This document discusses the Second Law of Thermodynamics and entropy. It begins by defining spontaneous processes as those that occur automatically without energy input and tend to proceed in one direction. It then reviews that the First Law concerns energy transfer and conservation. While the First Law doesn't determine spontaneity, entropy does - the greater the number of possible molecular arrangements (microstates), the higher the entropy and more disorder. The Second Law states that the total entropy change for any spontaneous process in the universe must be positive.

1. The Second Law of
Thermodynamics:
Spontaneous Reactions and
Entropy S (Pt 1)
By Shawn P. Shields, Ph.D.
This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons
Attribution-NonCommercial-ShareAlike 4.0 International License.

2. What is a Spontaneous Process?
Spontaneous reactions or processes
happen “automatically”, and do not
require an input of energy to occur.
Spontaneous processes have a great
tendency to proceed in one direction.

3. Recall: First Law of Thermodynamics
The transfer of energy between
a chemical reaction system and
its surroundings occurs and work
or heat.
Energy is conserved for the
transfer.

4. What Determines Whether a Reaction is
Spontaneous?
Energy is conserved in spontaneous
processes.
The First Law does not tell us whether
a given process is spontaneous or not.
So, how can it be determined whether a
process is spontaneous or not?

5. Enthalpy is Half of the Story…
Exothermic reactions release heat
(H) and are spontaneous according
to H.
Endothermic reactions absorb heat
(+H) and are nonspontaneous
according to H.

6. Predicting Spontaneous Processes
H is not the only factor to consider
in predicting a spontaneous process.
We need to consider another
thermodynamic quantity…
Entropy (S)

7. What is Entropy?
A measure of the disorder or
randomness in a system.
Entropy can also be thought of as the
number of possible arrangements or
“microstates” for a given state.

8. Entropy and Microstates
Analyze the 20 coins shown below:
20 coins are shown
10 coins are “heads up”
“Coins" from http://www.chem1.com/acad/webtext/thermeq/TE1.html
A state with half of
the coins “heads up”
is a “macrostate”.

9. Entropy and Microstates
Analyze the 20 coins shown below:
20 coins are shown with 10 coins “heads up”
“Coins" from http://www.chem1.com/acad/webtext/thermeq/TE1.html
Within the “macrostate”…
if information about the
particular coins that are
heads up is included, we
have a “microstate”!

10. Entropy and Microstates
The greater the number of
possible arrangements for a
given state (microstates), the
higher the entropy.
(More microstates indicates
greater disorder.)

11. Types of Entropy (Disorder)
Positional disorder- The
distribution of molecules
(particles) in space
Thermal disorder- The
distribution of energy states
among the molecules (particles)

12. The Second Law of Thermodynamics
The total entropy change of the
universe (Suniv) for any
spontaneous process is positive.
+ Suniv
Suniv = Ssys + Ssurr
Where Ssys and Ssurr are the entropy changes of
the system and the surroundings, respectively.

13. Next up…
Entropy and Microstates
(Pt 2)