting a gas “over water”water”

Collecting Gases over Collecting Gases over WaterWater

Total gas pressure = 769 mm Hg

Water vaporPressure @ 19C=17 mmHg

Total pressure –Water vapor pressure =Pressure of gas

769 – 17 = 752 mm Hg

Boyle’s LawBoyle’s Law Robert Boyle was among

the first to note the relationship between pressure and volume of a gas.

He measured the volume of air at different pressures, and observed a pattern of behavior which led to his mathematical lawDuring his experiments temperature and amount of gas weren’t allowed to change

Robert Boyle Robert Boyle (1627-1691). (1627-1691). Son of Earl of Son of Earl of Porrello, Porrello, Ireland.Ireland.

Boyle’s Law

The volume of a confined gas The volume of a confined gas varies inversely with the varies inversely with the pressure, if temperature pressure, if temperature remains constant.remains constant.

P goes up as V goes down.P goes up as V goes down.

PP11VV11 = P = P22 V V22

As the pressure increases

As the pressure increases

VolumedecreasesVolume

decreases

Boyle’s Law and Kinetic Molecular

Theory

Boyle’s Law and Kinetic Molecular

Theory

P proportional to 1/VP proportional to 1/V

Boyle’s Law:

since PV = k (k = constant)

P1V1 = P2V2P1V1 = P2V2

Ex: A gas has a volume of 3.0 L at 2 atm. What is its volume

at 4 atm?

What if we had a change in conditions?

1)determine which variables you have:

P and V = Boyle’s Law

2)determine which law is being represented:

P1 = 2 atm

V1 = 3.0 L P2 = 4

atm V2 = ?

3) Rearrange the equation for the variable you don’t know

4) Plug in the variables and chug it on a calculator:

P1V1 = V2

P2

(2.0 atm)(3.0L) = V2

(4atm)V2 = 1.5LV2 = 1.5L

How does Pressure and Volume of gases relate

graphically?

Volu

me

Volu

me

PressurePressure

PV = kPV = k

(Temperature, # of particlesremain constant)

(typical graph for an inverse proportion)

Charles’s LawCharles’s Law

Jacques Charles determined the relationship between temperature and volume of a gas.He measured the volume of air at different temperatures, and observed a pattern of behavior which led to his mathematical law.During his experiments pressure of the system and amount of gas were held constant.

Jacques Charles determined the relationship between temperature and volume of a gas.He measured the volume of air at different temperatures, and observed a pattern of behavior which led to his mathematical law.During his experiments pressure of the system and amount of gas were held constant.

Charles’s Charles’s LawLaw

The volume of a gas The volume of a gas varies directly varies directly with the absolute with the absolute temperature, if temperature, if pressure remains pressure remains constant.constant.

Pressure & Kelvin Pressure & Kelvin Temp. are directly Temp. are directly proportional!proportional!

V1 V2

Jacques Charles (1746-Jacques Charles (1746-1823). Isolated boron 1823). Isolated boron and studied gases. and studied gases. Balloonist.Balloonist.

T1 T2

=

Volume of balloon at

room temperature

Volume of balloon at

room temperature

Volume of balloon at 5°C

Volume of balloon at 5°C

Charles’s Law:Charles’s Law:

since V/T = k

Eg: A gas has a volume of 3.0 L at 127°C. What is its volume at

227 °C?

V1 V2

T1 T2

=

What if we had a change in conditions?

1)determine which variables you have: (ALWAYS CHANGE CELSIUS TO KELVIN!)

T and V = Charles’s Law

2)determine which law is being represented:

T1 = 127°C + 273 = 400K

V1 = 3.0 L T2 = 227°C + 273 =

5ooK V2 = ?

3) Rearrange to solve for unknown:

(500K)(3.0L) = V2

V2 = 3.75LV2 = 3.75L

V1 T2 V2 =

4) Plug in the variables

and chug

T1

(400K)

(400K

Temp

How does Temperature and Volume of gases relate

graphically?V

olu

me V/T = k

Pressure, # of particlesremain constant

(Typical graph for a direct proportion)

Gay-Lussac’s LawGay-Lussac’s LawThe pressure of a The pressure of a

gas is directly gas is directly proportional to proportional to the absolute the absolute temperature, if temperature, if volume is volume is constant.constant.

If pressure goes If pressure goes up, Kelvin Temp up, Kelvin Temp goes up!goes up!

