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Introduction and Section 19.1 In this chapter we examined some of the aspects of chemical thermodynamics,
the area of chemistry that explores energy relationships. Most reactions and chemical processes have an inherent
directionality: They are spontaneous in one direction and nonspontaneous in the reverse direction. The
spontaneity of a process is related to the thermodynamic path the system takes from the initial state to the final
state. In a reversible process, both the system and its surroundings can be restored to their original state by
exactly reversing the change. In an irreversible process the system can't return to its original state without there
being a change in the surroundings. Any spontaneous process is irreversible. A process that occurs at a constant
temperature is said to be isothermal.
Section 19.2 The spontaneous nature of processes is related to a thermodynamic state function called entropy,
denoted S. For a process that occurs at constant temperature, the entropy change of the system is given by the
heat absorbed by the system along a reversible path, divided by the temperature:
The way entropy
controls the spontaneity of processes is given by the second law of thermodynamics, which governs the change
in the entropy of the universe,
in an irreversible (spontaneous) process
joules per kelvin,
The second law states that in a reversible process
Entropy values are usually expressed in units of
Section 19.3 Molecules can undergo three kinds of motion: In translational motion the entire molecule moves
in space. Molecules can also undergo vibrational motion, in which the atoms of the molecule move toward and
away from one another in periodic fashion, and rotational motion, in which the entire molecule spins like a top. A
particular combination of motions and locations of the atoms and molecules of a system at a particular instant is
called a microstate. Entropy is a measure of how many microstates are associated with a particular macroscopic
state. If the total number of accessible microstates is W, the entropy is given by
The number of
available microstates, and therefore the entropy, increases with an increase in volume, temperature, or motion of
molecules, because any of these changes increases the possible motions and locations of the molecules. As a
result, entropy generally increases when liquids or solutions are formed from solids, gases are formed from either
solids or liquids, or the number of molecules of gas increases during a chemical reaction. The third law of
thermodynamics states that the entropy of a pure crystalline solid at 0 K is zero.
Section 19.4 The third law allows us to assign entropy values for substances at different temperatures. Under
standard conditions the entropy of a mole of a substance is called its standard molar entropy, denoted S°. From
tabulated values of S°, we can calculate the entropy change for any process under standard conditions. For an
isothermal process, the entropy change in the surroundings is equal to
Section 19.5 The Gibbs free energy (or just free energy), G, is a thermodynamic state function that combines
the two state functions enthalpy and entropy:
For processes that occur at constant temperature,
For a process or reaction occurring at constant temperature and pressure, the sign of
relates to the spontaneity of the process. When
is negative, the process is spontaneous. When
is
positive, the process is nonspontaneous; the reverse process is spontaneous. At equilibrium the process is
reversible and
is zero. The free energy is also a measure of the maximum useful work that can be performed by
a system in a spontaneous process. The standard free-energy change,
for any process can be calculated from
tabulations of standard free energies of formation,
enthalpies of formation,
The value of
which are defined in a fashion analogous to standard
for a pure element in its standard state is defined to be zero.
Sections 19.6 and 19.7 The values of
and
generally do not vary much with temperature. As a
consequence, the dependence of
with temperature is governed mainly by the value of T in the expression
The entropy term
has the greater effect on the temperature dependence of
and,
hence, on the spontaneity of the process. For example, a process for which
and
such as the
melting of ice, can be nonspontaneous
at low temperatures and spontaneous
at higher
temperatures. Under nonstandard conditions
is related to
and the value of the reaction quotient, Q:
At equilibrium
Thus, the standard free-energy
change is directly related to the equilibrium constant for the reaction. This relationship can be used to explain the
temperature dependence of equilibrium constants.