Please explain your answer Estimate the amount of heat that will be released by
ID: 503278 • Letter: P
Question
Please explain your answer Estimate the amount of heat that will be released by a rechargeable heat pack that uses the crystallization of a supersaturated solution of sodium acetate anhydrate? The solution volume is 50.0 ml. The solution is saturated at 95 degree C. The solution will typically cool to 40 degree C before people will stop holding the heat pack. The solution must be supersaturated at 25 Degree C. Delta H_hox = 19.7 kJ/mol. FW = 136.079 g/mol The solubility of sodium acetate trihydrate increases with temperature as shown in the graph below.Explanation / Answer
the heat pack contains a supersaturated solution of sodium acetate in water. Such heat packs are always stored in solution form. they are heated in boiling water and then allowed to cool to room temperature (RT). the supersaturated soluition is made by heating the saturated solution to higher temperature and then allowed to cool slowly after addition of more salt till the heated solution keeps on dissolving it. the extra sodium acetate or any other salt that dissolved at higher temperature, crystallises out when the solution cools down.
the solution remains in solution form untill crystallization is initiated by any force. in a heat pack, a metal disk provides this energy and initiate crystallization and thus heat is given out as the crystallization proceeds.
this is reversible process that means if you heat up the crysatllised pack it will go back to soluition form. the heat absorbed by crysatllised heat pack to get back into solution form is released when the solution is cooling down to recrysallization.
this amount of, q, is calculated by following formula,
q= m*T*Cp
where, m is the mass of the solvent in gm (here, solvent is water)
T is the change in temperature in kelvin (here, 95°C - 40°C=55°c= 328 K)
Cp is the specific heat capacity of the solvent (Cp of water is 4.18 J/g.K)
putting all the values,
q= m*T*Cp
q= 50*328*4.18 = 11.5 KJ
the heat given by the heat pack is about 11.5 kJ