Chemical Thermodynamics

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Chemical Thermodynamics

• BLB 11th Chapter 19

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Chemical Reactions

1. How fast will the reaction occur? Ch. 14
2. How far toward completion will the reaction
   proceed? Ch. 15
3. Will the reaction occur, i.e. is it spontaneous?
   Ch. 5, 19

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Review terms

• energy
• heat
• work
• pathway
• state function
• system
• surroundings

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Review terms

• exothermic
• endothermic
• enthalpy
• enthalpy change
• standard state
• std. enthalpy of formation
• 1st Law of Thermodynamics

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19.1 Spontaneous Processes

• Spontaneous proceeds on its own without any
  outside assistance
• product-favored
• not necessarily fast
• Nonspontaneous requires outside intervention
• reactant-favored
• not necessarily slow
• Spontaneity is temperature-dependent.

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Spontaneity and Energy

• Examples of spontaneous systems
• Brick falling
• Ball rolling downhill
• Hot objects cooling
• Combustion reactions
• Are all spontaneous processes accompanied by a
  loss of heat, that is, exothermic?

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Reversible Irreversible Systems

• Reversible a change in a system for which the
  system can restored by exactly reversing the
  change a system at equilibrium
• ex. melting ice at 0C
• Irrerversible a process that cannot be reversed
  to restore the system and surroundings to their
  original states a spontaneous process
• ex. melting ice at 25C
• See p. 806 (last paragraph of section)

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19.2 Entropy and the 2nd Law of Thermodynamics

• Entropy, S measure of randomness
• State function
• Temperature-dependent
• A random (or dispersed) system is favored due to
  probability.
• Entropy Is Simple If We Avoid the Briar
  Patches
• Frank Lambert, Occidental College, ret.
• http//www.entropysimple.com/content.htm

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Entropy Change

• ?S Sfinal - Sinitial (a state function)
• (isothermal) as for phase changes.
• ?S gt 0 is favorable

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Calculating ?S for Changes of State
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Problem 24
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System Surroundings

• Dividing the universe
• System dispersal of matter by reaction
  reactants ? products
• Surroundings dispersal of energy as heat

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2nd Law of Thermodynamics

• The total entropy of the universe increases for
  any spontaneous process.
• ?Suniv gt 0
• ?Suniv ?Ssys ?Ssurr
• For a reversible process ?Suniv 0.
• For an irreversible process
• Net entropy increase ?spontaneous
• Net entropy decrease ? nonspontaneous

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19.3 The Molecular Interpretation of Entropy

• Molecules have degrees of freedom based upon
  their motion
• Translational
• Vibrational
• Rotational
• Motion of water (Figure 19.6, p. 810)
• Lowering the temperature decreases the entropy.

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Boltzman Microstates

• S k ln W
• (W of microstates)
• If microstates ?, then entropy ?.
• Increasing volume, temperature, of molecules
  increases the of microstates.

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Examples of systems that have increased entropy

• Entropy increases for
• Changes of state solid ? liquid ? gas (T)
• Expansion of a gas (V)
• Dissolution solid ? solution (V)
• Production of more moles in a chemical reaction
  ( of particles)
• Ionic solids lower ionic charge
• S (J/molK)
• Na2CO3 136
• MgCO3 66

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Changes of State
H2O state S (J/molK)
l 69.91
g 188.83
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Dissolution
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Expansion of a Gas
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2 NO(g) O2(g) ? 2 NO2(g)
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Problem 20
TNT (trinitrotoluene) Detonation 4 C3H5N3O9(l) ?
6 N2(g) 12 CO2(g) 10 H2O(g) O2(g)

1. Spontaneous?
2. Sign of q?
3. Can the sign of w be determined?
4. Can the sign of ?E be determined?

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3rd Law of Thermodynamics

• The entropy, S, of a pure crystalline substance
  at absolute zero (0 K) is zero.

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19.4 Entropy Changes in Chemical Reactions

• Standard molar entropy values, S (J/molK)
• increase in value as temperature increases from 0
  K
• have been determined for common substances
  (Appendix C, pp. 1112-1114)
• increase with molar mass
• increase with of atoms in molecule

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Calculating ?Ssys

• ?Ssys ?nS(products) - ?mS(reactants)
• (where n and m are coefficients in the chemical
  equation)

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Problem 50
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Problem
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Entropy Changes in the Surroundings

• Heat flow affects surroundings.
• As T increases, ?H becomes less important.
• As T decreases, ?H becomes more important.

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Calculating ?Suniv

• ?Suniv ?Ssys ?Ssurr
• by obtaining ?Ssys and ?Ssurr
• If ?Suniv gt 0, the reaction is spontaneous.
• But there is a better way one in which only the
  system is involved.

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19.5 Gibbs Free Energy

• The spontaneity of a reaction involves both
  enthalpy (energy) and entropy (matter).
• Gibbs Free Energy, ?G makes use of ?Hsys and
  ?Ssys to predict spontaneity.
• ?Gsys represents the total energy change for a
  system.
• G H TS or ?G ?H T?S
• or, under standard conditions
• ?G ?H T?S

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Gibbs Free Energy

• If
• ?G lt 0, forward reaction is spontaneous
• ?G 0, reaction is at equilibrium
• ?G gt 0, forward reaction is nonspontaneous
• In any spontaneous process at constant
  temperature and pressure, the free energy always
  decreases.
• ?G is a state function.
• ?Gf of elements in their standard state is zero.

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Calculating ?Gsys

• ?Gsys ?Hsys - T?Ssys
• or
• ?Gsys ?n?Gf (products) - ?m?Gf (reactants)
• (where n and m are coefficients in the chemical
  equation)

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Problem 56
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19.6 Free Energy and Temperature
?H ?S -T?S ?G Reaction
- - always - spontaneous at all T K gt 1
- always nonspontaneous at all T K lt 1
- - - _at_ low T _at_high T spontaneous at low T nonspontaneous at high T
- _at_ low T - _at_high T nonspontaneous at low T spontaneous at high T
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Driving force of a reaction

• For a reaction where ?G lt 0
• Enthalpy-driven if ?H lt 0 and ?S lt 0 at low
  temp.
• Entropy-driven if ?H gt 0 and ?S gt 0 at high
  temp.
• cross-over point is where ?G 0

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Problem 66
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19.7 Free Energy and K

• If conditions are non-standard
• ?G ?G RT lnQ R 8.3145 J/molK
• If at equilibrium
• ?G ?G RT lnQ 0
• ?G -RT lnK

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Problem 78
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Problem
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