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Mr. ShieldsMr. Shields Regents Chemistry Regents Chemistry U07 L03 U07 L03

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Phase Changes

Let’s review what we’ve learned previously about Let’s review what we’ve learned previously about PHASE CHANGES.PHASE CHANGES.

First … What is a phase First … What is a phase change?change?A change from one stateA change from one stateof matter to another.of matter to another.

What does endothermic and exothermic mean?What does endothermic and exothermic mean?Absorbs heat Absorbs heat / Releases heat/ Releases heat

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Phase Change OverviewPhase Change Overview

Gas

Solid

LiquidEnergy AbsorbedEnergy Released

Endothermic

Exothermic

a

b

c

d

e

f

ENDOTHERMICPhase changes

a. Sublimation

b. Fusion

c. Vaporization

EXOTHERMICPhase Chages

d. Deposition

e. Condensation

f. Solidification

What are the names of the phase changes we’ve What are the names of the phase changes we’ve studied?studied?

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Phase ChangesPhase ChangesWhat actually happens to E as matter moves from What actually happens to E as matter moves from Solid Solid Liquid Liquid Gas: Gas:

Energy is absorbed (endo)Energy is absorbed (endo)Temp increasesTemp increasesKE increasesKE increasesmolecules move further apartmolecules move further apartPE increases (Why?)PE increases (Why?)

Remember … Remember … PE is a function of the positionPE is a function of the positionof two bodies relative to one anotherof two bodies relative to one another

SoSo … … PE inc as molecular separation increasesPE inc as molecular separation increases

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Heating CurveHeating CurveWhat happens when we keep adding energy to a solid ?What happens when we keep adding energy to a solid ?

Solid becomes a liquid then the liquid Solid becomes a liquid then the liquid becomes a gasbecomes a gas

As this energy is addedAs this energy is addedKE inc and so does Temp.KE inc and so does Temp.but does temp Uniformly inc.but does temp Uniformly inc.over time?over time?

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The answer is NO!The answer is NO!

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Heating CurveHeating Curve

So what does happen if the temperature does not So what does happen if the temperature does not Uniformly increase?Uniformly increase?

What happens is described by what is called What happens is described by what is called the the HEATING CURVEHEATING CURVE

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Heating CurveHeating Curve

If we heat a solid it’s temperature increases If we heat a solid it’s temperature increases Steadily until we finally reach the temperatureSteadily until we finally reach the temperatureat which the solid begins to meltat which the solid begins to melt

It begins to go thru a phase changeIt begins to go thru a phase changesolid solid Liquid Liquid

This phase change is calledThis phase change is called fusionfusion

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Solid/Liquid Phase Solid/Liquid Phase TransitionTransition

As the solid begins to melt somethingAs the solid begins to melt somethingUnusualUnusual Happens. Happens.

As we continue to add heat to the solidAs we continue to add heat to the solidthe temperature stops risingthe temperature stops rising as the solid as the solidcontinues to melt.continues to melt.

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Heat added over Time

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SolidSolid

Melting SolidMelting Solid

But But whywhy does this happen? does this happen?

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Solid/Liquid Phase Solid/Liquid Phase TransitionTransition

Solids exist in a rigid, closely packed, highlySolids exist in a rigid, closely packed, highlystructured patternstructured pattern

Liquids however have no such rigid structure.Liquids however have no such rigid structure.

As we reach the solids m.p. there is just enough energy toAs we reach the solids m.p. there is just enough energy tobegin overcoming the intermolecular forces betweenbegin overcoming the intermolecular forces betweenmolecules holding them together molecules holding them together in the solid state...in the solid state...

Molecules begin to separateMolecules begin to separate

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Heat of FusionHeat of FusionAdded additional heat energy goes intoAdded additional heat energy goes intoSeparating more and more moleculesSeparating more and more molecules

As molecules move from solid to liquid As molecules move from solid to liquid the PEthe PEIncreasesIncreases but since the temperature doesn’t but since the temperature doesn’t riseriseThe KE remains The KE remains constantconstant..

