Explaining why some reactions are spontaneous at certain temperatures

AI Thread Summary
The discussion centers on understanding spontaneity in chemical reactions, specifically through the Gibbs free energy equation ΔG = ΔH - TΔS. A key point is that a spontaneous reaction requires a negative ΔG, which depends on the signs of ΔH and ΔS and the temperature (T). Participants suggest plotting ΔH, -TΔS, and their sum to visualize how these components interact to determine spontaneity. Additionally, plotting temperature against the sum of these values is proposed as a way to understand the relationship better. The conversation highlights the importance of memorizing example reactions to illustrate different types of spontaneity.
ampakine
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On an exam I got this question:
[PLAIN]http://img849.imageshack.us/img849/5732/spontaneity.png
but at the time I had no idea how to answer it. I still don't really know if I can answer it. I can explain it mathematically with the equation: ΔG = ΔH - TΔS seeing as a spontaneous reaction requires a negative change in G and therefore if the sign of ΔH and ΔS are different then temperature will be a determining factor as to whether or not ΔG will be negative. I have no idea what I would need to plot on a graph though. As for examples of reactions of each type I suppose I needed to have a few example reactions memorised.
 
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ampakine said:
I have no idea what I would need to plot on a graph though.

How about plotting ΔH, -TΔS, and their sum?
 
Oh yeah didn't think of that. I could also plot T vs. their sum.
 

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