Thermochemistry

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Thermochemistry

• Chapter 6

• Outline
• Energy
• Types
• Law of Conservation of Energy
• First Law of Thermodynamics
• State Functions
• Work
• Enthalpy
• Specific Heat
• Calorimetry
• Hess Law

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Kinetic Potential Energy
3
What are the manifestations of energy?
Tro Chemistry A Molecular Approach, 2/e
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What is a system?
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Sign Conventions
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How does the internal energy change?

• - DEsystem DEsurroundings

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How does the internal energy change?

• DEsystem - DEsurroundings

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What is a state function?
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State Function
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How does the Law of Conservation of Energy apply
to an Ecosystem?
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How does a chemical reaction do work?
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How does a chemical reaction do work?
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Chapter 6 Examples Work

• Calculate the work associated with the expansion
  of a gas from 46 L to 64 L at a constant external
  pressure of 15 atm.

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Chapter 6 Examples Energy

• A 100 W electric heater (1 W 1 J/s) operates
  for 20 minutes to heat a gas cylinder. The gas
  expands from 2.04 L to 2.54 L against an
  atmospheric pressure of 1.0 atm. What is the
  change in internal energy of the gas?

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What is the sign of the enthalpy change?
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What is the sign of the enthalpy change?
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Where does the heat flow?

• Enthalpy transferred out of reactants ?
• exothermic ?
• ?H ?

• Enthalpy transferred into products ?
• endothermic ?
• ?H

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How are the enthalpy of a forward and a reverse
reaction related?

• H2O(g) ?? H2(g) 1/2 O2(g) ?H 241.8 kJ
• H2(g) 1/2 O2(g) ?? H2O(g) ?H ?241.8 kJ
• ?Hforward ??Hreverse (For reversible
  reactions)

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How does the amount of substance undergoing
change affect the enthalpy?

• H2O(g) ?? H2(g) 1/2 O2(g) ?H 241.8 kJ
• 2 H2O(g) ?? 2 H2(g) 1 O2(g) ?H 483.6 kJ

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Does the physical state of reactions and products
affect the enthalpy?

• H2O(g) ?? H2(g) 1/2 O2(g) ?H 241.8 kJ
• H2O(l) ?? H2(g) 1/2 O2(g) ?H 285.8 kJ

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Chapter 6 Examples Energy Stoichiometry

• If 5,449 kJ of energy is released when gaseous
  water is decomposed into its elements, how many
  grams of water was used?
• H2O (g) ? H2 (g) ½ O2 (g) ?H 241.8 kJ

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Chapter 6 Examples Energy Stoichiometry

• When 2.00 moles of sulfur dioxide gas reacts
  completely with 1.00 moles of oxygen gas to form
  2.00 moles of sulfur trioxide gas at 25 C and a
  constant pressure of 1.00 atm, 198 kJ of energy
  is released as heat. Calculate ?H and ?E for this
  process.

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Chapter 6 Examples Energy Stoichiometry

• Solid sulfur, S, reacts with carbon dioxide gas
  to produce sulfur dioxide gas and carbon solid,
  ?H - 75.8 kJ. If 12.9 g of sulfur react with
  9.70 g of carbon dioxide, how many kJ of energy
  are released or absorbed?

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Calorimetry constant pressure
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Chapter 6 Examples Calorimetry

1. A reaction known to release 1.78 kJ of heat takes
   place in a calorimeter containing 0.100 L of
   solution. The temperature rose by 3.65 C.
2. Next, 50 mL of hydrochloric acid and 50 mL of
   aqueous sodium hydroxide were mixed in the same
   calorimeter and the temperature rose by 1.26 C.
   What is the heat output of the neutralization
   reaction?

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Chapter 6 Examples Calorimetry

• A 55.0 g piece of metal was heated in boiling
  water to 99.8 C and dropped into an insulated
  beaker with 225 mL of water (? 0.997992 g/mL)
  at 21.0 C. The final temperature of the metal
  and the water is 23.1 C and cwater is 4.184 J/g
  C. Calculate the specific heat of the metal
  assuming that no heat was lost to the
  surroundings.

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Chapter 6 Examples Calorimetry

• A 33.14 g sample of copper (cCu 0.385 J/g
  C) and aluminum (cAl 0.902 J/g C) was
  heated to 119.25 C and dropped into a
  calorimeter containing 250.0 g of water at 21.00
  C. The temperature rose to 23.05 C. Assuming
  that no heat was lost to the surroundings, what
  is the mass of copper in the sample?

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Bomb Calorimeter constant volume
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Chapter 6 Examples Calorimetry

• Octane, C8H18, a primary constituent of gasoline
  burns in air. Suppose that a 1.00 g sample of
  octane is burned in a calorimeter that contains
  1.20 kg of water. The temperature of the water
  and the bomb rises from 25.00 C to 33.20 C. If
  the heat capacity of the bomb is 837 J/C,
  cwater is 4.184 J/g C, calculate the molar
  heat of reaction of octane.

