Chemical Thermodynamics
And the Thermodynamic Foundations of Life
Engines
1. capitalize on the fact that heat flows from hot to cold
2. convert some heat to work.
not all energy can be converted to
work, some heat is always lost
A cyclic process
• Takes you back to same P, T, U
• U = Q - W = 0
• Q=W
Here, Q = QC + QH
Efficiency e = W / QH
How much energy is available for work?
“Free energy”
Gibbs free energy
G
Same as U
First law of thermo
dE = dQ - dW
adding in entropy and P-V work
dE = T dS - P dV
Define Gibbs free energy
G = E + PV - TS
a derivative & some algebra
dG = V dP - S dT
At equilibrium,
P and T stay constant.
Therefore, what is dG?
dG = 0
(now think min, max, slope, entropy)
dG = V dP - S dT
G does the opposite of what entropy does:
G is minimized
G = how much useful work can be done (not just
heat released) at constant T and P.
At equilibrium, no more work can be done.
We can think of a cell as an engine…
dG = V dP - S dT +  dN
Chemistry!
 = chemical potential = how G depends on N (# molecules)
 dN = the sum of all  dN for all chemical
species involved in a reaction
Example: N2 + 3 H2 --> 2 NH3
.
 dN = 1 N2 + 3 H2 - 2 NH3
At equilibrium, G is at a minimum, so  dN = 0
Chemical reactions proceed in a certain
direction depending on T and P.
How does it “know” which way to go?
Rate = forward - reverse reaction rates
Rate constant k:
Experimental:
A catalyst lowers the activation energy