Joseph Louis Gay-Joseph Louis Gay-Lussac (1778-1850)Lussac (1778-1850)

P1 P2

T1 T2

=

Gas Pressure, Gas Pressure, Temperature, and Temperature, and

Kinetic Molecular TheoryKinetic Molecular Theory

Gas Pressure, Gas Pressure, Temperature, and Temperature, and

Kinetic Molecular TheoryKinetic Molecular Theory

P is directly proportional to Kelvin TP is directly proportional to Kelvin T

Temp

Pre

ssu

re

How does Pressure and Temperature of gases relate

graphically?

P/T = k

Volume, # of particlesremain constant

Gay-Lussac’s Law:Gay-Lussac’s Law:

What if we had a change in conditions?What if we had a change in conditions?

since P/T = ksince P/T = k

P1 P2

T1 T2

=

Eg: A gas has a pressure of 3.0 atm at 127º C. What is its pressure at 227º C?

Eg: A gas has a pressure of 3.0 atm at 127º C. What is its pressure at 227º C?

T and P = Gay-Lussac’s Law

T1 = 127°C + 273 =

400K P1 = 3.0 atm T2 = 227°C + 273 =

500K P2 = ?

1)determine which variables you have: (Celsius to Kelvin!!!)

1)determine which law is being represented:

3) Rearrange to solve for unknown:

(500K)(3.0atm) = P2 (400K)

P2 = 3.8atmP2 = 3.8atm

(P1)(T2) P2

=

=4) Plug in variables and

chug

T1

400K

LAWLAW RELATIONSHIPRELATIONSHIP LAWLAW CONSTANTCONSTANT

Boyle’sBoyle’s PP V V PP11VV1 1 = P= P22VV22 T, nT, n

Charles’Charles’ VV T T VV11/T/T11 = V = V22/T/T22 P, nP, n

Gay-Lussac’sGay-Lussac’s PP T T PP11/T/T11 = P = P22/T/T22 V, nV, n

Combined Gas LawCombined Gas Law The good news is that you The good news is that you

don’t have to remember all don’t have to remember all three gas laws! Since they three gas laws! Since they are all related to each other, are all related to each other, we can combine them into a we can combine them into a single equation. single equation. BE SURE BE SURE YOU KNOW THISYOU KNOW THIS EQUATION! EQUATION!

PP11 V V11 P P22 V V22

= = TT11 T T22

No, it’s not related to R2D2

Combined Gas LawCombined Gas Law

If you should only need one of If you should only need one of the other gas laws, you can the other gas laws, you can cover up the item that is cover up the item that is constant and you will get that constant and you will get that gas law!gas law!

==

P1V1

T1

P2 V2

T2

Boyle’s Law

Charles’ Law

Gay-Lussac’s Law

Combined Gas Law ProblemCombined Gas Law Problem

A sample of helium gas has a volume of 180 mL, A sample of helium gas has a volume of 180 mL, a pressure of 0.800 atm and a temperature of a pressure of 0.800 atm and a temperature of 29°C. What is the new temperature(°C) of the 29°C. What is the new temperature(°C) of the gas at a volume of 90.0 mL and a pressure of 3.20 gas at a volume of 90.0 mL and a pressure of 3.20 atm?atm?

Set up Data TableSet up Data Table

PP1 1 = 0.800 atm V= 0.800 atm V11 = 180 mL = 180 mL TT11 = 302 K = 302 K

PP22 = 3.20 atm V = 3.20 atm V22= 90 mL = 90 mL TT2 2 = ??= ??

CalculationCalculationPP1 1 = 0.800 atm V= 0.800 atm V11 = 180 mL = 180 mL T T11 = =

302 K302 KPP22 = 3.20 atm V = 3.20 atm V22= 90 mL T= 90 mL T2 2 = ??= ??

PP11 V V11 P P22 V V22

= = TT11 T T22

TT2 2 = P= P22 V V2 2 TT11

PP11 V V11

TT22 = 3.20 atm x 90.0 mL x 302 K = 3.20 atm x 90.0 mL x 302 K

0.800 atm x 180.0 mL 0.800 atm x 180.0 mL

TT22 = 604 K - 273 = 331 °C = 604 K - 273 = 331 °C

= 604 K

Learning CheckLearning Check

A gas has a volume of 675 mL A gas has a volume of 675 mL at 35°C and 0.850 atm at 35°C and 0.850 atm pressure. What is the pressure. What is the temperature in °C when the temperature in °C when the gas has a volume of 0.315 L gas has a volume of 0.315 L and a pressure of 802 mm Hg?and a pressure of 802 mm Hg?