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Chapter 6 Examples Hess Law

• Determine the ?Hsublimation of ice to water
  vapor
• H2O (s)? H2O (g)
• Given H2O (s)? H2O (l) ?H 6.02 kJ
• H2O (g)? H2O (l) ?H -40.7 kJ

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Chapter 6 Examples Hess Law

• Two forms of carbon are graphite, the soft,
  black, slippery material used in lead pencils,
  and as a lubricant for locks and diamond, the
  beautiful, hard gemstone.
• Using the enthalpies of combustion for graphite
  (-394 kJ/mol) and diamond (-396 kJ/mol),
  calculate ?H for the conversion of graphite to
  diamond
• C graphite (s)? C diamond (s)
• Given C graphite (s) O2 (g) ? CO2 (g) ?H
  -394 kJ
• C diamond (s) O2 (g) ? CO2 (g) ?H -396
  kJ

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Chapter 6 Examples Hess Law

• Calculate the enthalpy change of the formation
  of methane, CH4, from solid carbon as graphite
  and hydrogen gas
• C(s) 2 H2 (g) ? CH4 (g)
• Given
• C(s) O2 (g) ? CO2 (g) ?H -393.5 kJ
• H2 (g) ½ O2 (g) ? H2O (l)
  ?H -285.8 kJ
• CH4 (g) 2 O2 (g) ? CO2 (g) 2 H2 O (l) ?H
  -890.3 kJ
  
• 
  
  
  

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Chapter 6 Examples Hess Law

• Calculate the enthalpy change for
• S (s) O2 (g) ? SO2 (g)
  
  
• Given
• 2 SO2 (g) O2 (g) ? 2 SO3 (g) ?H -196 kJ
• 2 S (s) 3 O2 (g) ? 2 SO3 (g) ?H -790 kJ
  
  
  

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CH4(g) 2 O2(g)? CO2(g) H2O(g)
DH (DHf CO2(g) 2DHfH2O(g))- (DHf CH4(g)
2DHfO2(g))
CH4(g) ? C(s, graphite) 2 H2(g) D?H 74.6
kJ
C(s, graphite) 2 H2(g) ? CH4(g) DHf - 74.6
kJ/mol CH4
DH ((-393.5 kJ) 2(-241.8 kJ)- ((-74.6 kJ)
2(0 kJ)) -802.5 kJ
C(s, graphite) O2(g) ? CO2(g) DHf
-393.5 kJ/mol CO2
2 H2(g) O2(g) ? 2 H2O(g) DH -483.6 kJ
H2(g) ½ O2(g) ? H2O(g) DHf -241.8
kJ/mol H2O
CH4(g) 2 O2(g) ? CO2(g) 2 H2O(g) DH
-802.5 kJ
Tro Chemistry A Molecular Approach, 2/e
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Chapter 6 Examples Hess Law

• Calculate ?Hf for the combustion of methane
• CH4 (g) O2 (g) ? CO2 (g) 2 H2O (l)
  
  

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Chapter 6 Examples Hess Law

• Benzene, C6H6, is an important hydrocarbon.
  Calculate its enthalpy of combustion (?H)
• C6H6 (l) O2 (g) ? 6 CO2 (g) 3 H2O (l)
• Given ?Hf C6H6 (l) 49.0 kJ/mol
• ?Hf CO2 (g) -393.5 kJ/mol
• ?Hf H2O (l) -285.8 kJ/mol
• ?Hf O2 (g) 0 kJ/mol

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Chapter 6 Examples Hess Law

• Benzene, C6H6, is an important hydrocarbon.
  Calculate its enthalpy of combustion (?H)
• C6H6 (l) O2 (g) ? 6 CO2 (g) 3 H2O (l)
• Given
• 6 C (s) 3 H2 (g) ? C6H6 (l) ?H 49.0 kJ
• C (s) O2 (g) ? CO2 (g) ?H -393.5
  kJ
• H2 (g) ½ O2 (g) ? H2O (l) ?H -285.8 kJ

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Chapter 6 Examples Hess Law

• Calculate the enthalpy of formation of the
  fermentation of glucose to ethanol
• C6H12O6 (s) ? 2 C2H5OH (l) 2 CO2 (g)
• Given ?Hf C6H12O6 (s) -1,260 kJ/mol
• ?Hf CO2 (g) -393.5 kJ/mol
• ?Hf C2H5OH (l) -277.7 kJ/mol

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Chapter 6 Examples Hess Law

• Calculate ?Hf (in kilojoules) for the synthesis
  of lime (calcium oxide) from limestone (calcium
  carbonate), an important step in the manufacture
  of cement.
• CaCO3 (s) ? CaO (s) CO2 (g)
  
  
• Given ?Hf CaCO3 (s) -1,206.9 kJ/mol
• ?Hf CaO (s) -635.1 kJ/mol
• ?Hf CO2 (g) -393.5 kJ/mol

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Chapter 6 Examples Hess Law

• Nitroglycerin is a powerful explosive, giving
  four different gases when detonated
• 2 C3H5(NO3)3 (l) 3 N2 (g) ½ O2 (g) 6
  CO2 (g) 5 H2O (g)
  
• Given ?Hf C3H5(NO3)3 (l) -364 kJ/mol
• ?Hf CO2 (g) -393.5 kJ/mol
• ?Hf H2O (g) -241.8 kJ/mol
• ?Hf O2 (g) 0 kJ/mol
• ?Hf N2 (g) 0 kJ/mol
• Calculate the energy liberated when 7.00 g is
  detonated.

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Chapter 9 Examples Bond Energy

• Approximate the ?Hrxn for the production of
  ammonia by the Haber process
• N2 (g) 3 H2 (g) ? 2 NH3 (g)

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Chapter 9 Examples Bond Energy

• Approximate the ?Hrxn for the combustion of
  methane
• CH4 (g) 2 O2 (g) ? CO2 (g) 2 H2O (g)

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Chapter 9 Examples Bond Energy

• Approximate the ?Hrxn for the halogenation of
  acetylene gas
• C2H2 (g) 2 Cl2 (g) ? C2H2Cl4 (g)