Post on 21-Mar-2016
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Chemical ThermodynamicsAnd the Thermodynamic Foundations of Life
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
1. capitalize on the fact that heat flows from hot to cold2. convert some heat to work.
Engines
How much energy is available for work?
“Free energy”
Gibbs free energy G
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
Same as U
At equilibrium,
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
P and T stay constant.
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