Physics, Astronomy & Cosmology / Re: Is there a substance where entropy decreases at higher temperature?« on: 19/12/2022 05:10:37 »
It does seem to hinge around whether a supercooled liquid, or @chiralSPO 's suggestion of any chemical reaction that is only proceeding imperceptibly slowly, really is in an equilibrium state. Certainly in the case of a slow reaction, it isn't.
I wouldn't call either of these cases as being in an equilibrium state. I suppose we could say that an uninitiated heat pack could be in thermal equilibrium with another system (ie when it's on the shelf, it should have the same temperature as the surrounding air, and will roughly track the temperature of the air as it rises and falls during the day, absorbing and releasing heat to "re-equilibrate" as long as the energy involved is not enough to disrupt the metastable state and kickstart the transition.)
As far as "very slow" vs 0 reaction rate goes... Because molecules are discrete there can be reactions that are not strictly impossible (even thermodynamically favorable), but are still so improbable (at a given temperature) that even with moles of interacting molecules the molecular reactions would not take place even once in millions of years (or any arbitrary timeframe). But increase the temperature by a couple hundred degrees, and suddenly it's a fast reaction (temperature is usually a term in the exponent of the rate equation).
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