Free Energy & Chemical Thermodynamics Flashcards
(11 cards)
Free Energy
Energy available for work
Enthalpy
Energy of system plus work needed to make room for it.
H = U + PV
Helmholtz Free Energy
System with constant temperature and volume.
Total energy needed to create the system minus the heat you can get for free from environment at temperature T.
F = U - TS
Gibbs Free Energy
System with constant P and T.
System’s energy minus the heat term plus the atmospheric work.
G = U - TS + PV
Change in Helmholtz Free Energy
ΔF = ΔU - T ΔS = Q + W - TΔS
At constant temperature:
ΔF <= W
Change in Gibb’s Free Energy
ΔG = ΔU - T ΔS + P ΔV = Q + W - T ΔS + P ΔV
So ΔG = ΔH - T ΔS
W = -P ΔV + Wother
So at constant T, P:
ΔG <= Wother
Thermodynamic Potentials
U, H, F, and G
Thermodynamic Identities Given Enthalpy & Free Energy
Enthalpy:
dU = TdS - PdV + udN
dH = dU + PdV + VdP
dH = TdS + VdP + udN
Helmholtz Free Energy:
dF = dU - TdS - SdT
dF = -SdT - PdV + udN
Gibb’s Free Energy:
dG = dU + PdV - TdS
dG = -SdT + VdP + udN
Free Energy as a Force Toward Equilibrium
Relationship between:
- Entropy, Volume, Energy
- Helmholtz Free Energy, Temperature, Volume
- Gibb’s Free Energy, Temperature, Pressure
- At constant energy and volume, entropy tends to increase.
- At constant temperature and volume, Helmholtz free energy tends to decrease.
dStotal = dS - 1/T dU = -1/T(dU-TdS) = -1/T dF
- At constant temperature and pressure, Gibb’s free energy tends to decrease.
dStotal = dS - 1/T dU - P/T dV = -1/T(dU-TdS +PdV) = -1/T dF
Gibb’s Free Energy & Chemical Potential
If you add one particle to a system, holding the temperature and pressure fixed, the Gibbs free energy of system increases by u.
u = (dG/dN) T, P
G = Nu
(u is the Gibbs free energy per particle)
Extensive versus Intensive Properties
If a quantity doubles when you double the # of particles, it is an extensive quantity.
Extensive: V, N, S, U, H, F, G, mass
Intensive: T, P, u, density
