Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 1010375 • Letter: I
Question
Instant cold packs, often used to ice athletic injuries on the field, contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the following endothermic reaction:
NH4NO3(s)NH+4(aq)+NO3(aq)
In order to measure the enthalpy change for this reaction, 1.25 g of NH4NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 C and the final temperature (after the solid dissolves) is 21.9 C .
Calculate the change in enthalpy for the reaction in kilojoules per mole. (Use 1.0g/mL as the density of the solution and 4.18J/gC as the specific heat capacity.)
Explanation / Answer
Volume of the solution = 25ml, density = 1 g/ml, mass of the solution = 25*1= 25gm
Change in enthalpy = mass* specific heat* temperature difference= 25*4.18*(25.8-21.9)=407.55 Joules
since the reaction is endothermic, enthalpy change is +ve. This much heat is absorbed due to 1.25 gm of NH4NO3
Molecular weight of NH4NO3= 14+4+14+48= 80
moles of NH4NO3= 1.25/80 =0.0156 moles
Change in enthalpy per mole = 407.55/0.0156 = 26083.2 J/nole= 26.083 Kj/mole