Note: Volumes must be the same Note: Volumes must be the same unit and pressures must be the unit and pressures must be the same unit!same unit!

1. List variables:V1 = 675 mL = 0.675 LT1 = 35°C = 308 K°C = 308 KPP11 = 0.850 atm = 0.850 atmVV22 = .315 L = .315 LPP22 = 802 mmHg = 1.06 atm = 802 mmHg = 1.06 atm2.2.Decide on the appropriate gas law:Decide on the appropriate gas law:

Everything’s changing, so Combined!Everything’s changing, so Combined!3. Rearrange to solve for unknown:3. Rearrange to solve for unknown: TT22 = =

(P2) (V2) (T1)

(P1) (V1)= 179.2 K = (178.4K)

IF WE COMBINE ALL OF THE LAWS we’ve looked at TOGETHER - INCLUDING AVOGADRO’S LAW - WE GET:

PV=nRT

Ideal gas law

Ideal gas constant(R) R IS A CONSTANT THAT

CONNECTS THE 4 VARIABLES

R IS DEPENDENT ON THE UNITS OF THE VARIABLE FOR PRESSURE– TEMP IS ALWAYS IN KELVIN– VOLUME IS ALWAYS IN LITERS– PRESSURE IS IN EITHER atm OR mmHg OR kPa

Because of the different pressure units we use there are 3 different values for “R””

R=.0821L•atmmol•K

– IF PRESSURE IS GIVEN IN mmHg

R=62.4L•mmHgmol•K

– IF PRESSURE IS GIVEN IN kPa

R=8.314L•kPamol•K

– IF PRESSURE IS GIVEN IN atm

Learning CheckLearning Check

Dinitrogen monoxide (NDinitrogen monoxide (N22O), laughing O), laughing gas, is used by dentists as an anesthetic. gas, is used by dentists as an anesthetic. If 2.86 mol of gas occupies a 20.0 L If 2.86 mol of gas occupies a 20.0 L tank at 23°C, what is the pressure (in tank at 23°C, what is the pressure (in atm) in the tank in the dentist office?atm) in the tank in the dentist office?

Using Ideal gas lawUsing Ideal gas law

P

V T ?20.0 L 296K

R

=3.48 atm

0.0821L•atm

mol•K

(2.86 mol)(.0821)(296K)

20.0 L

PV = nRT

1. List variables:

2. Rearrange to solve for unknown:

P = nRTV

3, Plug & Chug:

n = 2.86 moles

GAS DIFFUSION AND EFFUSIONGAS DIFFUSION AND EFFUSION

DiffusionDiffusion is the is the movement of movement of molecules to fill a molecules to fill a containercontainer

EffusionEffusion is the is the movement of movement of molecules through molecules through a small hole into an a small hole into an empty container.empty container.

Graham’s LawGraham’s Law

Thomas Graham, 1805-1869. Thomas Graham, 1805-1869. Professor in Glasgow and Professor in Glasgow and London.London.

Rates of effusion of Rates of effusion of gases are inversely gases are inversely proportional to the proportional to the square root of their square root of their molar masses, at molar masses, at constant temp. & constant temp. & pressure.pressure.

Rates of effusion of Rates of effusion of gases are inversely gases are inversely proportional to the proportional to the square root of their square root of their molar masses, at molar masses, at constant temp. & constant temp. & pressure.pressure.

M of AM of B

Rate for B

Rate for A

M = molar mass & Gas B is the heavier gas!

Graham’s LawGraham’s LawMolecules effuse thru holes in a Molecules effuse thru holes in a

rubber balloon – that’s the main rubber balloon – that’s the main reason they get ‘whimpy’ after reason they get ‘whimpy’ after awhile! awhile!

They do this at a rate that is inversely They do this at a rate that is inversely proportional to molar mass.proportional to molar mass.

Therefore, He (4 g/mol) effuses more Therefore, He (4 g/mol) effuses more rapidly than Orapidly than O22 (32 g/mol) at same T. (It’s (32 g/mol) at same T. (It’s lighter!)lighter!)

HeHe

Lighter gases effuse faster than heavier ones

Graham’s Law We can use the entire equation to calculate

the actual speed of gas particles, however… We will just use the square root side to

COMPARE rates of effusion (speeds) Ex. Compare the rates of effusion of oxygen

gas & hydrogen gas. 1st – find their molar masses!

