Question

How would you exactly start the first problem? My thermodynamic skills is a bit rusty. I am assuming I have to use ΔH°f under the given reaction..? But then..we barely covered ΔG° in class. Am I to use this equation, ΔG° = ΔH° - T ΔS°? Umm..can someone explain in a detailed process how to go about this problem? Also for problem #2, I don't really understand what the question is asking for.


1. Consider the following reaction and thermodynamic data:

Sr(s) + H2O(g) --> SrO(s) + H2(g)

Substance________ΔH°f (/kJ/mol)________S° (J/mol K)
Sr(s)____0____52.3
H2O(g)____-241.8____188.7
SrO(s)____-634.9____54.4
H2(g)____0____130.6


A) What is ΔG° at 500.0 K for the above reaction?

B) Calculate the ΔG at 500.0 K if the reaction is run with PH2O=5.0 atm and PH2=1.0 atm.


2. An ideal fuel for the jet of a space vehicle should decompose with

A) ΔG < 0 and ΔH < 0
B) ΔG < 0 and ΔH > 0
C) ΔG > 0 and ΔH < 0
D) ΔG > 0 and ΔH > 0
E) ΔG = 0 and ΔH = 0

Answer 1

Yes, for #1 use dGo = dHo - TdSo. That will give you dGo. Then if you assume dH = dHo and dS = dSo and use T = 500K.

Answer 2

I think for #2 you are to use Keq expression with dG which is dG = -RTlnK. and go from there. So you know R, and T,
K = p(H2)/p(H2O) substitute and solve for dG.

Answer 3

What I stated as answer for #2 actually is B of #1.

Answer 4

Thank you DrBob22. :) But for #1, am I looking for dGo for H2(g)?

Answer 5

Or calculate dHo and dSo at standard enthalpies of formation and entropies?