How fast will it occur Kinetics

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Questions We Can Ask

• How fast will it occur Kinetics
• How much heat will it give off or absorb DH
  (enthalpy)
• Will it create more or less disorder DS
  (entropy)
• Will it occur at all? - DG (Gibbs Free Energy)

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Spontaneous Reactions

• Spontaneous Rxns occur without an outside
  source or energy
• Direction Rxns are spontaneous in one direction
  only (eggs breaking picture)
• Temperature has an effect
• Ice ? Water (spont. Above 0oC, reverse not
  spontaneous)

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Spontaneous Reactions

• Many spontaneous rxns
• 1. Exothermic Give off heat
• 2. Increase disorder
• Ex Person at end of day (warmer, more
  disordered)

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Entropy

• Deck of Cards example
• Entropy a measure of disorder
• The more disorder, the larger the entropy
• DS More disorder (shuffling a deck of cards)
• DS- Less disorder (ordering a deck of cards)

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• Boltzmann

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Entropy

• States of matter
• Solid ? Liquid ? Gas ? Plasma
• (Do drawing of atoms/molecules)
• Dissolving - DS
• NaOH(s) ? Na(aq) OH-(aq)

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Entropy

• Decomposing
• CaCO3(s) ? CaO(s) CO2(g)
• 4. Increase in gas molecules
• PCl5(g) ? PCl3(g) Cl2(g)

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Entropy

• Is DS is positive or negative for
• H2O(l) ? H2O(g)
• Ag(aq) Cl-(aq) ? AgCl(s)
• 2Fe2O3(s) ?4Fe(s) 3O2(g)
• CaO(s) CO2(g) ? CaCO3(s)
• N2(g) 3H2(g) ? 2NH3(g)
• N2(g) O2(g) ? 2NO(g)

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The First Law

• Energy is conserved
• Energy cannot be created or destroyed, it only
  changes form
• DE q w
• Battery in a discman
• Chemical ? Electrical ? Mechanical and light
• Work Done
• Some waste heat produced

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The Second Law

• Entropy is not conserved
• The universe is getting more disordered.
• Can only create local order.
• 4. Ex Cleaning locker You sweat and threw a
  lot of things away. Universe got more disordered.

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The Third Law

• The entropy of a pure crystal at absolute zero is
  zero
• Theoretical state of perfect order
• Above 0 K At 0 K
• atoms wiggle no molec.
• motion

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Laws of Thermodynamics
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Predicting Entropy

• Which of the following pairs has the higher
  entropy?
• H2O(s) or H2O(l)
• NaCl(s) or HCl(g)
• HCl(g) or Ar(g)
• N2(g) at 78 K or N2(g) at 298 K
• H2(g) or SO2(g)
• 1 mol of HCl(g) or 2 mol HCl(g)

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Calculating Entropy

• Standard Molar Entropies at 1 atm and 25oC (298
  K)
• Unit Joule/ mol K
• Standard molar entropies of element is not zero
• DHof DSo
• Fe(s) 0 27.2
• 4. DSor SnDSoprod S mDSoreactants

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Calculating Entropy

• Calculate DSo for
• N2(g) 3H2(g) ? 2NH3(g)
• (Ans -198.3 J/ mol K, more order)
• 2. Calculate DSo for
• Al2O3(s) 3H2(g) ? 2Al(s) 3H2O(g)
• (Ans 180.4 J/ mol K, more disorder)

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Gibbs Free Energy

• Josiah Gibbs 1st Ph.D. in science from a U.S.
  University (Yale, 1863)
• Free Energy Maximum amount of work you can
  get from a chemical reaction
• DG lt 0 Rxn will occur
• DG 0 Rxn at equilibrium
• DG gt 0 Rxn will NOT occur
• 4. DG NEVER tells you how fast a rxn will occur

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Gibbs Free Energy

• Combustion of Methane
• CH4 O2
• DG (-)
• CO2 H2O
• Reaction position

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Gibbs Free Energy

• DGor SnDGfoprod S mDGforxts
• Will the following rxn occur?
• N2(g) 3H2(g) ? 2NH3(g)
• (Ans DGor -33.32 kJ)
• 2. Will the following rxn occur?
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
• (Ans DGor -800.7 kJ)

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Gibbs Free Energy

• DG DH TDS
• T must be in Kelvin
• Will the following rxn occur at 500oC?
• N2(g) 3H2(g) ? 2NH3(g)
• (Ans DG 61 kJ)

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Gibbs Free Energy

• 2. Will the following rxn occur at 25 oC?
• 2SO2(g) O2(g) ? 2SO3(g)
• DG (-196.6) (298)(-0.1896)
• (Ans DG -140.1 kJ)

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Gibbs Free Energy

• DG DH TDS

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Gibbs Free Energy

• A certain rxn is exothermic and becomes more
  ordered. Will the rxn occur at any temperature?
• A certain rxn is endothermic and becomes less
  ordered. Will it occur?

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Gibbs Free Energy

• 3. A certain rxn is exothermic and occurs at
  any temperature. What does this reveal?

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Overview

• DH
• Tells you whether heat will be produced or if
  you must heat a rxn
• DHor SnDHoprod S mDHorxts
• DS
• Tells you whether the rxn produces more or less
  order
• DSor SnDSoprod S mDSorxts

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Overview

• DG
• Tells you whether a rxn will occur or not
• Considers DH, DS and temperature
• DG DH TDS

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Free Energy and K

• -DGo is at 298 K (25oC)
• Can calculate DG at other temperatures using
• DG DGo RT lnQ
• DGo Free energy change at 25oC
• R 8.314 J/mol-K
• T Temperature in Kelvin (Absolute temperature)
• Q Reaction Coefficient

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Example 1

• Calculate the DG at 298 K for a reaction mixture
  of 1.0 atm of N2, 3.0 atm of H2, and 0.50 atm of
  NH3.
• N2(g) 3H2(g) ?? 2NH3(g)

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• Q NH32 (0.50)2 0.0093
• N2H23 (1.0)(3.0)3
• DGo -33.32 kJ
• DG DGo RT lnQ
• DG -33,320 J (8.3.14 J/mol-K)(298
  K)(ln0.0093)
• DG - 44.9 kJ/mol
• (larger driving force for making NH3)

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Example 2

• Calculate the DG at 298 K for a reaction mixture
  of 0.50 atm of N2, 0.75 atm of H2, and 2.0 atm of
  NH3.
• (ANS -26.0 kJ/mol)

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Free Energy and K

• DG 0 at equilibrium
• DG DGo RT lnQ
• DGo RT lnK
• DGo -RT lnK or K e-DG/RT
• DGo negative Kgt1
• DGo zero K1
• DGo positive Klt1

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Example 3

• Calculate the value of K for the following
  equation if DGo -33.32 kJ
• N2(g) 3H2(g) ?? 2NH3(g)
• DGo -RT lnK
• lnK DGo -33,320 J
• -RT (-8.314 J/mol-K)(298K)
• lnK 13.4
• K e13.4 7 X 105

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Example 4

• Calculate DGo and K for the following reaction at
  298 K.
• H2(g) Br2(g) ?? 2HBr(g)
• ANS -109.6 kJ/mol, 1.6 X 1019