Geology Reference
In-Depth Information
Energy in chemical and mineral systems:
free energy
There is also an internal contribution to its total
energy from the individual motions of its constituent
atoms and molecules, which are continually vibrat-
ing, rotating and - in liquids and gases - migrating
about. This internal component, the aggregate of
the  kinetic energies of all the atoms and molecules
present, is what we mean by the enthalpy of the body.
Enthalpy is closely related to the concept of heat (and
was at one time referred to, rather misleadingly, as
'heat content'). Heat is one of the mechanisms by
which enthalpy can be transferred from one body to
another. The effect of heating a body is simply to
increase the kinetic energy of the constituent atoms
and molecules, and therefore to increase the enthalpy
of the body as a whole.
Natural processes continually convert energy from
one form into another. One of the fundamental axioms
of thermodynamics, known as the First Law, is that
energy can never be created, destroyed or 'lost ' in such pro-
cesses, but merely changes its form (Box 1.2). Thus the
energy given out by a reaction is matched exactly by
the amount of energy gained by the surroundings.
Experience tells us that mechanical systems in the
everyday world tend to evolve in the direction that
leads to a net reduction in total potential energy . Water
runs downhill, electrons are drawn toward atomic
nuclei, electric current flows from 'live' to 'neutral',
and so on. The potential energy released by such
changes reappears as other forms of energy or work:
for example, the kinetic energy of running water, the
light energy radiated by electronic transitions in atoms
(Chapter 6), or the heat generated by an electric fire.
Thermodynamics visualizes chemical processes in a
similar way. Reactions in chemical or geological sys-
tems arise from differences in what is called free energy ,
G , between products and reactants. The significance of
free energy in chemical systems can be compared to
that of potential energy in mechanical systems. A chem-
ical reaction proceeds in the direction which leads to a
net reduction in free energy , and the chemical energy so
released reappears as energy in some other form - the
Box 1.2 The First Law of Thermodynamics
the most fundamental principle of thermodynamics is that
energy is never created, lost or destroyed. It can be trans-
mitted from one body to another, or one place to another,
and it can change its identity between a number of forms
(as for example when the potential energy of a falling body
is transformed into kinetic energy, or when a wind turbine
converts the kinetic energy of moving air into electrical
energy). But we never observe new energy being created
from scratch, nor does it ever just disappear. accurate
energy bookkeeping will always show that in all known pro-
cesses total energy is always conserved . this cardinal prin-
ciple is called the First Law of Thermodynamics . the energy
given out by a reaction or process is matched exactly by
the amount of energy gained by the surroundings.
Implicit in the First Law is the recognition that work is
equivalent to energy, and must be accounted for in energy
calculations. When a compressed gas at room tempera-
ture escapes from a cylinder, it undergoes a pronounced
cooling, often to the extent that frost forms around the
valve. (a smaller cooling effect occurs when you blow on
your hand through pursed lips.) the cause of the cooling
is that the gas has had to do work during escaping: it
occupies more space outside the cylinder than when com-
pressed inside it, and it must make room for itself by dis-
placing the surrounding atmosphere. Displacing something
against a resisting force (in this case atmospheric pres-
sure) constitutes work, which the gas can only accomplish
at the expense of its enthalpy. this is related directly to
temperature, so that a drain on the gas's internal energy
reserves becomes apparent as a fall in temperature.
a similar cooling effect may operate when certain gas-
rich magmas reach high crustal levels or erupt at the sur-
face. an example is kimberlite, a type of magma that
commonly carries diamonds up from the mantle. Kimberlite
penetrates to the surface from depths where the assoc-
iated gases are greatly compressed, and the work that
they do in expanding as the magma-gas system bursts
through to the surface reduces its temperature; kimber-
lites found in subvolcanic pipes (diatremes) appear to
have been emplaced in a relatively cool state.
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