Calculating Energy Changes at Phase Changes

Calculating Energy Calculating Energy Changes at Phase Changes at Phase

ChangesChanges

heating rate = 150 J/min

If the substance takes 4 minutes to melt, how much heat energy was used to melt it?150J/min x 4min = 600J

What is the melting point of this substance?

50˚C

100˚CThe boiling point?

Heat of FusionHeat of Fusion

= amount energy required to change 1 = amount energy required to change 1 gram gram

of pure substance from solid to liquid at of pure substance from solid to liquid at its mpits mp

Heat of Fusion = HHeat of Fusion = Hff = physical constant = physical constant

HHff for water = 334 J/g for water = 334 J/g

(Table B)(Table B)

How much heat is absorbed How much heat is absorbed when 10 grams of ice melts when 10 grams of ice melts at 0at 0ooC?C?

Heat absorbed = Heat absorbed =

mass of substance x heat of fusionmass of substance x heat of fusion of of substancesubstance

Q = mHQ = mHff = (10 g)(334 J/g) = 3340 J = (10 g)(334 J/g) = 3340 J

Where does this energy go?Where does this energy go? Particles must overcome forces of attraction to Particles must overcome forces of attraction to

move farther apart during phase change (s → move farther apart during phase change (s → l)l)

Heat of VaporizationHeat of Vaporization

= amount energy required to convert 1 = amount energy required to convert 1 gram gram

of pure substance from liquid to gas at of pure substance from liquid to gas at its bpits bp

Heat of vaporization = HHeat of vaporization = Hvv = physical = physical constantconstant

HHvv for water = 2260 J/g for water = 2260 J/g

How much energy does it How much energy does it take to vaporize 10 g of take to vaporize 10 g of

water?water? Q = mHQ = mHvv

Q = (10 g)(2260 J/g) = 22600 JQ = (10 g)(2260 J/g) = 22600 J

It takes a lot more energy to go It takes a lot more energy to go from liquid to gas (22,600J) than from liquid to gas (22,600J) than from solid to liquid (3,340J). from solid to liquid (3,340J). Why?Why?

H2O changing from solid to liquid requires 3,340J/g

H2O changing from liquid to gas requires 22,600J/g

• greater energy required to change from liquid to gas because particles are spreading farther apart!

Heats of fusion & Heats of fusion & vaporizationvaporization

Determined in calorimetry experimentsDetermined in calorimetry experiments

Tem

pera

ture

Time

Q = mHf

Q = mHv

Q = mCliquidTQ =

mCsolidTQ = mCgasT

3 equations for Q3 equations for Q

1. Q = mC1. Q = mCTT

2. Q = mH2. Q = mHff

3. Q = mH3. Q = mHvv

figure out which to use figure out which to use depends on section of heating depends on section of heating

curvecurve look for hints in word problemlook for hints in word problem

Q = mCQ = mCTT

Temperature Temperature changedchanged

T T ↑↑ T ↓T ↓ Initial Initial

temperaturetemperature Start Start

temperaturetemperature

Final Final temperaturetemperature

Ending Ending temperaturetemperature

From __ ˚C to __ From __ ˚C to __ ˚C ˚C

WaterWater

Q = mHQ = mHff

IceIce FreezingFreezing MeltingMelting Occurs at 0Occurs at 0C (for HC (for H22O)O) At constant temperatureAt constant temperature

Q = mHQ = mHvv

SteamSteam BoilingBoiling CondensationCondensation Occurs at 100Occurs at 100C (for HC (for H22O)O) At constant temperatureAt constant temperature