Page 31 - Basic physical chemistry for the atmospheric sciences

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Chemical thermodynamics

Heat can be released or absorbed during a chemical reaction. This
provides a powerful method for studying chemical equilibrium by
means of chemical thermodynamics. Thermodynamics is based on a
e
few fundamental postulat s , called the first, second, and third laws o f
thermodynamics. We will discuss these laws first, and then return to
the subject of chemical equilibrium.

2.1 The first law of thermodynamics; enthalpy
In addition to the macroscopic kinetic and potential energy that a body
s
or system as a whole may posses , it also contains internal energy due
to the kinetic and potential energy of its molecules or atoms. Increases
in internal kinetic energy in the form of molecular motions are mani­
fested as increases in the temperature of the system, while changes in
the potential energy of the molecules are caused by changes in their
relative configurations.
u
Let s suppose that a system of unit mass takes in a certain quantity
of heat energy q (measured in joules). As a result, the system may do
a certain amount of external work w (also measured in joules). The
excess energy supplied to the system, over and above the external
q
work done by the system, is - w . Therefore, if there is no change in
the macroscopic kinetic and potential energy of the system, it follows
from the principle of conservation of energy that the internal energy of
i
q
the system must increase by - w . That s ,
q - w = u2 - u 1 (2. 1)
where u 1 and u2 are the internal energies of a unit mass of the system
before and after the change. In differential form Eq. (2. 1 ) becomes

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