O2 = 32.0 g/mol H2 = 2.0 g/mol

2nd -Put the heavier gas (Gas B) in the numerator!

M of AM of B

Rate for B

Rate for A4 32 g

2.0 gYou’re not done yet!

Graham’s Law

The number 4 is not much of a “comparison”! You must put your answer in sentence form! Try this: “_______ gas travels (or effuses) at a rate ___ times faster than _________ gas.”So, the answer is… “Hydrogen gas travels (or effuses) at a rate 4 times faster than oxygen gas.”

(Lighter gas)

(heavier)(#)

Graham’s Law You try it! Compare the rates of effusion of Ar

and nitrogen gas (N2)

M of AM of B

Rate for B

Rate for A

39.9 g/mol28.0 g/mol

39.928.0

1.19

“Nitrogen gas travels (or effuses) at a rate 1.19 times faster than argon gas.”

All of these gas laws work just ducky assuming the gases are ‘ideal”(Points with “no volume” & “no mutual attraction”Most of the time, gases conform to ideal conditions. So, when are gases not “ideal”?Under conditions of low temperature& high pressure (force molecules close enough to affect each other!)

Deviations from Ideal Gas Deviations from Ideal Gas LawLaw Real Molecules have Real Molecules have

volumes and volumes and attractive forces attractive forces between them.between them.

Gases are not Gases are not “Ideal” under “Ideal” under conditions of high conditions of high pressure & low pressure & low temperaturetemperature which which bring particles close bring particles close enough together to enough together to affect each other!affect each other!

STOP HERE

The other two The other two STATES OF MATTER!STATES OF MATTER!

LiquidsLiquids A decrease in the average kinetic A decrease in the average kinetic

energy of gas particles causes the energy of gas particles causes the temperature to decrease. temperature to decrease.

As it cools, the particles tend to move As it cools, the particles tend to move more slowly, if they slow down enough, more slowly, if they slow down enough, attractive forces - called van der Waal’s attractive forces - called van der Waal’s forces –pull them very close together forces –pull them very close together so they can only slip & slide past each so they can only slip & slide past each other.other.

It is now in liquid form!

Condensation –Change of a gas to a liquid

The Nature of LiquidsThe Nature of Liquids

The conversion of a liquid to a gas or The conversion of a liquid to a gas or vapor vapor at the at the surfacesurface of a liquid is of a liquid is called called VaporizationVaporization

The process is called The process is called EvaporationEvaporation

in an open containerin an open container Particles near the surface with enough Particles near the surface with enough

kinetic energy that happen to bounce kinetic energy that happen to bounce in the right direction escape!in the right direction escape!

If you were to add a drop of water below the tube to the left what would happen?It would rise to the top & evaporate.What would it do to the surface of the mercury?Vapor Pressure – pressure exerted by vapor!

The Nature of LiquidsThe Nature of Liquids Eventually the particles will lose energy and Eventually the particles will lose energy and

return to the liquid state, or condense.return to the liquid state, or condense. What are the odds that they will return to the What are the odds that they will return to the

original liquid?original liquid? What if we cover the container?What if we cover the container? So, the particles begin to evaporate, then So, the particles begin to evaporate, then

some begin to condense. Eventually, the some begin to condense. Eventually, the number of particles evaporating will equal the number of particles evaporating will equal the number condensing & the space above the number condensing & the space above the liquid will be saturated with vaporliquid will be saturated with vapor A A dynamic equilibriumdynamic equilibrium now exists now exists Rate of evaporation = rate of condensationRate of evaporation = rate of condensation

The Nature of LiquidsThe Nature of Liquids

Note that there will still be Note that there will still be particles that evaporate and particles that evaporate and condensecondenseBut, there will be no NET changeBut, there will be no NET change It will not appear that there are It will not appear that there are

changes taking place!changes taking place!

The Nature of LiquidsThe Nature of Liquids

A liquid will evaporate faster when A liquid will evaporate faster when heatedheated Because the added heat increases the Because the added heat increases the

average kinetic energy needed to average kinetic energy needed to overcome the attractive forces so overcome the attractive forces so more particles have enough energy to more particles have enough energy to ‘escape’!‘escape’!

But, But, evaporation is a cooling evaporation is a cooling processprocess

Cooling occurs because particles Cooling occurs because particles with the highest energy escape firstwith the highest energy escape first

The Nature of LiquidsThe Nature of Liquids

Particles left behind have lower Particles left behind have lower average kinetic energies; thus the average kinetic energies; thus the temperature decreasestemperature decreases Similar to removing the fastest Similar to removing the fastest

runner from a race- the remaining runner from a race- the remaining runners have a lower average speedrunners have a lower average speed

Evaporation helps to keep our skin Evaporation helps to keep our skin cooler on a hot day, unless it is very cooler on a hot day, unless it is very humid on that day. Why?humid on that day. Why?