The energy necessary to melt 1 GRAM of aThe energy necessary to melt 1 GRAM of asolid is called the …solid is called the …

It is unique for every substance.It is unique for every substance.For For waterwater, the heat of fusion = , the heat of fusion = 334 334 Joules/gramJoules/gram

HEAT OF FUSIONHEAT OF FUSION (H (Hff))

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Solid/Liquid Phase transitionSolid/Liquid Phase transitionRemember… heat Remember… heat ALWAYSALWAYS flows from hot to cold flows from hot to cold

Until the last piece of solid melts the temperature the Until the last piece of solid melts the temperature the Solid/liquid mixture Solid/liquid mixture remains constantremains constant

Any excess heat in the liquid immediately flowsAny excess heat in the liquid immediately flowsbackback into the colder solid into the colder solid

Once there’s no solid left all additional heat addedOnce there’s no solid left all additional heat addedbegins to begins to increaseincrease the temperature of the liquid. the temperature of the liquid.

CCHH

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Solid/Liquid Phase transitionSolid/Liquid Phase transition

Only a single solid (s) phase presentOnly a single solid (s) phase presentKE (T) increasesKE (T) increasesPE is ConstantPE is Constant

Dual L/S Phase presentDual L/S Phase presentKE (T) is ConstantKE (T) is ConstantPE increasePE increase

Only a single liquid (l) phase presentOnly a single liquid (l) phase presentKE (T) increasesKE (T) increasesPE is constantPE is constant

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SolidSolid

S & LS & L LiquidLiquid

When heat is added what happens to KE, T & PE ?When heat is added what happens to KE, T & PE ?

Heating curveHeating curve

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Liquid/Vapor Phase Liquid/Vapor Phase transitiontransition

Any additional heat addedAny additional heat addedTo the liquid inc. Temp. andTo the liquid inc. Temp. andVapor Pressure.Vapor Pressure.

Once VP = POnce VP = Patmatm the liquid the liquidBoils.Boils.

As in the transition from solidAs in the transition from solidTo liquid, 2 phases are now presentTo liquid, 2 phases are now present& the Temp. of the boiling water & the Temp. of the boiling water remains constantremains constant

B.PB.P

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S & LS & L LiquidLiquid

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Heat of VaporizationHeat of Vaporization

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Heat added over TimeT

emp

LiquidLiquidS & LS & L

SolidSolid

GasGasL & GL & G

Molecules in the liquid phase form a close butMolecules in the liquid phase form a close butloosely organized structureloosely organized structure

Molecules in the gasMolecules in the gasPhase have no structurePhase have no structureAnd are widely separated.And are widely separated.

To separate these molecules this much takesTo separate these molecules this much takeslots of energylots of energy..

This energy is called the This energy is called the HEAT OF HEAT OF VAPORIZATION (HVAPORIZATION (Hvv))

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Liquid &gas

Solid &liquidM.PM.P

B.P.B.P.

The Heating CurveThe Heating Curve

Heat of VaporizationHeat of Vaporization

As long as the liquid is As long as the liquid is boiling boiling T and KE will beT and KE will beConstantConstant

PE will IncreasePE will Increaseas moleculesas moleculesMove further apartMove further apartIn the gas phaseIn the gas phase

For water, Hv = 2,260 Joules/g Note HFor water, Hv = 2,260 Joules/g Note Hvv > H > Hff

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SummarySummary

T inc. / KE inc / PE Const.T inc. / KE inc / PE Const.( 1 phase )( 1 phase )

T const. / KE const. /PE incT const. / KE const. /PE inc( 2 phases )( 2 phases )

Hf = 334 J/g

(m.p.)

(b.p.)Hv = 2,260 J/g

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A Review: Specific HeatA Review: Specific Heat

The SPECIFIC HEAT of a substance is the amountof heat required to raise the temperature of 1 g of thematerial by 1 degree centigrade.

