Transcript Chapter 6

Chapter 6
Thermochemistry
Dr. S. M. Condren
Thermite Reaction
Dr. S. M. Condren
Thermite Reaction
Dr. S. M. Condren
Terminology
Energy
• capacity to do work
Kinetic Energy
• energy that something has because it is moving
Potential Energy
• energy that something has because of its position
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Kinetic Energy
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Chemical Potential Energy
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Chemical Potential Energy
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Internal Energy
• The sum of the individual energies of all
nanoscale particles (atoms, ions, or
molecules) in that sample.
• E = 1/2mc2
• The total internal energy of a sample of
matter depends on temperature, the type of
particles, and how many of them there are
in the sample.
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Energy Units
• calorie - energy required to heat 1-g of
water 1oC
• Calorie - unit of food energy; 1 Cal = 1-kcal
= 1000-cal
• Joule - 1-cal = 4.184 J = 1-kg*m2/sec2
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Law of Conservation of Energy
• energy can neither be created nor destroyed
• the total amount of energy in the universe is
a constant
• energy can be transformed from one form to
another
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First Law of Thermodynamics
• the amount of heat transferred into a
system plus the amount of work done
on the system must result in a
corresponding increase of internal
energy in the system
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Thermochemistry Terminology
system => that part of the universe under
investigation
surroundings => the rest of the universe
universe = system + surroundings
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Thermodynamic System
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Energy Transfer
Energy is always transferred from the hotter
to the cooler sample
Heat – the energy that flows into or out of a
system because of a difference in
temperature between the thermodynamic
system and its surroundings
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Thermochemistry Terminology
state properties => properties which depend
only on the initial and final states
=> properties which are path independent
non-state properties => properties which are
path dependent
state properties => E
non-state properties => q & w
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Thermochemistry Terminology
exothermic - reaction that gives off energy
endothermic - reaction that absorbs energy
chemical energy - energy associated with a
chemical reaction
thermochemistry - the quantitative study of
the heat changes accompanying chemical
reactions
thermodynamics - the study of energy and its
transformations
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Energy & Chemistry
2 H2(g) + O2(g) --> 2 H2O(g) + heat and light
This can be set up to provide
ELECTRIC ENERGY
in a fuel cell.
Oxidation:
2 H2 ---> 4 H+ + 4 eReduction:
4 e- + O2 + 2 H2O ---> 4 OHDr. S. M. Condren
Energy & Chemistry
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Enthalpy
• heat at constant pressure
qp = DH = Hproducts - Hreactants
Exothermic Reaction
DH = (Hproducts - Hreactants) < 0
H2O(l) -----> H2O(s)
DH < 0
Endothermic Reaction
DH = (Hproducts - Hreactants) > 0
H2O(l) -----> H2O(g)
DH > 0
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Enthalpy
H = E + PV
DH = DE + PDV
DE = DH – PDV
Where text uses U for internal energy
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Pressure-Volume Work
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First Law of Thermodynamics
heat => q
internal energy => E
internal energy change =>DE
work => w
DE = q - w
(Engineering convention)
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Specific Heat
• the amount of heat necessary to raise the
temperature of 1 gram of the substance 1oC
• independent of mass
• substance dependent
• s.h.
• Specific Heat of Water = 4.184 J/goC
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Heat
q = m * s.h. * Dt
where
q => heat, J
m => mass, g
s.h. => specific heat, J/g*oC
Dt = change in temperature, oC
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Molar Heat Capacity
• the heat necessary to raise the temperature
of one mole of substance by 1oC
• substance dependent
• C
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Heat Capacity
• the heat necessary to raise the temperature
1oC
• mass dependent
• substance dependent
• C
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Heat Capacity
C = m X s.h.
where
C => heat capacity, J/oC
m => mass, g
s.h. => specific heat, J/goC
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Plotted are graphs of
heat absorbed versus
temperature for two
systems. Which
system has the larger
heat capacity?
A, B
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Heat Transfer
qlost = - qgained
(m X s.h. X Dt)lost = - (m X s.h. X Dt)gained
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EXAMPLE If 100. g of iron at 100.0oC is
placed in 200. g of water at 20.0oC in an insulated
container, what will the temperature, oC, of the iron
and water when both are at the same temperature?
