Thermodynamics

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Thermodynamics

• Introduction

2
Entropy vs. Enthalpy
TS Arlene
Hurricane Katrina (Cat 5)
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Thermodynamics

• Transport, transformation and effect(s) of an
  organic compound depend on which environmental
  compartment it settles in.
• Examples
• Compounds can be transported long distances in
  air or water, but if they remain sorbed to
  organic matter, their movement is limited. (Like
  a Broken down Mac Truck)
• CCl2F2 may undergo reductive dehalogenation in
  anaerobic aqueous environments, but due to its
  very high vapor pressure, it stays in the
  atmosphere, which is oxidative. Thus CCl2F2 is
  very persistent and eventually reaches the
  stratosphere, where it causes ozone depletion.

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Thermodynamics

• Quantifies a compounds affinity for the
  environmental compartments which are available to
  it.
• Also implies the amount of energy necessary to
  move a chemical into a different compartment
• Can predict the direction of phase transfer
• Example Are PBDEs absorbing into or
  volatilizing out of the water of the Hudson River
  Estuary?
• Does not tell us anything about kinetics of phase
  transfer

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Equilibrium is related to kinetics
Simple reversible reaction
At equilibrium, there is no change in
concentrations
thus
Example sorption of PCBs to organic matter. Keq
is large, implying that sorption is fast and
desorption is slow
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In addition,

• Kinetic expressions for things like air-water
  exchange include the thermodynamic constant
  (Henrys law constant).
• Sometimes equilibrium can be related to kinetics
  via Linear Free Energy Relationships

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Chemical potential (m)

• Energy status of molecules in a system (e.g.
  Benzene in water)
• Internal energies
• Chemical Bonds, vibrations, flexations,
  rotations.
• External Energies
• Whole molecule transitions, orientations
• Interactions of molecule with surroundings
• Energy status is a function of
• Temperature
• Pressure
• Chemical composition
• Average energy per molecule

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Chemical potential free energy added to system
with each added increment of i
The total free energy is the sum of the
contributions from all the different components
present
Where m chemical potential (kJ/mol) DG
free energy (kJ) ni moles of component
(i) nj moles of component (j)
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• If two populations of chem (for example, the
  chemical coexists in two separate phases) each
  will have its own value of m1 and m2

m1
m2
Start liquid benzene (m1) and very little vapor
benzene (m2) Initial disequilibrium m1 ? m2 (m1
gt m2) Open stopcock. Benzene volatilizes. Net
movement of benzene to the right. m1
decreases, m2 increases until they are equal. No
net movement of benzene
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• No way to directly measure chemical potential.
• Can only determine differences in m, based on the
  tendencies of a chemical to move from one
  situation to another.
• Need a reference point, like sea level or
  absolute zero.
• often select pure liquid chem. as reference

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Fugacity urge to flee

• Fugacity is how happy the chemical is in its
  environment
• Fugacity is like temperature.
• At equilibrium, everything has the same fugacity
  (temperature) even though they may contain
  different concentrations (amounts) of the
  chemical (heat).

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Fugacity vs. Temperature
A liter of water Start T 0ºC fbenzene 0 (no
chemical present) Add 2100 Joules of heat to
raise temperature by 1 ºC 0.022 moles benzene to
raise fugacity to 104.1 Pa.
A liter of air Start T 0ºC fbenzene 0 (no
chemical present) Add 0.001 Joules of heat to
raise temperature by 1 ºC 0.0051 moles benzene
to raise fugacity to 104.1 Pa.
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Reference states

• For gases, ideal behavior is assumed, so a
  compounds fugacity is equal to its partial
  pressure

We use the gas phase as our reference state. As
a result, fugacity is given in units of pressure,
often Pa. Pressure (and therefore f ) is easy to
measure, unlike m
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• For pure solid (s) or liquid (L), the fugacity
  is

Where g describes the nonideal behavior resulting
from molecule-molecule interactions. For pure
solid or liquid, g is assumed to equal one.
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In a mixture (i.e. aqueous solution)
aqueous solutions of organic chemicals are
usually not ideal. g ? 1 (the means we are
using the pure liquid as our reference
state) Activity coefficients
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Activity coefficient and chemical potential

•  

chemical potential in solution
fugacity relative to ideal
chemical potential of pure liquid (ideal)
activity coefficient
mole fraction
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Phase transfer processes(or where the rubber
hits the road)
Consider a chemical (A) equilibrating between air
and water Aa Aw
Note still using pure liquid as reference state
At equilibrium, m is equal in the two phases
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After some rearranging you get
Recall
entropic and enthalpic terms
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contributions from enthalpy and entropy terms
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Temperature dependence of K