Geology Reference
In-Depth Information
The standard enthalpies of formation for many molecules are tabulated. Using
these data and applying the well-known Hess Law it is possible to obtain the stan-
dard enthalpy for any reaction as the difference between the formation enthalpies
of products and reactants.
A parallel analysis is applied to formation and reaction entropies. The Third
Law of Thermodynamics, allows for the entropy calculation of absolute values for
substances. These are tabulated and referred to as standard molar entropies. From
such data it is possible to obtain the entropy change for any reaction as the difference
between the standard molar entropy of products and reactants.
The relationship G = HTS can be used to tabulate the standard molar
free energies of formation G
f
. Typical values for H
f
and G
f
range from -
200 kJ/mol to -2000 kJ/mol
1
and are of the same order of magnitude and have the
same sign (see Pauling (1947) or Perry and Green (1984)). For instance:
CaO(s); H
f
= 636 kJ/molG
f
= 604 kJ/mol,
MgO; H
f
= 602 kJ/molG
f
= 570 kJ/mol,
SiO
2
(s); H
f
= 912 kJ/mol and G
f
= 857 kJ/mol,
Al
2
O
3
; H
f
= 1; 667 kJ/molG
f
= 1; 583 kJ/mol,
9.2.2 Stage II: solidification
In mineral formation, the change of phase from a gas to a solid does not consume as
much energy as the formation of a molecular species from its elements. At constant
temperature the variation of entropy is simply H=T and the Gibbs free energy
remains the same.
It is not easy to obtain data on sublimation enthalpies or entropies, since the solid
compound often decomposes before evaporating. In many cases only the enthalpy
and fusion temperature at atmospheric pressure are known. Vaporisation (l ! s)
and sublimation (s ! g) enthalpies are commonly greater than those of fusion
(s ! l) and typically range from 10 kJ/mol to 100 kJ/mol, i.e. one or two orders
of magnitude below the compound's formation enthalpy.
For instance, the standard formation enthalpy of water is -241.6 kJ/mol, whereas
its vaporisation enthalpy at 100
o
C is -40.7 kJ/mol.
The natural process of mineral formation almost exclusively progresses to the
solid phase. Therefore tabulated values for the majority of minerals comprise of
the direct formation to the solid phase. It is also common to modify them when
standard conditions are not the appropriate ones to describe the energy changes
occurring in mineral geochemical processes.
1
A negative sign means that its formation reaction is exothermic.
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