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I have two "rechargeable" heat packs which from my understanding operate on the phase change

$$ CH_3COONa_{(aq)} \rightleftharpoons CH_3COONa_{(s)} + \Delta H.$$

Triggering the crystallization releases heat, and leaving them in boiling water returns it to its supersaturated form. The packs say if they are left in solid form for more than two months, they may not be able to return to aqueous form. I left one solid for many months, and sure enough, I couldn't quite redissolve the crystals. It gets 90% dissolved, but the remaining crystals just trigger the forward reaction as the pack re-cools, even after multiple attempts. What mechanism prevents this?

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  • $\begingroup$ Writing sodium acetate as NaCH3COO is quite unusual, even if not necesserily wrong. The usual way is CH3COONa, following rather the organic way of writing structure formula than the inorganic ways to have the metal first. For the latter, it has the metal first, as there is the acidic hydrogen first. For CH3COOH, the acidic hydrogen is the last. It would be more correct, if the summary formula was used, instead of the structural one, like NaC2H3O2. $\endgroup$ – Poutnik Nov 24 '20 at 9:42
  • $\begingroup$ Ryan - Exactly how did you try to "redissolve the crystals"? $\endgroup$ – MaxW Nov 24 '20 at 19:57
  • $\begingroup$ @MaxW wrapped in a cloth, boiling them in water for 15 minutes. Those are the instructions on the product $\endgroup$ – Ryan Nov 24 '20 at 19:58
  • $\begingroup$ Ryan - Couple of followup questions. (1) Are you reasonable close to sea level? (ie not in mountains at 10,000 feet.) (2) You boiled them for 15 minutes after the water + pack started to boil? (ie you didn't start timing when you added cold pack to boiling water...) $\endgroup$ – MaxW Nov 24 '20 at 20:04
  • $\begingroup$ @MaxW yeah I’m at sea level. I dropped them in cold, but actually, one time I left them in the pot and forgot about them for like an hour and most of the water boiled away and the one bad one still didn’t fully revert. So I think it was thoroughly heated... $\endgroup$ – Ryan Nov 24 '20 at 20:09
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The sodium acetate that shows this behavior is not anhydrous $\ce{NaCH3COO}$ as you write. It is the hydrated salt $\ce{NaCH3COO·3H2O}$. The anhydrous salt does not show the phenomena of surfusion. Only the hydrated form does it. If you keep the solid form of the hydrated salt a long time, I think it will slowly loose water by evaporation through the plastic bag. So it will be partly transformed into anhydrous acetate. And by heating the whole system, it will never melt entirely. So it may never return to its supersaturated form. It is necessarily to add a few drops water to get supersaturation again.

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    $\begingroup$ It is trihydrate $\ce{CH3COONa . 3 H2O}$, not dihydrate. $\endgroup$ – Poutnik Nov 24 '20 at 9:39
  • $\begingroup$ Are not the packs supposed to be heated in water? Would storage in a humid place preserve the pack? $\endgroup$ – Alchimista Nov 24 '20 at 11:32
  • $\begingroup$ @Poutnik, You are right. It is a trihydrate. I am sorry. $\endgroup$ – Maurice Nov 24 '20 at 17:12
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    $\begingroup$ Huh, not sure how this works out. The pack surely contains residual unbound water. Do you have a reference that the hydrate is unstable at RT in water? $\endgroup$ – Karl Nov 24 '20 at 19:35
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    $\begingroup$ It doesn't seem that the manufacturer would choose a plastic that had a high permeability for water. However planned obsolescence isn't just a baseless conspiracy theory. $\endgroup$ – MaxW Nov 24 '20 at 20:10
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Why do reusable heat packs have a limited time frame before the reaction becomes irreversible?

This reaction should never become irreversible until and unless there is bacterial growth or loss of water for some reasons. For example, one manufacturer explains that the pads can re-used for 18 years! http://www.thermo-pad.com/frequently-asked-questions/

We normally think that plastics are impermeable to gas exchange or other small molecules. They are not just like our skin. Do remember that a balloon eventually gets deflated with time? Moisture is no exception.

Search Moisture permeability of polymers and you will find 100s of articles.

How many times can they be used?

There is no limit as to the number of times a Thermo-Pad can be used. There are pads still in use after eighteen years. The pad will not operate if there is a hole in the vinyl. Thermo-Pad is manufactured from the same vinyl as is used in water beds and comes with a one year warranty.

What are the white crystals that form in the liquid pad?

If the Thermo-Pad has been stored in its crystallized state for an extended period of time, a small amount of water can evaporate from the pad and cause the solution to become more concentrated. This higher concentration is indicated by “snowflake like” crystals in the liquid pad. The pad will still operate but at a slightly higher temperature. This water loss can be substantially reduced by storing the pad in its liquid state and may be reversed by extending the boiling times and by leaving the pad in water to cool.

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