Download - 1 Mr. ShieldsRegents Chemistry U07 L02 And Boiling Points

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Mr. ShieldsMr. Shields Regents Chemistry Regents Chemistry U07 L02 U07 L02

And Boiling PointsAnd Boiling Points

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Origins of Vapor PressureOrigins of Vapor PressureRecall, from our prior discussion of gases the following:Recall, from our prior discussion of gases the following:

1) Pressure is the result of the frequent collisions of1) Pressure is the result of the frequent collisions of molecules with the walls of the containermolecules with the walls of the container

2) The frequency of collisions is a function of temperature2) The frequency of collisions is a function of temperature

- Temperature is a measure of the avg. KE and- Temperature is a measure of the avg. KE and- The velocity of a molecule is a function of it’s KE- The velocity of a molecule is a function of it’s KE

3) As Temperature increases, velocity increases3) As Temperature increases, velocity increases so … the frequency of collisions increasesso … the frequency of collisions increases

- i.e. Pressure increases- i.e. Pressure increases

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KE and Temp – KMT Assumption 5KE and Temp – KMT Assumption 5Avg. KE of a gas is directly proportional to TempAvg. KE of a gas is directly proportional to Temp

This equation tells us that if mThis equation tells us that if m22 is > m is > m11 then thenvv22 must be < v must be < v11 to maintain the above equality. to maintain the above equality.

Let’s look at some data …Let’s look at some data …

KE = ½ mvKE = ½ mv22

So … At any specific temperatureSo … At any specific temperature

The KE of gas 1 = The KE of gas 2 so…The KE of gas 1 = The KE of gas 2 so…

½ m½ m11vv1122 = ½ m = ½ m22vv22

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Molecular Velocities Molecular Velocities Molecule H2 He H2O N2 O2 CO2

Avg. Speed(m/sec)

1960 1360 650 520 490 415

Molecular weight

2.0 4.0 18.0 28.0 32.0 44.0

KineticKinetic

EnergyEnergy

(x10(x1066))

3.8423.842 3.6993.699 3.8033.803 3.7863.786 3.8423.842 3.7893.789

But molecules do not have one specific velocityBut molecules do not have one specific velocity

KE is the same for eachKE is the same for each

incinc

decdec

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Distribution of Molecular Velocities At a fixed T

Note: molecules of a given gas are not moving atNote: molecules of a given gas are not moving atOne specific velocity. There is a distribution of velocitiesOne specific velocity. There is a distribution of velocities

(lighter)(lighter)

heavierheavier

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KE and Temp

And … as the temp increases Avg KE increasesAnd … as the temp increases Avg KE increasesAvg velocity shifts right and the whole Avg velocity shifts right and the whole distribution of velocities also shifts right anddistribution of velocities also shifts right andFlattens out.Flattens out.

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EvaporationEvaporation

This change in molecular velocity with changing T is This change in molecular velocity with changing T is True for True for all states of matterall states of matter, not just gases., not just gases.

And the And the distribution of velocitiesdistribution of velocities for any given temp for any given temp Is also true for any state of matter.Is also true for any state of matter.

As we will see, this is important in understanding As we will see, this is important in understanding Both liquid phase Both liquid phase evaporationevaporationand its and its vapor pressure.vapor pressure.

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EvaporationEvaporationWe’ve all seen puddles dry up on warm summer daysWe’ve all seen puddles dry up on warm summer daysOr the morning dew disappears shortly afterOr the morning dew disappears shortly afterThe rays of the sun warm the blades of grass.The rays of the sun warm the blades of grass.

How does this happen?How does this happen?

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Evaporation and VaporizationEvaporation and Vaporization

EvaporationEvaporation is a term that describes the escape of is a term that describes the escape ofMolecules from the surface of a liquid or solid intoMolecules from the surface of a liquid or solid intoThe Gas phase and it occurs at The Gas phase and it occurs at AllAll temperatures temperatures

This is not the same as This is not the same as boilingboiling! Why?! Why?

However, whether discussingHowever, whether discussingevaporation or boilingevaporation or boiling the thePhase change that occurs (liquid to gas) is known asPhase change that occurs (liquid to gas) is known asVaporizationVaporization..

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A word derived from the word vaporization is …A word derived from the word vaporization is …

VaporVapor is a term that describes is a term that describesthe the gaseous stategaseous state of a of a Substance that isSubstance that isnormallynormally liquid at room temperatureliquid at room temperature..

