Untitled - Kelly Walsh High School

256 CHEMISTRY FOR THE UTTERLY CONFUSED
Be Careful!
Be Careful!
Unlike heats of formation and Gibbs free energies of formation, the standard
entropy values for elements are not zero.
To finish this problem we need to use the following relationship:
For part a:
∆S ∆S products ∆S reactants
∆S [(1 mol H 2O)(188.7 J/molK)] [(1 mol H 2)(131.0 J/molK)
(1/2 mol O 2)(205.0 J/molK)]
∆S° 44.8 J/K
For part b:
∆S [(1 mol H 2O)(69.9 J/molK)] [(1 mol H 2)(131.0 J/molK)
(1/2 mol O 2)(205.0 J/molK)]
∆S 163.6 J/K
Many students make the mistake of only subtracting the first of the reactant
values from the product values. You must subtract the SUM of the reactant
values from the product values.
Determine the value of ∆G for the following reaction:
2 NH 4Cl(s) CaO(s) l CaCl 2(s) H 2O(l) 2 NH 3(g)
To begin the problem, we need the standard Gibbs free energy values for each
of the reactants and products. These are the values we’ll be using to complete
this problem: