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HEAT
THERMODYNAMICSBy: Engr. Lyza Marie G. Luchico
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THERMODYMANICS
Thermodynamics is thestudy of the motion of heatenergy as it is transferredfrom the system to thesurrounding or from thesurrounding to the system.
The transfer of heat could be due toa physical change or a chemical
change.
There are three laws of chemicalthermodynamics.
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CHEMICAL THERMODYMANICS
The first law of thermodynamics:
Energy and matter can be neither created nor destroyed;only transformed from one form to another. The energy
and matter of the universe is constant.
The second law of thermodynamics:
In any spontaneous process there is always an increase inthe entropy of the universe. The entropy is increasing.
The third law of thermodynamics:The entropy of a perfect crystal at 0 K is zero.
There is no molecular motion at absolute 0 K.
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HEAT (AFORMOFENERGY)The energy that flows into or out of a system
because of a difference in temperaturebetween the thermodynamic system and its
surrounding.
Symbolized by Q".When heat is evolved by a system, energy
is lost and Q is negative (-).
When heat is absorbed by the system, theenergy is added and Q" is positive (+).
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MOLECULESANDMOTION
The motion of moleculesproduces heat
The more motion, the moreheat is generated
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INTERNALENERGY
It is the grand total of all energies
inside a substance.
1. Molecular KE
Random motion of the
molecules in a substance
2. Rotational KE
3. KE due to internal movements of atoms
within molecules4. PE due to forces between
molecules
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There will be a transfer of
energyfrom one substance
to another.
Transferof energy
Heat Flow
Heat Transfer
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Does it mean that the flow of energyis from a
substance with more internal energyto a
substance with less internal energy?
Which has more internal energy?
A bowl of warm water
or
A red-hot thumb tack
Heat Flow:
Tto
T
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HOWMUCHENERGYREQUIREDTOHEAT
OBJECT?
Heat (energy) flows because of
temperature difference
Bigger temperature difference bigger
heat flow
Less insulation give more heat flow
for the same temperature difference
Heat will not flow between two bodies
of the same temperature
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HEAT FLOW
Heat can flow in one of two directions:
ExothermicTo give off heat; energy is lost from the system: (-
Q)
Endothermic
To absorb heat; energy is added to the system:(+Q)
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IFTHEHEATTRANSFERINVOLVESACHEMICALREACTIONTHENQISCALLED:
HEAT OF REACTION
The heat energy (
H; enthalpy) required toreturn a system to the given temperature at thecompletion of the reaction.
Q = H at constant pressure
The heat of reaction can be specific to a reaction like:
HEAT OF COMBUSTIONThe quantity of heat energy given off when aspecified amount of substance burns inoxygen.
UNITS: kJ/mol (kilojoules per mole) or kcal/mol (kilocalories per mole)
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HEAT CAPACITY & SPECIFIC HEAT
HEAT CAPACITY: The quantity of heat neededto raise the temperature of a substance one
degree Celsius (or one Kelvin).
Q = Cp T
SPECIFIC HEAT: The quantity of heat required
to raise the temperature of one gram of a
substance by one degree Celsius (or one
Kelvin).
Q = C m T
Both Cp & c are chemical specific constants
found in the textbook or Handbooks.
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SOMETHINGSAREEASIERTOHEAT(SPECIFIC
HEATCAPACITY)
More water in the kettle needs longer time to boil
Alcohol needs less energy to heat it than water
Energy required (Q) proportional desired change intemperature (DT) x mass (m) of material Q = mc DT
c called the specific heat cwater= 4190 J/(kg K) - very difficult to heat
cice= 2000 J/(kg K)
cmercury= 138 J/(kg K) - very easy to heat cethanol= 2428 J/(kg K) - very easy to heat
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UNITS FORHEAT ENERGY
Heat energy is usually measured in either Joules,given by the unit (J), and kilojoules (kJ) or in
calories, written shorthand as (cal), and kilocalories
(kcal).