The specific heat of iron is 0.106 cal/goC.
(100.g*0.106cal/goC*(Tf - 100.)oC) = qlost
- qgained = (200.g*1.00cal/goC*(Tf - 20.0)oC)
10.6(Tf - 100.oC) = - 200.(Tf - 20.0oC)
10.6Tf - 1060oC = - 200.Tf + 4000oC
(10.6 + 200.)Tf = (1060 + 4000)oC
Tf = (5060/211.)oC = 24.0oC
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EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice
+ DHfusion
+ DHwater
+ DHboil.
+ DHsteam
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
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Heat Transfer
Dr. S. M. Condren
EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
{
q = (10.0g*2.09J/goC*((0.0 – (-15.0))oC))
Mass of the ice
specific heat of ice
Temperature change
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EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
q = (10.0g*2.09J/goC*15.0oC)
+ (10.0g*333J/g) Melting of ice occurs at a
constant temperature
Mass of ice Heat of fusion
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EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
q = (10.0g*2.09J/goC*15.0oC)
+ (10.0g*333J/g)
+ (10.0g*4.18J/goC*((100.0-0.00)oC))
Mass of water Specific heat of
liquid water
Temperature change
of the liquid water
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EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
q = (10.0g*2.09J/goC*15.0oC)
+ (10.0g*333J/g)
+ (10.0g*4.18J/goC*100.0oC)
+ (10.0g*2260J/g)
Boiling of water occurs at a
constant temperature
Mass of water Heat of vaporization
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EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
q = (10.0g*2.09J/goC*15.0oC)
+ (10.0g*333J/g)
+ (10.0g*4.18J/goC*100.0oC)
+ (10.0g*2260J/g)
+ (10.0g*2.03J/goC*((127.0-100.0)oC))
Mass of steam Specific heat
of steam
Temperature change for the steam
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EXAMPLE: How much heat is required to
heat 10.0 g of ice at -15.0oC to steam at
127.0oC?
q = DHice + DHfusion + DHwater + DHboil. + DHsteam
q = (10.0g*2.09J/goC*15.0oC)
+ (10.0g*333J/g)
+ (10.0g*4.18J/goC*100.0oC)
+ (10.0g*2260J/g)
+ (10.0g*2.03J/goC*27.0oC)
q = (314 + 3.33X103 + 4.18X103 + 2.26X104 + 548)J
= 30.96 kJ
Dr. S. M. Condren
Spreadsheet of Previous Problem
10
10
10
10
10
2.09
333
4.18
2260
2.03
15
1
100
1
27
313.5
3330
4180
22600
548.1
314
3330
4180
22600
548
31
333
418
2260
54
x10
x10
x10
x10
x10
3096 x10
30.96 x10^3
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Bomb Calorimeter
calorimeter
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EXAMPLE
A 1.000g sample of a particular compound
produced 11.0 kJ of heat. The temperature
of the calorimeter and 3000 g of water was
raised 0.629oC. How much heat is gained by
the calorimeter?
heat gained = - heat lost
heatcalorimeter + heatwater = heatreaction
heatcalorimeter = heatreaction - heatwater
Dr. S. M. Condren
EXAMPLE
A 1.000g sample of a particular compound
produced 11.0 kJ of heat. The temperature
of the calorimeter and 3000 g of water was
raised 0.629oC. How much heat is gained by
the calorimeter?
heatcalorimeter = heatreaction - heatwater
heat = 11.0 kJ - ((3.00kg)(0.629oC)(4.184kJ/kgoC))
= 3.1 kJ
Dr. S. M. Condren
Example
What is the mass of water equivalent of the
heat absorbed by the calorimeter?
#g = (3.1 kJ/0.629oC)(1.00kg*oC/4.184kJ)
= 6.5 x 102 g
Dr. S. M. Condren
Example
A 1.000 g sample of ethanol was burned in the
sealed bomb calorimeter described above.
