Page 70 - Physical Chemistry

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one into the error of regarding heat and work as state functions. Heat and work are Section 2.4
defined only in terms of processes. Before and after the process of energy transfer The First Law of Thermodynamics
between system and surroundings, heat and work do not exist. Heat is an energy trans-
fer between system and surroundings due to a temperature difference. Work is an en-
ergy transfer between system and surroundings due to a macroscopic force acting
through a distance. Heat and work are forms of energy transfer rather than forms of
energy. Work is energy transfer due to the action of macroscopically observable
forces. Heat is energy transfer due to the action of forces at a molecular level. When
bodies at different temperatures are placed in contact, collisions between molecules of
the two bodies produce a net transfer of energy to the colder body from the hotter
body, whose molecules have a greater average kinetic energy than those in the colder
body. Heat is work done at the molecular level.
Much of the terminology of heat is misleading because it is a relic of the erro-
neous caloric theory of heat. Thus, one often refers to “heat flow” between system and
surroundings. In reality, the so-called heat flow is really an energy flow due to a tem-
perature difference. Likewise, the term “heat capacity” for C is misleading, since it
P
implies that bodies store heat, whereas heat refers only to energy transferred in a
process; bodies contain internal energy but do not contain heat.
Heat and work are measures of energy transfer, and both have the same units as
energy. The unit of heat can therefore be defined in terms of the joule. Thus the defi-
nition of the calorie given in Sec. 2.3 is no longer used. The present definition is
1 cal 4.184 J exactly (2.44)*

where the value 4.184 was chosen to give good agreement with the old definition of
the calorie. The calorie defined by (2.44) is called the thermochemical calorie, often
designated cal . (Over the years, several slightly different calories have been used.)
th
It is not necessary to express heat in calories. The joule can be used as the unit of
heat. This is what is done in the officially recommended SI units (Sec. 2.1), but since
some of the available thermochemical tables use calories, we shall use both joules and
calories as the units of heat, work, and internal energy.

Although we won’t be considering systems with mechanical energy, it is worthwhile to
consider a possible source of confusion that can arise when dealing with such systems.
Consider a rock falling in vacuum toward the earth’s surface. Its total energy is E K
V U. Since the gravitational potential energy V is included as part of the system’s energy,
the gravitational field (in which the potential energy resides) must be considered part of the
system. In the first-law equation E q w, we do not include work that one part of the
system does on another part of the system. Hence w in the first law does not include the work
done by the gravitational field on the falling body. Thus for the falling rock, w is zero; also,
q is zero. Therefore E q w is zero, and E remains constant as the body falls (although
both K and V vary). In general, w in E q w does not include the work done by con-
servative forces (forces related to the potential energy V in E K V U).
2
Sometimes people get the idea that Einstein’s special relativity equation E mc in-
validates the conservation of energy, the first law of thermodynamics. This is not so. All
2
2
E mc says is that a mass m always has an energy mc associated with it and an energy
2
E always has a mass m E/c associated with it. The total energy of system plus sur-
roundings is still conserved in special relativity; likewise, the total relativistic mass of
system plus surroundings is conserved in special relativity. Energy cannot disappear; mass
cannot disappear. The equation E q w is still valid in special relativity. Consider, for
example, nuclear fission. Although it is true that the sum of the rest masses of the nuclear
fragments is less than the rest mass of the original nucleus, the fragments are moving at high
speed. The relativistic mass of a body increases with increasing speed, and the total rela-
tivistic mass of the fragments exactly equals the relativistic mass of the original nucleus.