namely the volume, pressure, and internal energy, systems can be char-
acterized by additional state functions, including the enthalpy, H, which
is described below.
First law of thermodynamics
In considering the first law, we will assign to a system an internal energy,
U, which is the total energy available in the system at any given time.
Since we have said that energy leaves or enters a system in the form of
either heat or work, it follows that any change in the internal energy must
be due to either a heat flow or work being performed. Work and heat are
defined such that they are positive if they result in a net increase in the
internal energy of the system and negative if they result in a decrease in
the internal energy. To clarify the signs of these terms, consider the follow-
ing example. If a diathermic system is placed in contact with surroundings
that are colder than the system, then heat will flow from the system to the
surroundings. For a warm cup of water in an open cup, heat will flow
out of the cup into the air. Since the heat flow is out of the system the
sign is negative. If the diathermic system is placed in surroundings that are
warmer than the system, then heat will flow into the system and the sign
is positive. For example, heat is positive when an open cup of cold water
warms up. If work is done on a system, for example a gas is compressed,
the work is positive and the internal energy of the system increases. If the
system performs work, for example a gas is allowed to expand, the work
is negative.
The first law of thermodynamics states that, for an isolated, system the
internal energy of a system is constant.
Isolated systems cannot perform work or exchange heat with the surround-
ings. Any change in the internal energy, ΔU, must be due to the sum of
the work done on (or done by) the system, w, and the heat transferred
into or from the system, q:
ΔU=q+w (2.1)
Note that this equation deals with the change in the internal energy,
ΔU, that can be either positive, negative, or zero, and not the absolute
energy. Based upon this law, a perpetual-motion machine, which pro-
duces work without consuming an equivalent amount of energy, is an
impossible device to construct. Whenever work is performed, the internal
energy must decrease. In biological organisms, energy in the form of food
and nutrients is required for sustenance in order to perform the work
required to live.
26 PARTI THERMODYNAMICS AND KINETICS
