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Untitled - Kelly Walsh High School

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Entropy and Free Energy 255<br />

17-5 Utterly Confused About<br />

Thermodynamics<br />

Don’t Forget!<br />

Be Careful!<br />

Quick Tip<br />

Let’s work a few examples of problems employing information from this chapter.<br />

Determine the value of ∆S° for each of the following reactions.<br />

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<br />

equations.<br />

To begin the problem, we need the standard entropy values for each of the reactants<br />

and products. These are the values we’ll be using to complete this problem:<br />

H 2(g) 131.0 J/molK<br />

O 2(g) 205.0 J/molK<br />

H 2O(g) 188.7 J/molK<br />

H 2O(l) 69.9 J/molK<br />

Make sure you use the correct value from the table of values. As seen in this<br />

example, the value for liquid water is not the same as for gaseous water.<br />

Unless your instructor has told you to memorize specific thermochemical values,<br />

you will be provided the numbers you need. Most textbooks have a table of values<br />

in the chapter and an additional table in Appendix.

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