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Entropy and Free Energy 255
17-5 Utterly Confused About
Thermodynamics
Don’t Forget!
Be Careful!
Quick Tip
Let’s work a few examples of problems employing information from this chapter.
Determine the value of ∆S° for each of the following reactions.
a. H2(g) 1/2 O2(g) l H2O(g) b. H2(g) 1/2 O2(g) l H2O(l) These are thermochemical equations. Fractions are acceptable in thermochemical
equations.
To begin the problem, we need the standard entropy values for each of the reactants
and products. These are the values we’ll be using to complete this problem:
H 2(g) 131.0 J/molK
O 2(g) 205.0 J/molK
H 2O(g) 188.7 J/molK
H 2O(l) 69.9 J/molK
Make sure you use the correct value from the table of values. As seen in this
example, the value for liquid water is not the same as for gaseous water.
Unless your instructor has told you to memorize specific thermochemical values,
you will be provided the numbers you need. Most textbooks have a table of values
in the chapter and an additional table in Appendix.