thermiteA stoichiometric pow-
dered mixture of iron(III) oxide and
aluminium for the reaction:
2Al + Fe 2 O 3 →Al 2 O 3 + 2Fe
The reaction is highly exothermic
and the increase in temperature is
sufÜcient to melt the iron produced.
It has been used for localized weld-
ing of steel objects (e.g. railway lines)
in the Thermit process. Thermite is
also used in incendiary bombs.thermochemistryThe branch of
physical chemistry concerned with
heats of chemical reaction, heats of
formation of chemical compounds,
etc.thermodynamicsThe study of the
laws that govern the conversion of
energy from one form to another,
the direction in which heat willÛow,
and the availability of energy to do
work. It is based on the concept that
in an isolated system anywhere in
the universe there is a measurable
quantity of energy called the *inter-
nal energy (U) of the system. This is
the total kinetic and potential energy
of the atoms and molecules of the
system of all kinds that can be trans-
ferred directly as heat; it therefore
excludes chemical and nuclear en-
ergy. The value of U can only be
changed if the system ceases to be
isolated. In these circumstances U
can change by the transfer of mass to
or from the system, the transfer of
heat (Q) to or from the system, or by
the work (W) being done on or by the
system. For an adiabatic (Q = 0) sys-
tem of constant mass, ∆U = W. By
convention, W is taken to be positive
if work is done on the system and
negative if work is done by the sys-
tem. For nonadiabatic systems of
constant mass, ∆U = Q + W. This
statement, which is equivalent to the
law of conservation of energy, isknown as the Ürst law of thermody-
namics.
All natural processes conform to
this law, but not all processes con-
forming to it can occur in nature.
Most natural processes are irre-
versible, i.e. they will only proceed in
one direction (see reversible
process). The direction that a natural
process can take is the subject of the
second law of thermodynamics,
which can be stated in a variety of
ways. R. Clausius (1822–88) stated the
law in two ways: “heat cannot be
transferred from one body to a sec-
ond body at a higher temperature
without producing some other ef-
fect” and “the entropy of a closed sys-
tem increases with time”. These
statements introduce the thermody-
namic concepts of *temperature (T)
and *entropy (S), both of which are
parameters determining the direc-
tion in which an irreversible process
can go. The temperature of a body or
system determines whether heat will
Ûow into it or out of it; its entropy is
a measure of the unavailability of its
energy to do work. Thus T and S de-
termine the relationship between Q
and W in the statement of theÜrst
law. This is usually presented by stat-
ing the second law in the form ∆U =
T∆S – W.
The second law is concerned with
changes in entropy (∆S). The third
law of thermodynamics provides an
absolute scale of values for entropy
by stating that for changes involving
only perfect crystalline solids at *ab-
solute zero, the change of the total
entropy is zero. This law enables ab-
solute values to be stated for en-
tropies.
One other law is used in thermody-
namics. Because it is fundamental to,
and assumed by, the other laws of
thermodynamics it is usually known
as the zeroth law of thermodynamics.
This states that if two bodies are eachthermite 524t