Question

Sodium acetate is used in heat packs due to its ability to crystallize and re-dissolve. how is enthalpy part of this reversible reaction ?

Answer 1

Upon crystallization, heat is RELEASED....

Explanation:

Sodium acetate can form a supersaturated solution, i.e. a solution that contains a GREATER than EQUILIBRIUM quantity of solute. And to make it we take a mass of sodium acetate crystals, moisten them with a quantity of water, and then give them a good blast with a heat gun to get all the sodium acetate crystals in solution.

And we could represent this by the reaction....

`H_3C-CO_2^(-)Na^(+)(s) +Deltastackrel(H_2O)rarrH_3C-CO_2^(-)Na^+(aq)`

The supersaturated sodium acetate forms a metastable solution. If we add a seed crystal to the supersaturated solution, the MASS of sodium acetate crystallizes out, along with the evolution of heat as the crystals forms....(i.e. the same heat that was put in when we formed the solution).

In the commercial heat packs there is a supersaturated solution of sodium acetate, and you break a capsule inside the pack to introduce seed crystals, and ALL the sodium acetate crystallizes, along with the evolution of heat.

And we could represent this by the reverse equation with respect to the former...

`"H"_3"CCO"_2^(-)"Na"^(+)(aq) stackrel("seed crystal")rarr"H"_3"CCO"_2^(-)"Na"^+(s)+Delta`

See here for more details.