Chem 112 Class Guide: ENTROPY
Chapter 19, Sections 1 through 4 Learning Goals:
Upon completion of Chapter 19, Sections 1-4, you should be able to determine the following:
Determine whether a reaction is spontaneous or not by examining ΔS
Predict the change in entropy in a process
Calculate ΔS°rxn from a table of ΔS° values
Chapter reading Guide: Chapter 19, Sections 1 through 4
Section 1: SPONTANEOUS PROCESSES
Read Chapter 19.1
A spontaneous process is one that proceeds on its own without any assistance. Spontaneous
processes occur in one direction only. The reverse of a spontaneous process is a nonspontaneous process.
A non-spontaneous process needs some form of assistance in order to occur.
Generally, a reaction that is spontaneous in one direction is non-spontaneous in the other
direction. For example, if you drop an egg and it breaks, that is a spontaneous process.
However, the egg will not magically heal its shell and fly upwards, back into your hand. That
process would be non-spontaneous.
A reversible process is a process that can be exactly reversed. Generally, these processes
involve a VERY small change AND no change in the phase of the substance in question.
An irreversible process is a process that cannot be exactly reversed. 99% of all chemical
processes are irreversible, including those processes at equilibrium. Generally, all
spontaneous reactions are irreversible.
Try Practice exercise 19.1
Section 2: ENTROPY AND THE 2nd LAW OF THERMODYNAMICS
Read Chapter 19.2
Entropy, also termed S, is a measure of the randomness (or disorder) in a system.
ΔS is the change in entropy for a process and generally has the units of J/mol·K
The 2nd law of thermodynamics states that any process that is irreversible will result in a
change of entropy. Any process that is reversible will have no change in entropy. Another
way to think of this is that the entropy will always increase (become MORE disordered) for
any spontaneous process.
Section 3: MOLECULAR INTERPRETATION OF ENTROPY
Read Chapter 19.3
We can predict the change of entropy for a chemical reaction by looking at the reaction. In
general, a system gains entropy when:
1.)
2.)
3.)
4.)
Gas is formed from liquids and/or solids
Liquid is formed from solids
An aqueous solution is formed from liquids and/or solids
The number of gas molecules increases from reactants to products
Try Practice exercise 19.3
Try Practice exercise 19.4
Section 4: ENTROPY CHANGES IN MOLECULAR REACTIONS
Read Chapter 19.4
Given the following general reaction:
aA + bB  cC + dD
The change in entropy can be calculated as follows:
o
Srxn
 (cSC  d S D )  (aS A  bS B )
You can find ΔS values in Appendix C (pay attention to phases when choosing your values!).
Example:
Calculate ΔSrxn for the following reaction, using ΔS values found in Appendix C:
C2H4 (g) + 3 O2 (g)  2 CO2 (g) + 2 H2O (g)
Srxn   2 mol  S CO2   2 mol  S H 2O   1 mol  S C2H 4   3 mol  S O 2 
J


S rxn   2 mol  213.6
mol * K



  2 mol

J
J
J

 




 188.83
   1 mol  219.4
  3 mol  205.0

mol
*
K
mol
*
K
mol
*
K

 




J
J 
J
J

Srxn   427.2
 377.66    219.4
 615.0 
K
K 
K
K

J 
J
J

Srxn  804.9   834.4   29.5
K
K
K

 

Try Practice exercise 19.5
Learning Resources
Chapter Learning Goals
Chapter 19, Sections 1 through 4 Learning Goals
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete to the pre class assignment
(http://berks.psu.edu/clt/chem112/Entropy_HW.docx)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission:
Entropy”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor or the learning center
Chapter 19: 11, 15, 35, 41, 43, 53