Chapter 16: Thermodynamics

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Chapter 16 Thermodynamics

• Kinetics vs. Thermodynamics (speed vs.
  spontaneity)
• First Law of Thermodynamics (Review from Ch. 6)
• DE, q, w, DH, qp, qv
• Hess Law
• Entropy and the Second Law of Thermodynamics
• Definition of Entropy - Microstates
• Spontaneity and DS of the Universe
• Predicting and Calculating DS
• Predicting and Calculating DSsurr
• Free Energy, DG
• Predicting Spontaneity
• DG vs. DG
• Equilibrium
• Work

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Example 1

• Use Hess' Law and standard enthalpies of
  formation to
• determine DH for the following reaction.
• 4 NH3 (g)    5 O2 (g)  ?  4 NO (g)    6 H2O
  (g)
• Given

DHf(kJ/mol)
NH3 (g) -46
O2 (g) 0
NO (g)  90
H2O (g) -242
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Example 2

• The enthalpy of combustion of methane gas is
• -891 kJ/mole. Calculate DSsurr for the
  combustion of
• 1 mole of methane at 500C.

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Example 3

• Which of the following would you expect
• to be spontaneous at constant T and P?
• C12H22O11(s) ? 12 C(s) 11 H2O(g) exothermic
• N2(g) 3 H2(g) ? 2 NH3 (g) DH -91.8 kJ
• NaCl(s) ? Na(aq) Cl-(aq) DH 3.98 kJ

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Example 4

• Calculate DS, DSsurr, and DSuniv for
• 2 H2(g) O2(g) ? 2 H2O(l) at 25C.

DHf (kJ/mol) S (J/mol K)
H2 (g) 0 131
O2 (g) 0 205
H2O (l) -286 70.
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Example 5

• Find DG at 25C for
• 2 SO2 (g) O2 (g) ? 2 SO3 (g)
• Given

DHf (kJ/mol) S (J/K mol)
SO2 (g) -297 248
SO3 (g) -396 257
O2 (g) 0 205
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Example 6

• Find DG at 25C for
• 2 CO (g) O2 (g) ? 2 CO2 (g)
• Given
• 2 CH4(g) 3 O2 (g) ? 2 CO(g) 4 H2O(g)
• DG -1088 kJ
• CH4(g) 2 O2 (g) ? CO2(g) 2 H2O(g)
• DG -801 kJ

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Example 7

• Find DG at 25C for
• 2 CH3OH(g) 3 O2(g) ? 2 CO2(g) 4 H2O (g)
• Given

DGf (kJ/mol)
CH3OH(g) -163
O2(g) 0
CO2(g) -394
H2O (g) -229
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Example 8

• Estimate the temperature at which HgO
• decomposes to elemental mercury and O2 gas
• at 1 atm pressure.
• 2 HgO (s) ? 2 Hg (l) O2(g)
• For this process
• DH 304.2 kJ
• DS 414.2 J/K

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Example 9

• How much work can be obtained from the
• reaction of 10.0 g of hydrogen gas with 10.0 g
• of oxygen gas to produce liquid water? Assume
• the reaction occurs under standard conditions
• and 25C.
• 2 H2 (g) O2 (g) ? 2 H2O (l)
• For this process,
• DH -571.6 kJ
• DS -326.4 J/K

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Example 10

• Find DG for the reaction
• 2 H2S (g) SO2 (g) ? 3 S (s) 2 H2O (g)
• under the following conditions
• T 25C
• PH2S 1.0 x 10-4 atm
• PSO2 1.0 x 10-2 atm
• PH2O 3.0 x 10-3 atm
• Data from Appendix 4 DGf values in kJ/mole
• H2O(g) -229 S(s) 0 SO2(g) -300 H2S(g)
  -34

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Example 11

• Find K for the oxidation of ethanol to acetic
  acid
• at 25C
• CH3CH2OH (l) O2(g) CH3CO2H(l) 2 H2O(l)
• DG can be found from thermodynamic tables of
• DGf values. It is -455 kJ/mole.

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Example 12

• Use the data below to find Ksp for silver sulfide
• at 25C.

DHf(kJ/mol) DGf(kJ/mol) S(J/mol K)
Ag2S(s) -32 -40 146
Ag(aq) 105 77 73
S2-(aq) 33 86 -15
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Example 13

• The equilibrium constant for the reaction
• 2Fe3 (aq) Hg22(aq) 2Fe2 (aq)
  2Hg2(aq)
• is 9.1x10-6 at 298K.
• What is DG at this temperature?
• If standard state concentrations of the reactants
  and products are mixed, in which direction does
  the reaction proceed?
• Calculate DG when Fe30.20M, Hg220.010M,
  Fe20.010M, and Hg20.025M.

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Example 14

• Hydrogen gas is an excellent fuel. It combines
• With oxygen gas to produce water vapor.
• Calculate DH, DS, and DG for the combustion of
  1 mole of hydrogen gas at 25C if PO2PH2PH2O1
  atm.
• Is the spontaneity of this reaction dependent
  upon temperature? If so, at what temperatures is
  it spontaneous?

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Example 15

• Hemoglobin molecules carry oxygen from the lungs
  to
• tissue cells where the oxygen is then released
  for use in
• metabolic processes. The molecule can be
  represented
• as Hb in its unoxygenated form and as HbO2 in
  its
• oxygenated form. One reason why carbon monoxide
  is toxic is
• that it competes with O2 to bind to Hb
• HbO2 (aq) CO(g) HbCO(aq) O2(g)
• If DG-14kJ at 37C (body temp), what is the
  ratio of HbCO to HbO2 at 37C if O2CO?
• Use LeChateliers Principle to suggest how to
  treat a victim of CO poisoning.