The Nature of LiquidsThe Nature of Liquids

A liquid boils when its vapor pressure A liquid boils when its vapor pressure equals the external pressure, so the equals the external pressure, so the boiling point changes if the external boiling point changes if the external pressure changes.pressure changes. Bubbles form throughout the liquid, rise to Bubbles form throughout the liquid, rise to

the surface, and escape into the airthe surface, and escape into the air

Normal boiling pointNormal boiling point-- is when the is when the vapor pressure of a liquid equals vapor pressure of a liquid equals standard pressure. (1 atm)standard pressure. (1 atm)

The The boiling pointboiling point (bp) is the (bp) is the temperature at which the temperature at which the vapor vapor pressure of the liquid is equal to pressure of the liquid is equal to the external pressure on the liquidthe external pressure on the liquid

The Nature of LiquidsThe Nature of Liquids

Normal bp of water = 100 Normal bp of water = 100 ooCC However, in Denver = 95 However, in Denver = 95 ooC, since C, since

Denver is 1600 m above sea level Denver is 1600 m above sea level and average atmospheric pressure and average atmospheric pressure is about 85.3 kPa (Recipe is about 85.3 kPa (Recipe adjustments?)adjustments?)

In pressure cookers, which reduce In pressure cookers, which reduce cooking time, water boils above cooking time, water boils above 100 100 ooC due to the increased C due to the increased pressurepressure

Vapor Pressures of Vapor Pressures of LiquidsLiquids

Normal bp whencrossing here

At any pt. on a curve line, liquid is boiling

SOLIDSSOLIDS If you cool a liquid, the particles lose If you cool a liquid, the particles lose

kinetic energy and slow down & get kinetic energy and slow down & get closer together.closer together.

If they slow down enough, extra If they slow down enough, extra forces of attraction pull them in so forces of attraction pull them in so close together that they can only close together that they can only vibrate in place.vibrate in place.

FreezingFreezing – change of a liquid to a – change of a liquid to a solid.solid.

Types of SolidsTypes of Solids Molecular solidsMolecular solids Metallic solidsMetallic solids Ionic solidsIonic solids Covalent network solidsCovalent network solids

Crystals orCrystals or Crystalline Solids Crystalline Solids

Particles of crystals are arranged Particles of crystals are arranged in repeating geometric patternsin repeating geometric patterns

Crystalline SolidsCrystalline Solids

Table salt crystals are shaped Table salt crystals are shaped like cubes.like cubes.

Diamond, a form of carbon, is Diamond, a form of carbon, is also a crystalline solid. (network also a crystalline solid. (network solid)solid) the crystals are shaped the crystals are shaped

something like pyramids.something like pyramids.

Non-crystalline solidsNon-crystalline solids

Many solids do not form crystals- Many solids do not form crystals- AmorphousAmorphous

Their molecules do not arrange into Their molecules do not arrange into repeating patternsrepeating patterns often because they are too large.often because they are too large.

Examples:Examples: Glass - Glass - also called a super-cooled liquidalso called a super-cooled liquid many plastics, soot, asphalt, buttermany plastics, soot, asphalt, butter

PHASE CHANGESPHASE CHANGES PHASE CHANGES – PHASE CHANGES – change is physical change is physical

state (melting, freezing, boiling, state (melting, freezing, boiling, condensing, sublimation, deposition) condensing, sublimation, deposition)

BOTH PHASES present during a phase BOTH PHASES present during a phase changechange

Temperature remains constant during a Temperature remains constant during a phase change.phase change.

SublimationSublimation – change of a solid directly to – change of a solid directly to a gas (dry ice, iodine, snow)a gas (dry ice, iodine, snow)

Deposition Deposition – change of a gas directly to a – change of a gas directly to a solid.solid.

Phase Diagrams show relationship Phase Diagrams show relationship between energy, temperature, & phases.between energy, temperature, & phases.

Heat (kilojoules)

Tem

pera

ture

(

C°)

0°

0-20°

20°

60°

Phase DiagramPhase Diagram

water

Freezing/Melting occur along this line

Boiling/condensingOccur along this line

Sublimation/depositionoccur along this line