Spontaneity

[Kittenz]

I seem to be confused with Spontaneity and overall thermodynamics at this point. I think it's due to the fact that we use enthalpy synonymously as heat. I understand that if ΔG is negative, the reaction is always spontaneous. Also meaning it is exothermic favoring the products. But if a reaction gives a negative ΔH, it is exothermic, releasing heat and also spontaneous. Is this ΔH also like the enthalpy like we see in the following equation and Spontaneity cases? If so, how is that we can ever have a reaction with negative enthalpy and be non-spontaneous at high temperatures like what we see below in the 4th case? Thanks!

using ΔG= ΔH - T ΔS, we see the cases:

- ΔH & + ΔS = Spon @ all T

+ ΔH & - ΔS = Non-Spon @ all T

+ ΔH & + ΔS= Spon @ High T

- ΔH & -ΔS = Spon @ Low T

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[derbear]

A (-H) minus a larger (-S) due to a high temperature will result in a positive number, thus being spontaneous. By minimizing temperature, the magnitude of the (-S) will be less thus allowing a possible (-G) and a spontaneous reaction. Vice versa for the 3rd case.

 

[Kittenz]

I understand the equations, but I'm wondering how exothermic reactions can always be classified as spontaneous. Exothermic reactions, as I understand, are defined as releasing heat and giving a - ΔH. But at high Temperatures, a - ΔH doesn't necessarily mean a - ΔG, and so they can differ. And since ΔG seems to be the actual verdict of spontaneity, exothermic reactions are then not always spontaneous?

 

[derbear]

No they are not always spontaneous, you have to take into account ΔS. Exothermic and endothermic will give you the sign of ΔH, that's it.

 

[discensdentibus]

Exothermic is often called thermodynamically favorable because your products are in a lower energy state. Typically, I believe energy diagrams are labeled with gibbs free energy anyway. An example to your question is condensation 🙂
Think for a sec: condensation goes from a gas to a liquid (negative entropy) and releases heat (exothermic). Remember that gases behave more ideally at high T, and if a gas gets what it wants going back down to liquid phase is not preferred. Hope this helps. Try tying in concepts together, and it will stick. Hope this helps