Intersection 6: Thermochemistry

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Intersection 6 Thermochemistry
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• 10/10/05
• 6.4 p228-232 6.7 p240-242 6.10-6.11 p 248-256
• 11.3-11.4 p494-507

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A Review of Thermochemistry (from Studio)
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• Thermochemical Equations
• C(graphite) O2(g)? CO2(g)  ?H -393.509 kJ/mol
• Endothermic and exothermic
• Heats of reactions are additive (Hesss Law)
• Calorimetry
• How heats of reaction are measured
• Heat capacity q mCDT

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Problem 1
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• Suppose you burn 0.300 g C(graphite) in an excess
  of O2(g) to give CO2(g). The temperature of the
  calorimeter which contains 775 g of water
  increases from 25.00oC to 27.38oC.
• What is the quantity of thermal energy evolved
  per mole of graphite?

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Question 1
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• 100 mL of water at 25C and 100 mL of alcohol at
  25C are both heated at the same rate under
  identical conditions. After 3 minutes the
  temperature of the alcohol is 50C. Two minutes
  later the temperature of the water is 50C.
• Which liquid received more heat as it warmed to
  50C?
• The water.
• The alcohol.
• Both received the same amount of heat.
• It is impossible to tell from the information
  given.

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Do all substances have the same heat capacity?
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It takes 145 J to raise the temperature of 1 mole
of air by 5oC. Is the heat capacity of air
higher, lower, or the same as water?

• Explain why at the beginning of the summer, the
  air is really warm, but the water at the beach is
  not.

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A
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Thermochemistry (in IS)
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• Heat and bonds
• Breaking bonds takes energy
• Making bonds releases energy
• Trends in bond enthalpy
• Calculate Heats of Reactions
• Heats of reaction can be calculated using bond
  enthalpy
• Heat of reactions can be calculated using heats
  of formation
• Change of state and heat energy

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Bonds
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• Why do bonds form?

Does it take energy or give off energy to break a
bond?
Picture from www.nios.ac.in/ sc10/ch5sc10.htm
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• The energy given off to make H-H bonds is shown
  below.
• H(g) H(g) ? H2(g)     ? H -435 kJ/mol   
•      
• The opposite process, breaking H-H bonds requires
  the input of the same amount of energy that was
  given off in making the bond  
• H2(g) ? H(g) H(g)      ? H 435 kJ/mol   
• Which process is endothermic? Exothermic?  

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Bond Enthalpy (kJ/mol) (average energy needed to
break bonds)
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H-F 566 CC 598 C-O 336
H-Cl 431 CN 418 CO 695
H-Br 366 CO 695-803 CO 1073
H-I 299
C-C 356
C-Si 301 C-C 356 CC 598
C-P 264 C-N 285 CC 813
C-S 272 C-O 336
C-Cl 327 C-F 486
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Energy terms
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• Heat energy enthalpy, DH, q
• Enthalpy (H) is the quantity of thermal energy
  transferred into a system at constant pressure. 
•  ? E ? H wexpansion
• "?H accounts for all the energy transferred
  except the quantity that does the work of pushing
  back the atmosphere, which is usually relatively
  small.  Whenever heat transfer is measured at
  constant pressure, it is ?H that is determined."
  (Moore, p232)  

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Holy Contradiction!
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• It takes energy to break bonds.and yet we get
  energy from fats (and carbohydrates and alcohol)
  by breaking them down ???

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How do we get energy from food?
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• Food is broken down by adding O2 and converting
  all of the carbon and hydrogen in the food
  molecules to CO2 and H2O.

