Geoscience Reference
In-Depth Information
energy (dU) is equal to the heat added to the system (dq) plus the work (dw) done
on the system:
dU ¼ dq
þ
dw
:
ð
2
:
1
Þ
The first law of thermodynamics applied to an adiabatic system may be
expressed as the work done on a system by an adiabatic process, which is equal to
the increase in its internal energy, and a function of the state of the system.
In any natural system, the system boundary is prescribed as a nonadiabatic
(insulated) wall that allows the passage of heat to and from the surrounding
system. The change in internal energy is not equal to the heat supplied when the
system is open and free to change its volume. Under this condition, part of the
energy supplied is returned to the surroundings, and the heat supplied at constant
pressure is equal to the enthalpy (H) of the system, which is defined as
H ¼ U
þ
PV
;
ð
2
:
2
Þ
where U is the internal energy, P is the pressure, and V is its volume. Because U,
P, and V are state functions, the enthalpy also is a state function.
The second law of thermodynamics, as formulated by Kelvin, states that ''no
process is possible in which the sole result is the absorption of heat from a
reservoir and its complete conversion into work;'' in other words, in any natural
process involving a transfer of energy, some energy is converted irreversibly into
heat that cannot be involved in further exchange. The second law of thermody-
namics, therefore, is a recognition of spontaneous and nonspontaneous processes
and the fact that natural processes have a sense of direction.
The second law of thermodynamics can be expressed in terms of another state
function, the entropy (S). The thermodynamics definition considers the change in
entropy dS that occurs as a result of a physical or chemical change and is based on
the expression
dS ¼ dq
rev
=
T
;
ð
2
:
3
Þ
in which dq is an infinitesimal amount of heat gained by a body at temperature T in
a reversible (rev) process. A reversible change in thermodynamics is a change that
can be reversed by an infinitesimal modification of a variable. Because real pro-
cesses are never completely reversible, the entropy is a measure of the degree in
which a system has lost heat and therefore part of its capacity to do work. In
general, a local decrease in entropy is possible but must be accompanied by an
increase in entropy in the surrounding system. In real geochemical (open) systems,
a change in entropy must be equal to (in the case of reversible change) or greater
than zero:
dS
dq
=
T
:
ð
2
:
4
Þ
A fundamental equation combines the first and second laws of thermodynamics
and, in this manner, addresses the behavior of matter. For a reversible change in a
