Energy Changes & Phase Changes

It takes It takes energyenergy to heat to heat stuff up!stuff up!

For pure substance in For pure substance in single phasesingle phase, , cancan calculate how much using Q = calculate how much using Q = mCmCT T Q = energy in JoulesQ = energy in Joulesm = mass in gramsm = mass in gramsC = specific heat capacityC = specific heat capacityT = change in temperature = TT = change in temperature = Tff - -

TTii

On the other hand, when something On the other hand, when something cools down, energy is released!cools down, energy is released!

Q = mCQ = mCTT C = specific heat capacity = C = specific heat capacity =

amount heat required to amount heat required to raise temperature of 1 gram raise temperature of 1 gram of pure substance by 1of pure substance by 1C C

C is physical constant C is physical constant unique for every pure unique for every pure

substancesubstance CCH2OH2O = 4.18 J/g = 4.18 J/gC

Heat Flow: hot to coldHeat Flow: hot to cold

SysteSystem1 m1

phase phase

Environment

But what about phase But what about phase changes?changes?

Sometimes more than one phase Sometimes more than one phase of substance is presentof substance is present

Example: melting ice Example: melting ice both liquid water and solid ice are both liquid water and solid ice are

presentpresent temperature is constant, so temperature is constant, so T = T =

0, even though beaker ice water 0, even though beaker ice water is absorbing heat is absorbing heat

All chemical & physical All chemical & physical changes are accompanied changes are accompanied by energy changes.by energy changes. Phase changes are physical Phase changes are physical

changeschangessometimes energy is absorbed sometimes energy is absorbed sometimes energy is releasedsometimes energy is released

energy changeenergy change for given for given phase phase changechange can be can be measured or measured or calculatedcalculated

What are 6 possible phase changes and their names?

Potential EnergyPotential Energy Energy of relative positionEnergy of relative position

Molecules always attracted to one Molecules always attracted to one anotheranother

need to put energy into system to pull need to put energy into system to pull molecules apart from one anothermolecules apart from one another

farther apart molecules are, higher their farther apart molecules are, higher their PE isPE is

Melting IceMelting Ice As beaker of ice water sits on hot As beaker of ice water sits on hot

plate, ice is melting plate, ice is melting system absorbing heat from hot system absorbing heat from hot

plate and using it for phase changeplate and using it for phase change temperature is constant:temperature is constant:

heat energy from hot plate going into heat energy from hot plate going into phase change or PE of systemphase change or PE of system

heat energy heat energy notnot going to KE going to KE

Identify a phase change as Identify a phase change as endothermic or exothermic.endothermic or exothermic.

PE

Solid

Liquid

GasEndothermic

Exothermic

FusionVaporizationSublimation

CondensationFreezingDeposition

Heating & Cooling Heating & Cooling CurvesCurves

way to investigate energy way to investigate energy changeschanges

Measure temperature as Measure temperature as function of time at constant function of time at constant heating or cooling rateheating or cooling rate

Time

Tem

pera

tur

eI II III IV V

Solid

Solid & Liquid

Liquid

Liquid & Gas

Gas

K.E. K.E. K.E.

K.E.↔K.E.↔P.E.↔

P.E.↔P.E.↔

P.E. P.E.Melt pt.

Boil pt.

Melting & Boiling PointsMelting & Boiling Points PPlateaus = lateaus = PPhase changes = hase changes = PPE E

changeschanges

as long asas long as 2 phases are present, 2 phases are present, temptemperature is erature is constantconstant

Melting Melting pointpoint, Boiling, Boiling pointpoint specific temperature value – not specific temperature value – not

rangerange

What happens to the What happens to the temperature as heat is temperature as heat is

added at the boiling point?added at the boiling point?

Nothing, until only 1 phase Nothing, until only 1 phase present!present!

To analyze a To analyze a heating/cooling curve:heating/cooling curve:

Does curve go uphill or downhill?Does curve go uphill or downhill? Label phase(s) present in each regionLabel phase(s) present in each region Describe what happens to KE in each Describe what happens to KE in each

regionregion Describe what happens to PE in each Describe what happens to PE in each

regionregion Locate melting point and boiling pointLocate melting point and boiling point

Heating curve of H2O.

The heating rate is 150 J/min

If substance takes 4 minutes to melt, how much heat energy used to melt it?

What is melting point of substance?The boiling point?

Heat of FusionHeat of Fusion Amount energy required to change Amount energy required to change

1 gram of pure substance from 1 gram of pure substance from solid to liquid at its melting pointsolid to liquid at its melting point

Heat of Fusion = HHeat of Fusion = Hff = physical = physical constantconstant

HHff for water = 333.6 J/g (Table B) for water = 333.6 J/g (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 substance x heat of fusionmass 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 energy go?Where does energy go? Particles use it to overcome forces Particles use it to overcome forces

of attraction to move farther apartof attraction to move farther apart

Heat of VaporizationHeat of Vaporization Amount energy required to convert Amount energy required to convert

1 gram of pure substance from 1 gram of pure substance from liquid to gas at its boiling pointliquid to gas at its boiling point

Heat of vaporization = HHeat of vaporization = Hvv = = physical constantphysical constant

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 lot more energy to go It takes lot more energy to go from liquid to gas than from from liquid to gas than from solid to liquid. Why?solid to liquid. Why?

The particles are spreading out a lot more!

Heats of fusion & Heats of fusion & vaporizationvaporization

Determined in calorimetry experimentsDetermined in calorimetry experiments

Tem

pera

ture

Time

Q = mHf

Q = mHv

Q = mClTQ =

mCsT

Q = mCgT

3 equations for Q3 equations for Q Q = mCQ = mCTT Q = mHQ = mHff Q = mHQ = mHvv figure out which one to use for figure out which one to use for

given problemgiven problem depends on section of heating depends on section of heating

curvecurve look for hints in problemlook for hints in problem

Q = mCQ = mCTT Temperature Temperature

changedchanged Temperature Temperature

increasedincreased Temperature Temperature

decreaseddecreased Initial Initial

temperaturetemperature Start Start

temperaturetemperature

Final Final temperaturetemperature

Ending Ending temperaturetemperature

From ____ to ____From ____ to ____ WaterWater

Q = mHQ = mHff

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

Q = mHQ = mHvv

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