Another Question about Quiz 2014 #9

[Douglas Dean]

I understand most of the problem and how to get the right answer, you use deltaG = deltaH - TdeltaS, but how do you know that deltaH ends up being a positive value? The value given is negative, thats why it confuses me. Thanks :D

[Chem_Mod]

Hi,
Please post the question in full

[Douglas Dean]

Hi,
Please post the question in full

Sorry about that!

9. Calculate DeltaGr for the balanced reaction showing the decomposition of mercury (II) oxide at 298K.

HgO(s) --> Hg(l) + O2(g)
DeltaHf in kj/mol: -90.83
S in j/kmol: 70.29 76.02 205.14

[Michael Okene 3B]

Hi Douglas,

To find the enthalpy change, it's products minus reactants, so it would look like this
0+0 - (-90.83) = 90.83
So deltaH = +90.83 kJ/mol

[104607508]

I understand the formula for the problem but when doing the summation of the deltaS it calls for the summation of the compound minus the reactants but in this problem the compound is the reactant so how would you go about doing the calculations? Thanks

[Douglas Dean]

Thanks so much Michael! That makes sense, I was just using deltaH not considering it as a reactant.

And for the other question, I'm pretty sure deltaS refers to the summation of the entropy of the products minus the summation of the entropy of the reactants. Thus in this problem, you would do deltaS= 205.14 + 76.02 x 2 - ( 70.29 x 2).