Re: How could sodium acetate & water be used for both heating &cooling?

There is a lot of information in your question/explanation! And I, too, am a bit of the "teacher that everyone goes to with strange questions". Indeed, a couple of years ago, one of my first year students used to spend time between classes trying to think up strange questions to ask!

But yours is not such a strange question. However, it does confound two different ideas - sortof. The phenomena involved in both cooling and heating with sodium acetate are essentially the same but they come about in different ways.

Dissolution of sodium acetate in water is an endothermic process. That is, it requires the addition of heat. If you take a lump of solid sodium acetate and add it to a beaker of water, the water will get colder. This is because the heat that is required to break up the sodium acetate must come from somewhere and the water is the only immediate source.

This is the same phenomena that you were observing with ammonium acetate and can be seen with a number of salts. Endothermic dissolution is quite common and is a simple physical transformation. Personally, my favorite is an endothermic reaction between barium hydroxide octahydrate and ammonium thiocyanate. It is a bit stinking as it releases some ammonia but when you mix the two solids and stir them up, they give a solution that is super cold - enough to readily freeze water!

But to get back to your question, if it takes heat going into the solid for it to dissolve into the water, then when the solid reforms from the solution, that heat must be given off. After all, heat is a conserved quantity. So, if you were to take your solution of sodium acetate and let it evaporate, over time it would release all of the heat that was added to it to make it into a solution.

This is what the hot pads do with a few modifications. They use supercooled solutions and they require an initiating action but in essence, all they are doing is releasing the heat that was added by the surroundings during the process of dissolving the sodium acetate. The commercial versions of this that I have seen work by boiling the sack with the sodium acetate in water where the higher temperature results in a much different solubility for the sodium acetate. It is the heat of the boiling water that ultimately is tied up in the resulting solution, but it is still the heat that was added upon dissolving the sodium acetate.

The supercooled part is critical to making it work as the solution has to be sufficiently concentrated that it is well beyond its solubility limit at room temperature. Agitation then ensures that the crystalline sodium acetate will reform and that the heat will be released.

Hope that this helps explain what is going on.