The temperature of the water rose from
24.284oC to 26.225oC. Determine the heat
for the reaction.
m = (3000 + "647")g H2O
q = m X s.h. X Dt
= (3647g)(4.184J/goC)(1.941oC) = 29.61 kJ
Dr. S. M. Condren
When graphite is burned to yield CO2, 394 kJ of
energy are released per mole of C atoms burned.
When C60 is burned to yield CO2 approximately
435 kJ of energy is released per mole of carbon
atoms burned. Would the buckyball-to-graphite
conversion be exothermic or endothermic?
exothermic, endothermic
Dr. S. M. Condren
Laws of Thermochemistry
1.
The magnitude of DH is directly
proportional to the amount of reactant or
product.
s --> l
l --> g
DH => heat of fusion
DH => heat of vaporization
Dr. S. M. Condren
Laws of Thermochemistry
2. DH for a reaction is equal in magnitude but
opposite in sign to DH for the reverse
reaction.
H2O(l) -----> H2O(s)
DH < 0
H2O(s) -----> H2O(l)
DH > 0
Dr. S. M. Condren
Laws of Thermochemistry
3. The value of H for the reaction is the same
whether it occurs directly or in a series of
steps.
DHoverall = DH1 + DH2 + DH3 + · · ·
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Hess' Law
• a relation stating that the heat flow in a
reaction which is the sum of a series of
reactions is equal to the sum of the heat
flows in those reactions
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EXAMPLE
CH4(g) + 2 O2(g) -----> CO2(g) + 2 H2O(l)
CH4(g) -----> C(s) + 2 H2(g)
DH1
2 O2(g) -----> 2 O2(g)
DH2
C(s) + O2(g) -----> CO2(g)
DH3
2 H2(g) + O2(g) -----> 2 H2O(l)
DH4
--------------------------------------------CH4(g) + 2 O2(g) -----> CO2(g) + 2 H2O(l)
DHoverall = DH1 + DH2 + DH3 + DH4
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Standard Enthalpy of
Formation
the enthalpy associated with the formation of
a substance from its constituent elements
under standard state conditions
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Calculation of DH
o
DHo = Sc*DHfoproducts - Sc*DHforeactants
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Example
What is the value of DHrx for the reaction:
2 C6H6(l) + 15 O2(g) --> 12 CO2(g) + 6 H2O(g)
from Appendix J Text
C6H6(l) DHfo = + 49.0 kJ/mol
O2(g) DHfo = 0
CO2(g) DHfo = - 393.5
H2O(g) DHfo = - 241.8
D Hrx = [S c* D Hfo]product - [S c* D Hfo]reactants
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Example
What is the value of DHrx for the reaction:
2 C6H6(l) + 15 O2(g) --> 12 CO2(g) + 6 H2O(g)
from Appendix J Text
C6H6(l) DHfo = + 49.0 kJ/mol; O2(g) DHfo = 0
CO2(g) DHfo = - 393.5; H2O(g) DHfo = - 241.8
D Hrx = [S c* D Hfo]product - [S c* D Hfo]reactants
D Hrx = [12(- 393.5) + 6(- 241.8)]product
- [2(+ 49.0 ) + 15(0)]reactants kJ/mol
= - 6.271 x 103 kJ
Dr. S. M. Condren
Example
What is the value of DHrx for the reaction:
Fe2O3(s) + 2 Al(s) --> 2 Fe(l) + Al2O3(s)
from Appendix J Text
Fe2O3(s) DHfo = -825.5 kJ/mol; Al(s) DHfo = 0 kJ/mol
Al2O3(s) DHfo = - 1675.7 kJ/mol; Fe(l) DHfo = +12.4 kJ/mol
D Hrx = [S c* D Hfo]product - [S c* D Hfo]reactants
D Hrx = [1(- 1675.7) + 2(- 12.4)]product
- [1(-825.5 ) + 2(0)]reactants kJ/mol
= - 2.4764 x 103 kJ
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Thermite Reaction on Saturday
~150g Fe2O3 ~1 mol Fe2O3
~2.5x106J
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~2.5MJ
Fossil Fuels
coal
petroleum
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natural gas
Energy Sources
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Based on 1998 Data
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