Ch. 9 K&K: Gibbs Free Energy and Chemical Reactions

Recall (ch. 8):

Free energy conditions . Helmholtz F isothermal Enthalpy H constant pressure Gibbs G const. pressure and temp

Ch. 9 K&K: Gibbs Free Energy and Chemical Reactions

Recall (ch. 8):

Free energy conditions . Helmholtz F isothermal Enthalpy H constant pressure Gibbs G const. pressure and temp

Recall (ch. 3)

pg. 68: “The Helmoltz free energy will be a minimum for a system S in thermal contact with a reservoir R

if the volume of the system is constant.”

Chemical reactions (and other experiments) are often carried out at constant pressure and constant temperature:

Gibbs free energy

G(U,P,V) = U – + PV(Gibbs free energy;Thermodynamic potential)

Chemical reactions (and other experiments) are often carried out at constant pressure and constant temperature:

Gibbs free energy

G(U,P,V) = U – + PV

dG = dU – d – d + PdV + VdP

(Gibbs free energy;Thermodynamic potential)

Chemical reactions (and other experiments) are often carried out at constant pressure and constant temperature:

Gibbs free energy

G(U,P,V) = U – + PV

dG = dU – d – d + PdV + VdP

(Gibbs free energy;Thermodynamic potential)

Isothermal dIsobaric dP = 0

So, for a system S at equilibrium:

dGS = dUS – dS + PdVS

dG = 0 at equilibrium

Comparing to eqn. 5.39 (dUS = dS – PdVS + dNS) shows

dGS = dUS – dS + PdVS

dGS = dNS = 0, since dNS = 0 at

equilibrium

Chemical reactions in equilibrium

H2 + Cl2 = 2HCl

1A1 + 2A2 + 3A3 + ….mAm = 0

A1 = H2 1 = 1 A2 = Cl2 1 = 1 A3 = HCl 1 = -2

jAj = 0

Want to find expression of chemical equilibrium under conditions of constant temperature and pressure

dG = jdNj

The unknown details of the chemical reactions are summarized by the various chemical potentials.

j

Chemical reactions in equilibrium

At equilibrium, dG = 0

Chemical reactions in equilibrium

K(t) for an ideal gas with internal degrees of freedom

K(t) for an ideal gas with internal degrees of freedom

K(t) for an ideal gas with internal degrees of freedom

K(t) for an ideal gas with internal degrees of freedom

K(t) for an ideal gas with internal degrees of freedom

K(t) for an ideal gas with internal degrees of freedom

K(t) for an ideal gas with internal degrees of freedom

Chemical reaction rates