Each substance has it’s own unique specific heat…

Recall from our earlier discussions that …Recall from our earlier discussions that …

The The lower lower the specific heat the the specific heat the better the conductorbetter the conductor

And …. q = Cp x m x (Tf – Ti)

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Specific Heat Specific Heat

The Heating CurveThe Heating Curve

One of the variables in Specific heat calculationsOne of the variables in Specific heat calculationsInvolves TemperatureInvolves TemperatureChange.Change.

What phases of the What phases of the The heating curveThe heating curveInvolve changes in T?Involve changes in T?

Phase 1, 3, 5Phase 1, 3, 5

Phase 1 Phase IIPhase 1 Phase II

Phase III Phase IVPhase III Phase IVPhase VPhase V

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The Heating CurveThe Heating Curve

Phase 1 Phase IIPhase 1 Phase II

Phase III Phase IVPhase III Phase IVPhase VPhase V

Specific Heat Specific Heat It’s in these phases that specific heat calculationsIt’s in these phases that specific heat calculationsare used to determine how much heat us neededare used to determine how much heat us neededto raise the temperatureto raise the temperatureof the sampleof the sample

But how do we findBut how do we findthe heat necessary tothe heat necessary toto fully melt or vaporizeto fully melt or vaporizea sample of mattera sample of matterIn regions In regions where T doeswhere T doesNot changeNot change (phase II & IV) (phase II & IV)

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Melting/Vaporization Melting/Vaporization Calculations Calculations

It’s even easier than specific heat calculations sinceIt’s even easier than specific heat calculations sinceTemperature is not a variable.Temperature is not a variable.

In phase II:In phase II:Q= m x HQ= m x Hff

m=total massm=total massHHff=heat of fusion=heat of fusion

In phase IV:In phase IV:Q=m x HQ=m x Hvv

The Heating CurveThe Heating Curve

Phase 1 Phase IIPhase 1 Phase II

Phase III Phase IVPhase III Phase IVPhase VPhase V

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Melting/Vaporization Melting/Vaporization Calculations Calculations

Problem 1: How much heat is necessary to melt 100g of Problem 1: How much heat is necessary to melt 100g of ice? ice?

Problem 2: How much heat is necessary to vaporize this Problem 2: How much heat is necessary to vaporize this water?water?

Q = m x HQ = m x Hff

Q = 100g x 334 J/gQ = 100g x 334 J/gQ = 33,400 JQ = 33,400 J

Q = m x HQ = m x Hvv

Q = 100g x 2,260 J/gQ = 100g x 2,260 J/gQ = 226,000 JQ = 226,000 J

The Heating CurveThe Heating Curve

Phase 1 Phase IIPhase 1 Phase II

Phase III Phase IVPhase III Phase IVPhase VPhase V

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Melting/Vaporization Melting/Vaporization Calculations Calculations

Note that these calculations are true for all Note that these calculations are true for all sorts of matter, sorts of matter, not just waternot just water..

For example here are some For example here are some HHvv as a function of as a function of the intermolecular force: the intermolecular force:

HH22OO Hydrogen BondingHydrogen Bonding 2260 J/g2260 J/gNHNH33 Hydrogen BondingHydrogen Bonding 1276 J/g1276 J/gHH22SS Dipole-DipoleDipole-Dipole 553 J/g 553 J/gFF22 London DispersionLondon Dispersion 155 J/g 155 J/g

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Melting/Vaporization Melting/Vaporization Calculations Calculations

Problem: What is the heat of vaporization of a liquid if it takesProblem: What is the heat of vaporization of a liquid if it takes6500 J to totally vaporize 8g?6500 J to totally vaporize 8g?

Q = m x HQ = m x Hvv

6500 J = 8g x H6500 J = 8g x Hv v

HHvv = 6500 / 8 = 812.5 J/g = 6500 / 8 = 812.5 J/g

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Heat of Fusion and Heat of Fusion and Vaporization from heating Vaporization from heating

graphsgraphsIf heat is added at a constant rate over time to a specific If heat is added at a constant rate over time to a specific mass we can determine both Hmass we can determine both Hff and H and Hvv by noting both by noting both the initial onset of melting or vaporization and the the initial onset of melting or vaporization and the completion point.completion point.