1 cal = 4.186 J
NOTE: This conversion correlates to thespecific heat of water which is 1 cal/g oC or
4.186 J/g oC.
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SIRJAMESJOULE
James Joule 1818-1889
Stirring water made itwarm Change in temperature
proportional to work done
Showing equivalence ofheat and energy
Also that electricalcurrent flow through aresistor gives heating
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For a weight watcher, a peanut
contains 10 Calories;But to a physicist, a peanut can
release 10,000 calorieswhen
burned or consumed.
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ctivity Calorie burned per pound per
minute
Badminton 0.044Stationary Bike 0.0227
Calisthenics 0.0265
Stair Machine 0.0529
Aerobics, general 0.0492
Yoga 0.0303
Jump Rope 0.074
Swimming, slow 0.058Walking, normal,
asphalt
0.036
Sitting Still 0.009
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The quantity of heat, Q, required to increase
the temperature of a mass, m, of a certainmaterial is proportional to the change in
temperature.
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The quantity of heat, Q, is also proportional
to the mass, m of the material.
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where:
Q is heat, J
m is mass, kgDT is the change in temperature, Co
c is the specific heat capacity of the
substance, J / (kgCo)
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SPECIFIC HEAT PROBLEMS
Determine the energy (inkJ) required to raise the
temperature of 100.0 g ofwater from 20.0 oC to 85.0oC?
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EQUILIBRIUM
+ =Hot black
coffee
Cold
milk
Warm white
coffee
Two objects of different temperature when
placed in contact will reach the same
temperature
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HEATTRANSFER= ENERGYTRANSFER
Energy measured in Joules but heat often
measured in Calories
One cal raises one gram of water from 14.5C to
15.5C 1 cal - 4.186J
Doing work on something usually makes it
hot
Splash in the bath and the water will get warm!
1st law of thermodynamics heat and work
are both forms of energy
LAW OF CONSERVATION OF
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LAW OF CONSERVATION OF
ENERGYThe law of conservation of energy (the
first law of thermodynamics), whenrelated to heat transfer between twoobjects, can be stated as:
The heat lost by the hot object = the heat
gained by the cold object
-Qhot= Qcold
-mh ch Th= mc cc Tcwhere T = Tfinal- Tinitial
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LAW OF CONSERVATION OF ENERGY
Assuming no heat is lost, what
mass of cold water at 0.00oC is
needed to cool 100.0 g of waterat 97.6oC to 12.0oC?
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SPECIFIC HEAT PROBLEMS
LAW OF CONSERVATION OF ENERGY
1. Determine the specific heat of anunknown metal that required 2.56 kcalof heat to raise the temperature of150.00 g of the metal from 15.0oC to
200.0
o
C?2. Calculate the specific heat of anunknown metal if a 92.00 g piece at100.0oC is dropped into 175.0 mL ofwater at 17.8 oC. The final temperatureof the mixture was 39.4oC.
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ORIGINAL
GROUP
1 yellow paper
[accessed on July28, 2014]
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PRACTICE PROBLEMS 21. Iron metal has a specific heat of 0.449 J/goC. How much heat
is transferred to a 5.00 g piece of iron, initially at 20.0 oC, when
it is placed in a beaker of boiling water at 1 atm?2. How many calories of energy are given off to lower the
temperature of 100.0 g of iron from 150.0 oC to 35.0 oC?
3. If 3.47 kJ were absorbed by 75.0 g H2O at 20.0oC, what would
be the final temperature of the water?
4. A 100. g sample of water at 25.3 oC was placed in a
calorimeter. 45.0 g of lead shots (at 100 oC) was added to the
calorimeter and the final temperature of the mixture was 34.4oC. What is the specific heat of lead?
5. A 17.9 g sample of unknown metal was heated to 48.31 oC. It
was then added to 28.05 g of water in an insulted cup. The
water temperature rose from 21.04 oC to 23.98oC. What is the
specific heat of the metal in J/goC?
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EXPLANATION
OFANSWERS
Write the solution on the
yellow paper as you explain.
