Exothermic Crystallization

[X-Wulf]

I'm sure most of you have seen those small liquid-filled plastic packets with a metallic clicker in them.
When the clicker is clicked, the liquid begins to crystallize in an exothermic reaction (and thus these devices are used as hand warmers etc.) from the clicker outwards.
After the solid has then cooled, it can be placed into the microwave, liquified, and the process repeated.
Does anyone know what exactly is in these packets? I've heard that a large proportion of it is MSG (monosodium glutamate), but a super-saturated msg solution does not work. Therefore there must be one or more ingredients missing.

Any ideas?

[Pu239 Stuchtiger]

How do those heat packets work? What happens is when the packet is appropriately shocked, the dissolved sodium glutamate crystalizes out of solution; this is a transition from a less stable state to a more stable state; therefore heat is released. With your super-saturated solution, the problem is that the crystals that were not able to be dissolved provide a starting point for the crystalization; therefore, the heat is slowly released over a period of time, not over a second or so. This would be perceived simply as the solution taking an unusually long period of time to cool.

[wantsomfet]

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[Azazel]

The forward reaction is exothermic... but when warmed up the reaction is forced backwards, producing the original reactants as products. It continues in a cycle... when the reactants release energy, chemical bonds are broken and heat is released in this process.. when it is reheated the products gain enough energy to form those bonds again...

is that correct ? boh

[kingspaz]

Azazel, i beleive you are slightly wrong but have the right idea <img border="0" title="" alt="[Wink]" src="wink.gif" />
as the stuff comes out of solution it makes bonds, releasing energy.
when heated up again enough energy is given to break the bonds which previously formed. this restores it to its original state.

[Azazel]

lol thought it was like that
ahhhhhhhhh :( {me stresses} i wish i knew everything... it would make uni exams so much easier

ahhhh sigh :o

[Pu239 Stuchtiger]

"Bonds" really aren't formed... it's more like heat of fusion. I prefer to refer to what occurs in the topic of this thread as "heat of crystalization" because it really isn't the same as "typical" examples of heat of fusion.