Transcript Slide 1

Chapter 17
Thermochemistry
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Energy Transformations
When fuel is burned in a car engine, chemical energy
is released and is used to do work.
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Measuring and Expressing Enthalpy Changes
• A burning match
releases heat to its
surroundings in all
directions.
• Chemical energy → …
Quick-write : Describe the
energy conversions for this
scenario.
How much heat does this exothermic rxn release???
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Energy Transformations
Thermochemistry:
study of heat changes (ΔH; change in heat
content) that occur during
 chem rxns and
 changes in physical states.
chemical potential energy (chemical energy)
• energy stored in the chem bonds of a sub
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Calorimetry
Calorimetry
 the precise measurement of the heat flow into
or out of a system for chemical (e.g. burning)
and physical processes (e.g. boiling, melting—
phase changes).
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Calorimetry
In calorimetry,
 heat released by the system =
heat absorbed by surroundings
 Conversely,
heat absorbed by a system = heat released by
surroundings
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Calorimetry
Calorimeter
• insulated
device used to
measure the
absorption or
release of heat
in chem or
physical
processes.
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Calorimetry
Enthalpy (H) of the system.
 The heat content of a system
 cannot be measured directly with
instruments.
 can measured enthalpy change (ΔH);
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Calorimetry
Calorimetry expts can be performed at a constant volume
using a bomb calorimeter.
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Thermochemical Equations
A thermochemical eqn
• chem eqn that includes the enthalpy change
(ΔH).
exothermic
In a chem eqn, the ΔH for the rxn can be written
as either a reactant or a product.
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Thermochemical Equations
• The heat of rxn is the ∆H.
Exothermic
Enthalpy change
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Thermochemical Equations
Exothermic Rxn
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Thermochemical Equations
Endothermic Rxn
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Thermochemical Equations
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Thermochemical Equations
• The (molar) heat of combustion is the heat of
rxn for the complete burning of 1 mole of a sub.
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Energy Transformations
Energy Transformations
 In what direction does heat flow?
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Exothermic and Endothermic Process
In an endothermic process,
• system gains heat as the surroundings cool
down.(+ve)
In an exothermic process,
• system loses heat as the surroundings heat up.
(-ve)
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Exothermic and Endothermic Process
A system
• part of the universe on which you focus your
attention.
The surroundings
• include everything else in the universe.
Law of conservation of energy
• states that in any chem or physical process,
energy is neither created nor destroyed.
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Exothermic and Endothermic Processes
An endothermic process is one that absorbs heat from
the surroundings.
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Exothermic and Endothermic Processes
An exothermic process is one that releases heat to its
surroundings.
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Units for Measuring Heat Flow
2 common units for heat flow
 calorie ; Calorie (food)
 joule.
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Specific Heat Capacity
The specific heat of a sub (pure)
• is the amt of heat it takes to raise the temp of 1 g of the
sub 1°C.
C 
q
J/(g∙°C)
mT
q: qty of heat (J)
m: mass of sub (g)
∆T: change in temperature (°C);
∆T = Tfinal – Tinitial
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Heat Capacity and Specific Heat
 Water releases a lot of heat as it cools.
 During freezing
weather,
farmers protect
citrus crops by
spraying them
with water.
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Specific Heat
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Heat Capacity and Specific Heat
Because it is
mostly water,
the filling of a
hot apple pie is
much more
likely to burn
your tongue
than the crust.
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Calculate specific heat of a metal
Problem 1: The temp of a 95.4-g piece of Cu
increases from 25°C to 48.0°C when the Cu absorbs
849 J of heat. What is the specific heat of Cu ?
C 
q
mT
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Calculate Amt of heat to …
Problem 2: How much heat is required to raise the
temp of 250.0g of Hg 52°C? (CHg = 0.14J/(g·°C).
C 
q
mT
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Latent Heat
Latent heat for phase change
• melting, freezing, condensation,
vaporization, involved heat changes.
• the temperature of the sub remain constant;
• only phase change.
• Heat of fusion (ΔHfus), heat of solidification
(ΔHsolid), heat of vaporization (ΔHvap), heat of
condensation (Δhcond)
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Latent Heats
Latent heat (for phase
change)
abbreviation
Sign of ΔH
Heat of fusion
ΔHfus
+ve
Heat of solidification
ΔHsolid
-ve
Heat of vaporization
ΔHvap
+ve
Heat of condensation
ΔHcond
-ve
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Latent Heat
Heat of fusion (ΔHfus)
 ΔH for melting (+ve; gain heat);
 e.g. ΔHfus of ice = 6.01 kJ/mol
 temp of sub remains constant
 e.g. ice at 0°C remains 0°C while melting
though it keeps on absorbing heat
 refer to graph in the next slide.