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Energy from Ethanol
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• CH3CH2OH    O2 ?   CO2 H2O

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2
3
moles Bonds broken Enthalpies kJ/mol moles Bonds made Enthalpies kJ/mol
C-H 416 CO 803
C-C 356 O-H 467
C-O 336
O-H 467
OO 498
Total
5 1 1 1 3
4 6
4733 kJ
(-) 6014 kJ
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A Picture of Breaking Down Ethanol
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Total D H
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4733 kJ into reactants 6014 kJ out of products
? Hreaction S Hreactants - S Hproducts
-1281 kJ Watch your
signs!!!!!!!!!!.!!!!!
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Breaking Down Glucose
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6
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C6H12O6 ___O2 ____CO2 ___H2O
moles bonds kJ/mol moles bonds kJ/mol
C-H 416 CO 803
C-C 356 O-H 467
C-O 336
O-H 467
OO 498
Total
7 5 7 5 6
1212
12217 kJ
15240 kJ
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Inflaming Nitrocellulose
HNO3/H2SO4
Cellulose Oxidation
Nitrocelluose Oxidation
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DHreaction Comparison
Nitrocellulose Combustion Nitrocellulose Combustion Nitrocellulose Combustion Nitrocellulose Combustion Nitrocellulose Combustion Nitrocellulose Combustion Nitrocellulose Combustion
Reactants Reactants Reactants Products Products Products
Moles Bonds kJ/mol Moles Bonds kJ/mol
28 C-H 416 48 CO 803
20 C-C 356 28 O-H 467
28 C-O 336 6 NN 946
- O-H 467
9 OO 498
24 N-O 201
12 NO 607
Total 44,776 57,296
Cellulose Combustion Cellulose Combustion Cellulose Combustion Cellulose Combustion Cellulose Combustion Cellulose Combustion Cellulose Combustion
Reactants Reactants Reactants Products Products Products
Moles Bonds kJ/mol Moles Bonds kJ/mol
7 C-H 416 12 CO 803
5 C-C 356 10 O-H 467
7 C-O 336
3 O-H 467
6 OO 498
Total 11,433 14306
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Conversion to food Calories
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• 1 Calorie 1 kilocalorie 1000 calories
• 1 calorie 4.184 J
• Convert the enthalpies for glucose and ethanol to
  Cal and Cal/g

For glucose
3023 kJ 1000J 1 cal 1Cal
mol 1 kJ 4.184 J 1000 cal
722.5 Cal/mol
722.5 Cal 1 mol glucose
mol glucose 180 g
4.0 Cal/g
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Comparing Energy Sources
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Bond Energy Nutritional Guidelines
Ethanol 6.7 Cal/g 7 Cal/g
Glucose 4.0 Cal/g 4 Cal/g
Stearic Acid 8.9 Cal/g 9 Cal/g
C18H36O2 26 O2 18 CO2 18 H2O DH
-10622 kJ
Stearic acid
http//www.unlimited-health.com/nutrition/
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Evolution due to fat storage..
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• "Animals, though they store small amounts of
  glycogen in the muscles and liver, and rely on
  glucose in the bloodstream for immediate energy
  needs, bank most of their energy reserves in fat.
   Economy in weight is important for an organism
  that must work not only to move, but simply to
  stand up against the force of gravity.  The body
  of a typical woman is about 25 fat by weight (a
  man's body is closer to 15).  This means that if
  her fat supply were converted to its energy
  equivalent in carbohydrates, a 120-pound woman
  would weigh 150 pounds.  In a world where
  swiftness and agility increase an animal's
  chances for survival, fat is clearly the
  preferable means of energy storage. (Harold
  McGee, On Food and Cooking. Scribners, New York
   1984, p. 597.)

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Who wants to count up bonds every time?
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• ? Hof standard enthalpy of formation is the
  energy given off or needed to form a substance
  from its elements in their standard states ie,
  O2(g), H2(g), N2(g), Br2(l), Na(s)..etc.  
• The o stands for standard conditions of 1 atm and
  298K.   

The formation reaction of gaseous water would be
written H2(g)   1/2 O2(g)  ?  H2O(g)        ?
Hrxn (1 atm, 298K) ? Hof  H2O(g) -241.83
kJ/mol
? Hof found in Appendix J of Moore or back of
coursepack
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Table 6-2, p.250
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Standard Enthalpy of Formation
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• H2(g)   1/2 O2(g)  ?  H2O(g)     ? Hof H2O(g)
  -241.83 kJ/mol
• Do you get a different answer from bond
  enthalpies?
•  

moles bonds kJ/mol moles bonds kJ/mol
H-H 436 O-H 467
OO 498
Total
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DHof Signs and Multiplicity
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• C(graphite) O2(g)? CO2(g)   ?H
  -393.509 kJ/mol
• CO2(g) ? C(graphite) O2(g)   ?H
  393.509 kJ/mol
• 2C(graphite) 2 O2(g)? 2CO2(g)  
  ?H -787.018 kJ/mol
• 4CO2(g) ? 4C(graphite)
  4O2(g)   ?H 1574.04 kJ/mol

