Chapter 19
Entropy, Spontaneity
and Free Energy
First Law of Thermodynamics
• Law of Conservation of Energy.
• This law suggests that energy can
be transferred from one system to
another in many forms. However, it
can not be created nor destroyed.
• Thus, the total amount of energy
available in the Universe is
constant.
This is also evident in Enthalpy Rxns
• Cdiamond + O2(g) CO2(g) ΔH = -396 kj
• CO2(g)  Cdiamond + O2(g) ΔH= +396 kj
• What are the forms of Energy?
First Law of Thermodynamics
• The first law is all about the accounting of
energy.
• How much energy is involved in the
change?
• Does the energy flow in or out of a
system?
• What form does the energy finally
assume?
Spontaneous Processes
• A process that will occur
without any energy
input from the
surroundings. A process
that will occur on its
own.
• Thermodynamics
considers only the initial
and final states of a
reaction and not the
pathway.
Spontaneous does not mean fast!
Spontaneous reactions: these
reactions never occur in the reverse.
• Heat moves from warm object to cold.
• Wood burns in an exothermic
reaction.
• Below 0 ° water freezes; above 0 °
water melts.
Spontaneous movement of gases
Entropy
Entropy symbol: S
• The driving force for a spontaneous
reaction is an increase in entropy in the
universe.
• Entropy is randomness, or the measure of
molecular disorder.
Entropy is the associated with
probability
• Nature proceeds spontaneously towards
the states that have the highest probability
of existing.
• So… which arrangement is most likely to
occur?
Probability
1
2
3
4
5
Probability
• The probability is that in
an arrangement of 4
molecules and two
places that
arrangement 3 is the
most likely.
• The likelihood of all 4 in
one chamber is low.
Probability of finding all gas
molecules in left bulb as a function
of the total number of gas particles
•
•
•
•
•
# molecules Probability
1
½
2
½x½=¼
3
½ x ½ x ½ = 1/(2)3
5
½x½ x½x½x½
= 1/(2)5
• n
½ x ½ x ½ x…1/(2)n