Posted by Christian on .
Give the enthalpy change and entropy change values shown below calculate the free energy change for this reaction at 25 degrees C using Gibbs free energy equation. How does this value compare to the free energy change using the standard free energy of formation data?
delta H=196.0 kJ and delta S=125.7J/K
Is the decomposition of hydrogen peroxide spontaneous at 25 degrees?
My answer: delta G= (196.0kJ)25(125.7J/K)
so the free energy change at 25 degrees is:2946.5 but I'm not sure what units to put on it.Then the decomposition of hydrogen peroxide at 25 degrees would be spontaneous because the free energy is negative.
First, the T must be put into the equation in Kelvin and note that delta H is listed in kJ (I presume for the rxn, as in 2 mols) while delta S is listrd in J; you can't add those two together without changing one or the other.
To put 25 degrees into Kelvin all I do is add 25 to 273 right? So them plug 298 into my equation instead of 25 but then how do I change delta H or delta S so I can add them together?
Was the first answer (the one this one is to be compared with) in kJ. I think so; therefore, let's change this to kJ. Delta H is already in kJ. S is in J/K, to change that to kJ, just divide S by 1000 and it will have the units kJ/K. Yes, add 273 to 25 for 298.
I used 273.16 (since S has that many s.f.) and delta G = 233.5 kJ from the data you have. I don't know how to compare this with the other since I don't know what they problem means "enthalpy change" and "entropy change"' that is, I don't know if those changes are for 1 mol or 2 mols. Anyway, I think the answer is that the reaction is spontaneous. Check my thinking. I don't think it will change the answer but you might want to check with the prof or another student and see how they interpret the 1 mol vs 2 mol thing.
Ok so my new equation is: delta G=(196.0)(298)(0.1257)
then the free energy change at 25 degrees is 233.4586 kJ.So compared with the previous equation the free energy change is the same because in my previous post the answer was 233.4kJ.And the reaction is still spontaneous. Can you check this for me? Thanks

Chemistry 
ben,
Right
firstly entropy is 125.7j/molk. Thus it is 0.1257kj/molk. Now try using the legendre relationship again an you should get an answer of 233.5KJ/mol, which is the same as you get if you calculate it with gibbs free energies of formation. Hope this helps  have fun with thermodynamics