1. Thermodynamics
Introduction

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

3. 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.

4. Thermodynamics
Quantifies a compound’s 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

5. 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

6. In addition,
Kinetic expressions for things like air-water exchange include the thermodynamic constant (Henry’s law constant).
Sometimes equilibrium can be related to kinetics via Linear Free Energy Relationships

7. 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”

8. The total free energy is the sum of the contributions from all the different components present:
Chemical potential = free energy added to system with each added increment of i
Where m = chemical potential (kJ/mol)
DG = free energy (kJ)
ni = moles of component (i)
nj = moles of component (j)

9. 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 > 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

10. 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

11. 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).

12. Fugacity vs. Temperature
A liter of air
Start:
T = 0ºC
fbenzene = 0
(no chemical present)
Add:
0.001 Joules of heat to raise temperature by 1 ºC
moles benzene to raise fugacity to Pa.
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 Pa.

13. Reference states
For gases, ideal behavior is assumed, so a compound’s 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

14. 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.

15. 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:
solute =>
hexane
benzene
diethylether
ethanol
solvent 
n-hexadecane ~1 35
CHCl3
1.8
0.8
0.3
4.5 12
5.4
n.a. 1
water
460,000
2500
130
3.6

16. Activity coefficient and chemical potential
chemical potential
in solution
fugacity relative to ideal
chemical potential
of pure liquid (ideal)
activity coefficient
mole fraction

17. 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

18. After some rearranging you get:
Recall
entropic and enthalpic terms

19. contributions from enthalpy and entropy terms
Water-building of H-bonded cage around each solute molecule accounts for large positive (unfavorable) entropy change
Note how most things have a similar entropy of vaporization, except ethanol, which undergoes extensive H bonding in the liquid phase.

20. Temperature dependence of K