[hupy90]

4. Use values of the Gibbs free energy change of formation, given below, to deduce whether the cis– or trans– isomer of but-2-ene is the more stable at 25 °C. At what temperature will the two isomers have the same stability?

I've been stuck on this question for a while. For part 1 is it trans is more stable because it has a lower enthalpy change and therefore lower energy/more stability? And for part 2 what is the difference (in the data provided) between Gf, Hf, and S and the corresponding letters in the formula ΔG = ΔH - TΔS? I understand one is formation and the other is the change, but is there a way to go from one to the other, or are they equal? I attempted plugging them in with 25 degrees (298K) but it doesn't work out. I've attempted re-arranging for T and combining the formulas (When T = T) but it hasn't worked out. Any help would be greatly appreciated! Thanks!

Data:

                             ∆Gf (kJ mol–1) ∆Hf (kJ mol–1) S (J mol–1K–1)

cis–but-2-ene               67.1                 –5.7                  301

trans–but-2-ene           64.1                 –10.1                 296

[mjc123]

For part 1 is it trans is more stable because it has a lower enthalpy change and therefore lower energy/more stability?

Read the question: "Use values of the Gibbs free energy change of formation..." Why do you use ΔG rather than ΔH?

For part 2: consider the reaction cis  trans. What are the values of ΔG (at 25°C), ΔH and ΔS for this reaction? What is the criterion for the two isomers having the same stability?

[hupy90]

Read the question: "Use values of the Gibbs free energy change of formation..." Why do you use ΔG rather than ΔH?

For part 2: consider the reaction cis  trans. What are the values of ΔG (at 25°C), ΔH and ΔS for this reaction? What is the criterion for the two isomers having the same stability?

Thanks! Just a few clarifications/checks.

For part 1: I see I misread the question there, but I'm not entirely sure as to why you would use ΔG rather than ΔH. Is it, similarly to what I said, the trans isomer has a lower required free energy to initialize the reaction (Due to ΔGf being positive)?

For part 2: If I look at the cis  trans reaction and I find the values for ΔG, ΔH, ΔS and plug them into ΔG = ΔH -TΔS, and I rearrange for T (since at a certain unknown temperature they will have equal stability) I get 280K. Does that seem correct (No answers to the questions in the booklet) or have I misinterpreted something else? Also, (for my knowledge) how does this reaction (if correct) imply equal stability, is there some way to equate them (Like ΔGf = ΔGf₂ for equal stability and solve for T) or is this the only method (Once more, if correct)

Thanks you very much!