Why do reusable heat packs have a limited time frame before the reaction becomes irreversible? I have two &quot;rechargeable&quot; 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?
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.