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Heat of Fusion and Heat of Fusion and Vaporization from heating Vaporization from heating

graphsgraphsFor example in this graph the onset of vaporization For example in this graph the onset of vaporization occurs after 875 Joules have been added and is complete occurs after 875 Joules have been added and is complete after 3100 J have been added. Thereforeafter 3100 J have been added. ThereforeIt took 3100-875 KJ toIt took 3100-875 KJ toVaporize this sample.Vaporize this sample.

If the mass of the If the mass of the Sample is 300g then theSample is 300g then theHHvv = 2225KJ/300g = 2225KJ/300g =7.417 KJ/g=7.417 KJ/g

The same can be doneThe same can be doneTo calculate the HTo calculate the Hff

875 KJ 3,100 KJ

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Heat of Fusion and Vaporization Heat of Fusion and Vaporization from heating graphsfrom heating graphs

Time (min)1 2 3 4 5 10 11 12 13 14

Time = 3.8min Time = 14.5 min

If we add heat to a sample at a fixed rate, say 422.4 J/min, weIf we add heat to a sample at a fixed rate, say 422.4 J/min, weCan then calculate both HCan then calculate both Hff and H and Hvv if we know the mass if we know the mass Of the sample.Of the sample.

In this example the waterIn this example the waterstarts to boil at 3.8 minstarts to boil at 3.8 minAnd is complete at 14.5 minAnd is complete at 14.5 min

So we’ve added 422.4J/minSo we’ve added 422.4J/minFor 10.7min = 4520 JFor 10.7min = 4520 J

Since the mass is 2 gram then Since the mass is 2 gram then HHv. v. = 4520/2 = 2260 J= 4520/2 = 2260 J

(Mass = 2 gram)

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Cooling CurveCooling Curve

Since the sample is cooling it must be Since the sample is cooling it must be releasing heatreleasing heat..

As Temp decreases KE dec and as a sample goes from gas As Temp decreases KE dec and as a sample goes from gas To liquid to solid the PE must be decreasing.To liquid to solid the PE must be decreasing.

Instead of Heat of vaporization and heat of fusion we haveInstead of Heat of vaporization and heat of fusion we havethe the Heat of condensationHeat of condensation and the and the Heat of solidificationHeat of solidification. They. TheyAre equal to HAre equal to Hvv and H and Hff but are opposite in value but are opposite in value

-H-Hvv = H = Hcc & & -H-Hff = H = Hss

The opposite of a heating Curve The opposite of a heating Curve is ais a COOLING CURVE.COOLING CURVE.

Brrrrr…Brrrrr…

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Cooling CurveCooling Curve

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Gas & Liquid

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I II III IV V

Gas

Condensation

Solidification

Phase I: T decPhase I: T dec KE decKE dec PE constantPE constant

Phase II: T constantPhase II: T constant KE constantKE constant PE decPE dec

HHcc = -H = -Hvv

Phase III: T decPhase III: T dec KE decKE dec PE constantPE constant

Phase IV: T constantPhase IV: T constant KE constantKE constant PE decPE dec HHss = -H = -Hff

Phase V: same as I and IIIPhase V: same as I and III

Note: This is the mirror image of a heating curveNote: This is the mirror image of a heating curve

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Cooling Curve CalculationsCooling Curve Calculations

Problem: How much heat is released when 100gProblem: How much heat is released when 100gOf water solidifies? How much heat is released whenOf water solidifies? How much heat is released when100g of water condenses?100g of water condenses?

a)a)Q = m x HQ = m x Hs s (i.e. –H(i.e. –Hff ) )Q = 100g x Q = 100g x --334 J/g334 J/gQ = Q = --33,400 J (i.e. 33,400 joules are released)33,400 J (i.e. 33,400 joules are released)

b)b)Q = 100g x -2260 J/g = -226,000 JQ = 100g x -2260 J/g = -226,000 J

The negative sign means this much heat is released (and not absorbed)The negative sign means this much heat is released (and not absorbed)