““VAPORVAPOR””

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The term The term “Gas”“Gas” is usually used only for those is usually used only for thoseSubstances that normallySubstances that normallyexist as a exist as a gas a room tempgas a room temp..

And a And a liquid that canliquid that canEvaporate quicklyEvaporate quickly at atroom Temp is said beroom Temp is said bevolatile.volatile.

A word that is sometimes alsoA word that is sometimes alsoused to describe someone’sused to describe someone’sPersonality! Right? Personality! Right?

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Evaporation and VaporizationEvaporation and VaporizationSo … What is needed to So … What is needed to vaporizevaporize a molecule from the a molecule from theLiquid Phase into the gaseous phase?Liquid Phase into the gaseous phase?

(Think back to the (Think back to the phase change diagramphase change diagram).).

Would this phase transition be exo or endothermic?Would this phase transition be exo or endothermic?

Endothermic – absorption of energy. Endothermic – absorption of energy. Why do we need to provide energy?Why do we need to provide energy?

EnergyEnergy is needed to overcome the intermolecularis needed to overcome the intermolecularAttractive between molecules.Attractive between molecules.

EE

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EvaporationEvaporationThese forces of attraction are known collectively as theThese forces of attraction are known collectively as thevan der Waal’s forcesvan der Waal’s forces

London dispersion (induced dipole)London dispersion (induced dipole)Dipole-dipoleDipole-dipoleHydrogen bondHydrogen bond

They are the forces that hold molecules together in theThey are the forces that hold molecules together in theliquid phase that must be overcome to move moleculesliquid phase that must be overcome to move moleculesfrom the liquid phase into the gas phasefrom the liquid phase into the gas phase

The energy necessary to overcome these forces isThe energy necessary to overcome these forces issupplied by the…supplied by the…

KE of the moleculeKE of the molecule

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Evaporation

Remember…Remember…Molecules at a given T have a distribution ofMolecules at a given T have a distribution ofVelocities. Those at the far right are moving fasterVelocities. Those at the far right are moving fasterThan those further to the left.Than those further to the left.

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Evaporation

Those molecules with sufficient KE can Those molecules with sufficient KE can overcome theovercome theAttractive forcesAttractive forces between it’s neighbors and therefore between it’s neighbors and thereforeLeave the liquid phase.Leave the liquid phase.

At higher temps thereAt higher temps thereIs a greater % of Is a greater % of Molecules with sufficientMolecules with sufficientEnergy to escape.Energy to escape.

What do you think happens to average KE of theWhat do you think happens to average KE of theRemainingRemaining molecules in the liquid molecules in the liquidwhen higher KE molecules leave?when higher KE molecules leave?

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EvaporationEvaporation1.1. Avg Velocity of the molecules decreasesAvg Velocity of the molecules decreases2.2. As Avg. Velocity decreases so does avg KEAs Avg. Velocity decreases so does avg KE3.3. Therefore Temperature of the liquid decreasesTherefore Temperature of the liquid decreases

Have you ever rubbed alcohol on your skin?Have you ever rubbed alcohol on your skin?What happens?What happens?

The same is true when youThe same is true when youperspire. Evaporating sweatperspire. Evaporating sweatcools down remaining sweat cools down remaining sweat

It in turn cools the skin.It in turn cools the skin.

Why?Why?

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EvaporationEvaporation

Remember … Remember … Evaporation can occur at any tempEvaporation can occur at any temp. Even. EvenWhen it’s cold outside puddles eventually “dry up”. ThereWhen it’s cold outside puddles eventually “dry up”. ThereAre always some no. of molecules with enough energyAre always some no. of molecules with enough energyTo overcome the attractive forces.To overcome the attractive forces.

But what happens if the liquid is confined in a But what happens if the liquid is confined in a sealedsealedcontainer?container?

In a short while, a In a short while, a DYNAMIC EQUILIBRIUMDYNAMIC EQUILIBRIUM is reached. is reached.

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Dynamic EquilibriumDynamic Equilibrium

Molecules with sufficient energy leave the liquid. In an Molecules with sufficient energy leave the liquid. In an Open containerOpen containerThis continues This continues Until all the liquidUntil all the liquidIs gone.Is gone.

In a closed system In a closed system The molecules inThe molecules inThe gas phaseThe gas phaseRemain above Remain above The liquid.The liquid.

