CHEM
Fall 2016.pdf

A4 2015 ls brown 4 the reaction shown below is used

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A4 © 2015 L.S. Brown 4. The reaction shown below is used to remove hydrogen sulfide from natural gas. 2 H 2 S(g) + SO 2 (g) 3 S(s) + 2 H 2 O(g) Δ H ° = –145.6 kJ Compound H f ° (kJ/mol) S ° (J/mol/K) G f ° (kJ/mol) H 2 S(g) –20.6 ? –33.6 SO 2 (g) –296.8 248.1 –300.2 S(s) 0 31.8 0 H 2 O(g) –241.8 188.7 –228.6 (6 pts) (a). S ° for the reaction above is –186.7 J/K. Find S ° for H 2 S(g). Δ S ° = –186.7 = 3 S ° (S) + 2 S ° (H 2 O) – 2 S ° (H 2 S) – S ° (SO 2 ) S ° (H 2 S) = 1 2 [3 S ° (S) + 2 S ° (H 2 O) – S ° (SO 2 ) + 186.7] = 1 2 [3(31.8) + 2(188.7) – (248.1) + (186.7)] = 205.7 J/K/mol (6 pts) (b). Calculate G ° for the reaction at the standard temperature of 298 K. (There are two possible ways that you could do this with the information given above.) We can do this from H and S , or from the G f ° values. I’ll do both here. Δ G ° = 3 G f ° (S) + 2 G f ° (H 2 O) – 2 G f ° (H 2 S) – G f ° (SO 2 ) = 3(0) + 2(–228.6) – 2(–33.6) – (–300.2) = –89.8 kJ Δ G ° = H ° – T S ° = –145.6 kJ –(298 K)(–0.1867 kJ/K) = –89.96 kJ

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NAME:_________________________________ © 2015 L.S. Brown A5 (Problem #4, continued) (7 pts) (c). At what temperatures would this reaction be thermodynamically spontaneous? Both H and S are negative, so the reaction will be spontaneous at low temperature but not high. To find the transition T we set G = 0 and solve for T. Δ G = Δ H T Δ S = 0 Δ H = T Δ S T = Δ H Δ S = –145.6 kJ –0.1867 kJ/K = 780 K So the reaction is spontaneous for T < 780 K
A6 © 2015 L.S. Brown (8 pts) 5. Use the data in the table below to estimate H ° for the following reaction between methane and chlorine. CH 4 (g) + 3 Cl 2 (g) CHCl 3 (g) + 3 HCl(g) Type of bond Average bond energy (kJ/mol) C—H 413 C—Cl 330 Cl—Cl 242 H—Cl 432 H = Σ (bond energies in reactants) – Σ (bond energies in products) = 4(C–H) + 3(Cl–Cl) – (C–H) – 3(C–Cl) –3(H–Cl) = 4(413) + 3(242) – (413) –3(330) –3(432) = –321 kJ (If you start with products minus reactants here, you would just flip all of the signs and get +321 kJ. That should have gotten you 4 points, as long as your numbers and units were all OK.)

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NAME:_________________________________ © 2015 L.S. Brown A7 6.
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