Entropy, Free Energy and Equilibrium
Spontaneous Process
One of the main objectives in studying
thermodynamics, as far as chemists are
concerned, is to be able to predict whether
or not a reaction will occur when reactants
are brought together under a specific set
of conditions (for example, at a certain
temperature, pressure, and concentration).
A reaction that does occur under the given
set of conditions is called a spontaneous
Reaction. If a reaction does not occur under
specified conditions it is Non-spontaneous.
Examples:
- A waterfall runs downhill
- A lump of sugar dissolves in a cup of
coffee
- At 1 atm, water freezes below 0 0C
and ice melts above 0 0C
- Heat flows from a hotter object to a
colder object
- A gas expands in an evacuated bulb
- Iron exposed to oxygen and water
forms rust
*Spontaneously in one direction cannot,
under the same conditions, also take place
spontaneously in the opposite direction. *
Exothermicity- spontaneity of a reaction
but does not guarantee it.
It is possible for an endothermic reaction to
be spontaneous, it is
possible for an exothermic reaction to be
nonspontaneous
Entropy(S) - a measure of how spread out
or dispersed the energy of a system is
among the different possible ways that
system can contain energy. It is a measure
of the randomness or disorder of a
system.
Greater the dispersal, the greater is the
entropy.
For any substance, the solid state is more
ordered than the liquid state and the liquid
state is more ordered than gas state
Most processes are accompanied by a
change in entropy
Processes that lead to an increase in
entropy (ΔS > 0)
Microscopic states or microstates – eleven
possible ways of distributing the molecules.
Distribution- each set of similar microstates
Probability of occurrence of a particular
distribution (state) depends on the
number of ways (microstates) in which the
distribution can be achieved.
1868 Boltzmann showed that the entropy
of a system is related to the natural
log of the number of microstates (W)
S = k ln W
k is called the Boltzmann constant (1.38 3
10223 J/K)
the larger the W, the greater is the entropy
of the system
Entropy is a state function
The entropy change for the
process, DS, is
ΔS = Sf - Si
From Equation (18.1) we can write
ΔS = k ln Wf
Wi
How does the entropy of a system change
for each of the following processes?
(a) Condensing water vapor
Randomness decreases
Entropy decreases (ΔS < 0)
(b) Forming sucrose crystals from a
supersaturated solution
Randomness decreases
Entropy decreases (ΔS < 0)
(c) Heating hydrogen gas from 600C to
800C
Randomness increases
Entropy increases (ΔS > 0)
