Page 69 - Instant notes

B5
ENTROPY AND CHANGE

Key Notes
A spontaneous process has a natural tendency to occur without
the need for input of work into the system. Examples are the
expansion of a gas into a vacuum, a ball rolling down a hill or
flow of heat from a hot body to a cold one.
A non-spontaneous process does not have a natural tendency to
occur. For a non-spontaneous process to be brought about, energy
in the form of work must be put into a system. Examples include
the compression of a gas into a smaller volume, the raising of a
weight against gravity, or the flow of heat from a cold body to a
hotter one in a refrigeration system.
The second law of thermodynamics states that the entropy of an
isolated system increases for irreversible processes and remains
constant in the course of reversible processes. The entropy of an
isolated system never decreases.
Because entropy is a state function, entropy changes in a system
may be calculated from the standard entropies of the initial and
final states of the system:

The standard entropy of reaction is be calculated from:

Related topics The first law (B1) Entropy (B6)
Enthalpy (B2) Free energy (B6)
Thermochemistry (B3) Statistical thermodynamics (G8)

Spontaneous process
Any process may be defined as being either spontaneous or non-spontaneous. A
spontaneous process has a natural tendency to occur, without the need for input of work
into the system. Examples are the expansion of a gas into a vacuum, a ball rolling down a
hill or flow of heat from a hot body to a cold one (Fig. 1).
It is important to note that the word ‘spontaneous’ is a formal definition and is not
used in the colloquial sense. If a process is described as spontaneous, it does not imply