The Gibbs free energy G is the thermodynamic potential that tells us which way a
reaction goes at a given set of physical conditions-neither the enthalpy change nor the
entropy change for a reaction alone can provide us with this information. The two
meas
The partial molar volume is defined as
i
If you mix two compounds A and B together and find a volume of mixing that is non-ideal, how
can you determine the contribution that A and B each make to the volume? That is, what are
the partial molar volumes of A
If we say that rG = 0 at equilibrium, then we can write our solids-only and constant-heatcapacity approximations as
0 = rG1,T - rS1,Tref(T - Tref) + rVsP
0 = rH1,Tref - T rS1,Tref + rVsP
and thus we can calculate the pressure of a reaction at different
Adding heat Q to a crystal increases its internal energy U:
dU
dQ
( indicates 'proportional') but if the crystal is allowed to expand, some of the added energy will
be consumed by expansion dV, so the total energy of the crystal is reduced:
dU = dQ - Pd
We have discussed the intuitive statement that reactions probably proceed because the
reactants can decrease their internal energy by reacting. We also noted that internal
energy scales with enthalpy, suggesting that reactions might 'go' because of a decr
There is a useful relation between the slope of a reaction in PT space (i.e., dP/dT) and the
entropy and volume changes of the reaction that follows from
VrdP - SrdT
rG =
At equilibrium G = 0, such that
rVdP =
rSdT
or
=
So, the P-T slope of a reaction is
Net-transfer reactions are those that cause phases to appear or disappear. Geobarometers are
often based on net-transfer reactions because rV is large and relatively insensitive to
temperature. A popular one is GASP:
=
+
3CaAl2Si2O8
+ SiO2
Ca3Al2Si3O12 2