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Molar Heat of Fusion
ΔHfus of ice = 6.01 kJ/mol
Heat change (to
melt 1 mole of ice) =
qty of Heat
#(kJ)
moles of sub
Heat absorbed (+ve)
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Heats of Fusion and Solidification
Molar heat of fusion (∆Hfus)
 amt of heat absorbed by 1 mole of a solid sub
as it melts to a (l) at a constant temp.
 e.g. ΔHfus of ice = 6.01 kJ/mol
Molar heat of solidification (∆Hsolid)
 amt heat lost when 1 mole of a (l) solidifies at a
constant temp.
 e.g. ΔHsolid of water (l) = - 6.01 kJ/mol
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Heats of Vaporization and Condensation
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vaporizing
(e)
liquid water temp
increasing
(d)
(100°C)
Temp increasing
All ice 0°C
melting
Boiling water 100°C
(c)
(Liquid H2O + steam)
All liquid water 0°C
(b)
(0°C)
(Ice+water) 0°C
(a)
(Ice) temp increasing
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Quick-write
Why f.p. of water = 0°C and m.p. of ice is also
0°C?
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Latent Heat
Heat of fusion (ΔHfus)
 melting
 ΔH is +ve
Heat of solidification (ΔHsolid)
 freezing
 ΔH is -ve
Heat of vaporization
(ΔHvap)
 ΔH is +ve
Heat of condensation
(ΔHcond)
ΔH is -ve
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Heats of Fusion and Solidification
The qty of heat absorbed by a melting (s) is
exactly the same as the qty of heat released
when the (l) solidifies
∆Hfus = –∆Hsolid
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Heats of Vaporization and Condensation
How does the qty of heat absorbed by a vaporizing
(l) compare to the qty of heat released when the
vapor condenses?
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Heats of Vaporization and Condensation
Molar heat of vaporization (∆Hvap).
amt of heat necessary to vaporize 1 mole of a
given (l) at constant temp.
Molar heat of condensation (∆Hcond).
 amt of heat released when 1 mol of vapor
condenses at the normal b.p.
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Heats of Vaporization and Condensation
The qty of heat absorbed by a vaporizing (l) is
exactly the same as the qty of heat released
when the vapor condenses;
∆Hvap = –∆Hcond
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Heats of Vaporization and Condensation
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Heats of Vaporization and Condensation
ΔH accompany changes in state.
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Heat of Soln
Heat of Solution
 During the formation of a soln, heat is either
released or absorbed.
Molar heat of soln (∆Hsoln)
 The enthalpy change caused by dissolution
of 1 mole of sub.
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Heat of Solution
When NH4NO3 crystals and water mix inside the cold
pack, heat is absorbed as the crystals dissolve.
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Problem 3: The graph below shows a pure sub which
is heated by a constant source of heat supplying
2000.0 j/min. Identify the area described in the
questions below and complete the necessary
calculations.
UV = 0.36 min, VW = 3.6 min, WX = 3.6 min, XY
= 19.4 min, YZ = 0.6 min
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a. being warmed as a solid ___________
b. being warmed as a liquid __________
c. being warmed as a gas ____________
d. changing from a solid to a liquid _____
e. changing from a liquid to a gas ______
f. What is its boiling temp? _________________
g. What is its melting temp? _________________
h. How many joules were needed to change the
liquid to a gas? ____________
i. Where on the curve do the molecules have the
highest KE? ______
j. If the sample weighs 10.0 g, what is its heat of
vaporization in J/g? ______
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Example 4
How many g of ice at 0°C could be melted by the
addition of 5.75 kJ of heat? (ΔHfus of ice = 6.01
kJ/mol)
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CST problem 1
The specific heat of Cu is about 0.4 J/(g·°C). How
much heat is needed to change the temperature of a
30-g sample of Cu from 20.0°C to 60.0°C?
A 1000 J
B 720 J
C 480 J
D 240 J
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CST problem 2
Which of these is an example of an exothermic
chemical process?
A
B
C
D
evaporation of water
melting ice
photosynthesis of glucose
combustion of gasoline
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The End
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Example 5
How much energy is required to convert 300.0 g of
water at 60°C completely to steam at 100.0 °C?
(Given: ΔHvap of water = 0.9 kJ/mol;
Cwater = 4.18 J/g· °C))
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Calculating the Enthalpy Change in Solution
Formation
How much heat (in kJ) is released when 2.500 mol
NaOH(s) is dissolved in water?
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How many moles of NH4NO3(s) must be dissolved in
water so that 88.0 kJ of heat is absorbed from the
water?
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1. The molar heat of condensation of a
substance is the same, in magnitude, as its
molar heat of
A. formation.
B. fusion.
C. solidification.
D. vaporization.
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2. The heat of condensation of ethanol (C2H5OH)
is 43.5 kJ/mol. As C2H5OH condenses, the
temperature of the surroundings
A. stays the same.
B. may increase or decrease.
C. increases.
D. decreases.
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3. Calculate the amt of heat absorbed to
liquefy 15.0 g of methanol (CH3OH) at its
m.p. The molar ∆Hfus for methanol is 3.16
kJ/mol.
A. 1.48 kJ
B. 47.4 kJ
C. 1.52  103 kJ
D. 4.75 kJ
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4. How much heat (in kJ) is released when 50 g
of NH4NO3(s), 0.510 moles, are dissolved in
water? Hsoln = 25.7 kJ/mol
A. 12.85 kJ
B. 13.1 kJ
C. 25.7 kJ
D. 1285 kJ
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