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• Calculate the DHrxn using heats of formation
• CH3CH2CH3 (g)    5 O2 (g)   ?   3 CO2 (g) 4
  H2O (g)

3C(s,graphite) 4H2(g) ? CH3CH2CH3 (g) DH -
103.82 kJ/mol
O2 (g) DH 0
kJ/mol
C(s,graphite) O2 (g) ? CO2 (g) DH
- 393.5 kJ/mol
H2(g) ½ O2(g) ? H2O(g) DH -
241.82 kJ/mol
CH3CH2CH3(g) ? 3C(s,graphite) 4H2(g) DH
103.82 kJ/mol
3C(s,graphite) 3O2 (g) ? 3CO2 (g)
3(DH - 393.5 kJ/mol)
4H2(g) 2 O2(g) ? 4H2O(g)
4(DH - 241.82 kJ/mol)
-2043.96 kJ/mol
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Calculate the DHrxn using heats of
formation CH3CH2CH3 (g)    5 O2 (g)   ?   3 CO2
(g) 4 H2O (g)
? Hrxn S nHof products - S nHof reactants
Form the products (keep sign) Unform the
reactants (change sign)
 S n? Hof products -393.5 kJ/mol (3 mol CO2)  
 -241.83 kJ/mol (4 mol H2O)
- 2147.82 kJ
 S n? Hof reactants -103.82 kJ/mol (1 mol
propane)    0 kJ/mol (5 mol O2)
-103.82 kJ
? Hrxn S nHof products - S nHof
reactants            -2147.82 kJ - (-103.82
kJ)          -2044 kJ
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Iron Oxidation
DHf for Fe2O3 -824.2 kJ/mol What is DHf for Fe
and O2??
? Hrxn S nHof products - S nHof reactants
? Hrxn S nHof products - S nHof
reactants            2(-824.2 kJ/mol) -
0          -1648 kJ/mol
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Problem 2
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• 2 C8H6(l) 25 O2(g) ?16 CO2(g) 18 H2O(g) ?H
  -10,992 kJ
• Write a thermochemical equation for
• The production of 4.00 mol CO2(g)
• The combustion of 100. mol C8H6
• How much heat energy is produced when 50 g of
  C8H6 is burned?

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Problem 3
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• The standard molar enthalpy of formation of
  AgCl(s) is 127.1 kJ/mol. Write a balanced
  thermochemical equation for which the enthalpy of
  reaction is 127.2 kJ

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Problem 4
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• The Romans used CaO as mortar in stone
  structures. The CaO was mixed with water to give
  Ca(OH)2, which slowly reacted with CO2 in the air
  to give limestone.
• Ca(OH)2(s) CO2(g) ? CaCO3(s) H2O(g)
• Calculate the enthalpy change for this reaction

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Question 2
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• Heat is given off when hydrogen burns in air
  according to the equation 2H2 O2  2H2O
• Which of the following is responsible for the
  heat?
• Breaking hydrogen bonds gives off energy.
• Breaking oxygen bonds gives off energy.
• Forming hydrogen-oxygen bonds gives off energy.
• Both (a) and (b) are responsible.
• (a), (b), and (c) are responsible.

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Debate _________is the safest and best
artificial sweetener
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• Friday, October 13th in studio
• Splenda (sucralose)
• Aspartame
• Saccharin
• Acesulfame K

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A
Chips Points Time
Opening 3 9 6 min
Rebuttal 3 5 3 min
Question 1 1 2 30 sec
Response 1 1 3 1 min
Question 2 1 2 30 sec
Response 2 1 3 1 min
Question 3 1 2 30 sec
Response 3 1 3 1 min
Question 4 1 2 30 sec
Response 4 1 3 1 min
Closing 3 6 3 min