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Dynamic equilibriumDynamic equilibrium

In the closed system some of the gas molecules return In the closed system some of the gas molecules return To the liquid phase. This is called To the liquid phase. This is called condensation.condensation.

At first condensation is At first condensation is close to zeroclose to zero since there are since there areFew molecules in the gas phase. Few molecules in the gas phase.

As more molecules moveAs more molecules moveInto the gas phase theInto the gas phase theRate of condensationRate of condensationRapidly increaseRapidly increase as as Molecules lose energy toMolecules lose energy toThe system.The system.

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At any given T the rate of evaporation is constant. TheAt any given T the rate of evaporation is constant. TheRate of Rate of condensationcondensation rapidly increases until it is rapidly increases until it isEqual toEqual to the rate of the rate of evaporationevaporation

When the two processesWhen the two processesAre equal we have Are equal we have Reached a state ofReached a state ofDynamic EquilibriumDynamic Equilibrium

Rate of evap. at given TRate of evap. at given T

Time Time

RateRate

condensation

* *

** * * * * * Fig B * Fig B * *

* Fig A* Fig A

Fig CFig C

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Dynamic EquilibriumDynamic EquilibriumIncreasing Gas phaseIncreasing Gas phaseConcentrationConcentration

Time 0Time 0

Fig AFig A Fig BFig B Fig C Fig C

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Vapor PressureVapor Pressure

Remember … molecules in the gas phase have aRemember … molecules in the gas phase have aspecific KE Which is a function of the temperaturespecific KE Which is a function of the temperature

These molecules These molecules collidecollide with the with thecontainer walls andcontainer walls andProduce a Produce a pressurepressure

This pressure produced by theThis pressure produced by theliquid’s molecules in theliquid’s molecules in theGas phase is called the…Gas phase is called the…

VAPOR PRESSUREVAPOR PRESSURE of the liquid of the liquid

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VP of HVP of H2200

760

Normal BP = 100 deg

BP at 290 mm = 75 deg

Vapor Pressure & B.P.Vapor Pressure & B.P.VP has a VP has a fixed valuefixed valueFor a given Temperature.For a given Temperature.

The higher the Temp theThe higher the Temp theHigher the VPHigher the VP

When the VP When the VP equalsequalsAtmospheric pressureAtmospheric pressureBoilingBoiling occurs occurs

The The ““Normal BPNormal BP”” is the is theTemp at which a liquid Temp at which a liquid Boils at Boils at Std PressureStd Pressure (1 atm)(1 atm)

Note how BP changes with Pressure!Note how BP changes with Pressure!

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VP & BP of Various LiquidsVP & BP of Various Liquids

VP is VP is dependentdependentOnly UponOnly Upon the Temp of the Temp of The liquid andThe liquid andThe The forces of forces of AttractionAttraction that are that areProminent for thatProminent for thatLiquid & Liquid & NOT HOWNOT HOWMUCH LIQUID THERE ISMUCH LIQUID THERE IS

Forces of attractionForces of attractionFor these liquids For these liquids inc.inc.From Left to right soFrom Left to right soVP dec.VP dec. in the same in the sameDirection.Direction.

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HO-CH2-CH2-OH x

Ethylene Glycol

VP & BP of Various LiquidsVP & BP of Various Liquids

H- BondH- Bond plays playsA key role in A key role in DeterminingDeterminingThe BP of theseThe BP of theseLiquids with theLiquids with theexception of exception of Diethyl ether.Diethyl ether.

Why?Why?

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BP and Molecular WeightBP and Molecular WeightLastly, as mentioned earlier Intermolecular forces Lastly, as mentioned earlier Intermolecular forces Increase with Increase with increasing MWincreasing MW

Therefore, Therefore, within the same family of compoundswithin the same family of compounds, as , as MW increases VP dec. and BP inc. For example…MW increases VP dec. and BP inc. For example…

AlcoholsAlcohols MWMW BP deg CBP deg C AlkanesAlkanes MWMW BP deg CBP deg C

MethanolMethanol 3232 64.764.7 MethaneMethane 1616 -162-162

EthanolEthanol 4646 78.378.3 EthaneEthane 3030 -89-89

PropanolPropanol 6060 97.297.2 PropanePropane 4242 -42-42

ButanolButanol 7474 117.7117.7 ButaneButane 5454 -0.5-0.5

Download - 1 Mr. ShieldsRegents Chemistry U07 L02 And Boiling Points

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