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Untitled - Kelly Walsh High School

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256 CHEMISTRY FOR THE UTTERLY CONFUSED<br />

Be Careful!<br />

Be Careful!<br />

Unlike heats of formation and Gibbs free energies of formation, the standard<br />

entropy values for elements are not zero.<br />

To finish this problem we need to use the following relationship:<br />

For part a:<br />

∆S ∆S products ∆S reactants<br />

∆S [(1 mol H 2O)(188.7 J/molK)] [(1 mol H 2)(131.0 J/molK)<br />

(1/2 mol O 2)(205.0 J/molK)]<br />

∆S° 44.8 J/K<br />

For part b:<br />

∆S [(1 mol H 2O)(69.9 J/molK)] [(1 mol H 2)(131.0 J/molK)<br />

(1/2 mol O 2)(205.0 J/molK)]<br />

∆S 163.6 J/K<br />

Many students make the mistake of only subtracting the first of the reactant<br />

values from the product values. You must subtract the SUM of the reactant<br />

values from the product values.<br />

Determine the value of ∆G for the following reaction:<br />

2 NH 4Cl(s) CaO(s) l CaCl 2(s) H 2O(l) 2 NH 3(g)<br />

To begin the problem, we need the standard Gibbs free energy values for each<br />

of the reactants and products. These are the values we’ll be using to complete<br />

this problem:

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