Untitled - Kelly Walsh High School

258 CHEMISTRY FOR THE UTTERLY CONFUSED
Quick Tip
Don’t Forget!
The H f (and the G f) for elements in their standard states are zero. It is not
necessary to search for these values. If you are in doubt, you can look for the
values in a table.
We need to use the following relationships before using the designated equation:
Solving these two relationships gives:
∆H ∆H products ∆H reactants
∆S ∆S products ∆S reactants
∆H [(1 mol Al 2(SO 4) 3)(3441 kJ/mol)] [(2 mol Al)(0 kJ/mol)
(3 mol S)(0 kJ/mol) (6 mol O 2)(0 J/molK)]
∆H 3441 kJ
∆S [(1 mol Al 2(SO 4) 3)(239 J/molK)] [(2 mol Al)(28.32 J/molK)
(3 mol S)(31.88 J/molK) (6 mol O 2)(205.0 J/molK)]
∆S 1143.28 J/K (unrounded)
We can now enter these results into the given equation. We will also need a
kJ/J unit conversion.
∆G ∆H rxn T∆S rxn
∆G 3441 kJ (298 K)(1143.28 J/K)(1 kJ/10 3 J)
∆G 3100.30 3100. kJ
The degree symbol indicates standard conditions. This means you should use
298 K as the temperature.