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PRACTICE PROBLEMS 21. Iron metal has a specific heat of 0.449 J/goC. How much heat is transferred
to a 5.00 g piece of iron, initially at 20.0 oC, when it is placed in a beaker ofboiling water at 1 atm?
2. How many calories of energy are given off to lower the temperature of100.0 g of iron from 150.0 oC to 35.0 oC?
3. If 3.47 kJ were absorbed by 75.0 g H2O at 20.0oC, what would be the final
temperature of the water?
4. A 100. g sample of water at 25.3 oC was placed in a calorimeter. 45.0 g oflead shots (at 100 oC) was added to the calorimeter and the finaltemperature of the mixture was 34.4 oC. What is the specific heat of lead?
5. A 17.9 g sample of unknown metal was heated to 48.31 oC. It was thenadded to 28.05 g of water in an insulted cup. The water temperature rosefrom 21.04 oC to 23.98oC. What is the specific heat of the metal in J/goC?
0.792 J/goC
180. J
1.28 J/go
C
1.23 x 103cal
31.1 oC
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REFERENCES:
Padgett, Miles. 2013. Thermal Physics. Physics 9.
Available at [email protected]
[accessed on July 28, 2014]
No Author. 2013. 11 Heat. Available at
http://faculty.lacitycollege.edu[accessed on July 28,2014]
mailto:[email protected]://faculty.lacitycollege.edu/http://faculty.lacitycollege.edu/mailto:[email protected]8/10/2019 2 All About Heat (Students_ Copy)
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PHASE
CHANGEEngr. Lyza Marie G. Luchico
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PHASECHANGES(E.G. SOLIDTOLIQUID)
time
Tempe
rature
ice
water
steam
MP
BP
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ENERGYREQUIREDFORPHASECHANGE
Heat of fusion (Q), solid TOliquid
Q = mLf
(Lf is latent heat of fusion)Lf (water) = 334 x10
3J/kg
Lf (mercury) = 11.8 x103
J/kg
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ENERGYREQUIREDFORPHASECHANGE
Heat of vaporization (Q), liquid TO gas
Q = mLv(Lv is latent heat of vaporization)Lv (water) = 2256 x10
3J/kg
Lv (mercury) = 272 x103J/kg
Heat of sublimation (Q), solid TO gas
Q = mLs(Ls is latent heat of sublimation)
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EXAMPLE
A 20-g ice cube at 0 0C is
heated until 20.0 g has
become water at 1000
C and5.0 g has been converted to
steam. How much heat is
added to do this system?
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TRY ME (PROBLEM1)
What mass of steam [c = 2010
J/(kgCo)] initially at 130oC is
needed to warm 200-g of water [c= 4186 J/(kgCo)] in a 100-g glass
container [c = 837 J/(kgCo)] from
20o
C to 50o
C? The latent heat ofvaporization of steam is 2.26 x 106
J/kg
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PAIR SHARE
What are the similarities
between heat and God?
How about differences?
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Heat is being used for
some other purpose thanraising the temperature of
the ice-water mixture.
Heat is being used to melt
the ice by separatingthe
ice molecules from each
other.
There is a change in phasebecause the potential
energyof the ice molecules changes but its
average kinetic energy remains the same.
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Phaseit describes a state of matter ---
solid, liquid, gas or plasma
Phase Changea physical change fromone phase to another.
A phase change always require a transfer
of energy.
Energy-in-Transit Heat
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Fusion/Melting
Solidification/
Freezing
Evaporation
Condensation
Sublimation
Deposition
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The amount of heat that must be added or
removed depends on: Type of material
Nature of the phase change
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The amount of heat that must be supplied or
removed to change the phase of a mass of a
substance is called Latent Heat.
where:Q is heat, J
m is mass, kg
L is latent heat, J/kg
Solid & LiquidLatent Heat of Fusion, Lf
Liquid & GasLatent Heat of Vaporization, Lv
Solid & GasLatent Heat of Sublimation, Ls
+, heat gained
-, heat lost
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ENERGYREQUIREDFORPHASECHANGE
Heat of fusion (Q), solid TOliquid
Q = mLf
(Lf is latent heat of fusion)Lf (water) = 334 x10
3J/kg
Lf (mercury) = 11.8 x103
J/kg
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ENERGYREQUIREDFORPHASECHANGE
Heat of vaporization (Q), liquid TO gas
Q = mLv(Lv is latent heat of vaporization)Lv (water) = 2256 x10
3J/kg
Lv (mercury) = 272 x103J/kg
Heat of sublimation (Q), solid TO gas
Q = mLs(Ls is latent heat of sublimation)
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A plot of temperature versus energy added when 1.00 g of ice
initially at -30oC is converted to steam at 120oC.
cice= 2090 J/kg Co
csteam= 2010 J/kg Co
Lf= 3.33 x 105J/kg at 0 oC
Lv= 2.26 x 106
J/kg at 100o
C
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PROBLEMNO. 2A 7-kg glass bowl [c = 840 J/(kgCo)] contains 16-kg
punch at 25oC. Two-and-a-half kilograms of ice [c =
2090 J/(kgCo)] are added to the punch. The ice
has an initial temperature of -20oC. The punchmaybe treated as if it were water [c = 4186
J/(kgCo)] and that there is no heat flow from punch
bowl to its surroundings. The latent heat of fusion
for water is 3.33 x 105J/kg. When thermalequilibrium is reached, all ice has melted.
Determine the temperature at thermal equilibrium.
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ASSIGN P N 4
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ASSIGN: PROBLEMNO. 4 CROSSWISE
1. What mass of steam at 100oC must be added to
1.0-kg of ice at 0oC to yield liquid water at 20oC?
2. What mass of steam at 120oC must be added to
1.0-kg of ice at -3.0oC to yield liquid water at 20oC?
Substance Melting
Point(oC)
Lf
(J/kg)
Boiling
Point(oC)
LV
(J/kg)
H20 0 3.33 x 105 100 2.26 x 106
cice= 2090 J/kg Co
csteam= 2010 J/kg CocH20= 4186 J/kg C
o
PRACTICE SET (HEAT)
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PRACTICE SET (HEAT)1. A 250 g of water at 376.3 oC is mixed with 350.0 mL (D of
water is 1.00 g/mL) of water at 5.0 oC. Calculate the final
temperature of the mixture.2. If 40.0 kJ were absorbed by 500.0 g H2O at 10.0
oC, what
would be the final temperature of the water?
3. What mass of liquid water initially at 3oC is needed to
warm a 0.2-kg aluminum bar from -155oC to 0oC?4. A 1-kg solid mercury at its melting point of -39oC is placed in a
0.62-kg aluminum calorimeter with 0.4-kg water at 12.8oC.
Determine the latent heat of fusion of mercury if the resulting
equilibrium temperature of the mixed system is 5.06o
C5. What mass of steam (in g) at 100OC is needed to heat 500
g of water at 10OC in a 250-g aluminum container to a final
equilibrium temperature of 30OC?
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CONSTANTS
cice= 2090 J/kg Co
csteam= 2010 J/kg Co
cH20
= 4186 J/kg Co
cAl= 900 J/kg Co
cHg= 138 J/kg Co
Substance Melting
Point
(oC)
Lf
(J/kg)
Boiling
Point
(oC)
LV
(J/kg)
Al 660 3.97 x 105 2450 1.14 x 107
H20 0 3.33 x 105 100 2.26 x 106
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REFERENCES:
Padgett, Miles. 2013. Thermal Physics. Physics 9.
Available at [email protected]
[accessed on July 28, 2014]
No Author. 2013. 11 Heat. Available at
http://faculty.lacitycollege.edu[accessed on July 28,2014]
mailto:[email protected]://faculty.lacitycollege.edu/http://faculty.lacitycollege.edu/mailto:[email protected]