ENERGY IN GEOCHEMICAL PROCESSES - Chemical Fundamentals of Geology and Environmental Geoscience - page 13

planktonic gastropods (pteropods) are constructed
of aragonite precipitated directly from seawater.)
The explanation for this appearance of stability is
that aragonite is stabilized by a free-energy 'hur-
dle', just like its mechanical analogue.

G Activation energy ( E a ) Chemical kinetics

Aragonite

Thermodynamics

Δ G

The free-energy path followed by aragonite (under
the conditions shown by X in Figure 1.3a), as it under-
goes transformation into calcite, is shown Figure 1.4.
The energy hurdle exists because rearranging the
crystal structure of aragonite into that of calcite
involves some work being done in breaking bonds
and moving atoms about. Although this energy invest-
ment is recovered several times over in the net release
of free energy as the reaction proceeds, its importance
in determining whether the reaction can begin is con-
siderable. In recognition of this influence, the height
of the energy hurdle is called the activation energy of
the reaction (symbol E a ).
Figure 1.4 illustrates an important distinction
between two major domains of geochemical science.
Thermodynamics is concerned with the free-energy
changes associated with chemical equilibrium between
phases, and provides the tools for working out which
mineral assemblages will be stable under which condi-
tions. Only the initial and final states are of interest in
thermodynamics, and attention is confined to net
energy differences between reactants and products
(Δ G , Δ H , Δ S ), as Chapter 2 will show. The science of
chemical kinetics deals with the mechanics of the reac-
tions that lead to equilibrium and the rates at which
they occur. In this area, as we shall see in Chapter 3, the

Calcite

Progress of reaction

Figure 1.4 The free-energy path during the recrystallization
of aragonite into calcite at the P-T conditions shown by X in
Figure 1.3.

activation energy asserts a dominant role, accounting
for the strong influence of temperature on many
geological processes.

Further reading

Atkins, P. (2010) The Laws of Thermodynamics: A Very Short
Introduction . Oxford: Oxford University Press (especially
chapters 1 and 2).
Goldberg, D. (2010) Beginning Chemistry . 2nd edn. New York:
McGraw Hill.
Pauken, M. (2011) Thermodynamics for Dummies . Chichester:
John Wiley and Sons, Ltd.
Smith, E.B. (2004) Basic Chemical Thermodynamics , 5th edn.
London: Imperial College Press.
Williams, L. (2003) Chemistry Demystified . New York:
McGraw-Hill.