Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 798316 • 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)+NO?3(aq)
In order to measure the enthalpy change for this process, 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 kJ/mol. (Use 1.0g/mL as the density of the solution and 4.18J/g??C as the specific heat capacity.) Calculate the change in enthalpy for the reaction in kJ/mol. (Use 1.0g/mL as the density of the solution and 4.18J/g??C as the specific heat capacity.)
Explanation / Answer
Mass of solution = volume x density of solution
= 25.0 x 1.0 = 25 g
Heat absorbed by reaction = heat released by solution
= mass x specific heat x temperature change of solution
= 25 x 4.18 x (25.8 - 21.9) = 407.55 J
Moles of NH4NO3 = mass/molar mass of NH4NO3
= 1.25/80.04 = 0.01562 mol
Enthalpy change of reaction = heat absorbed/moles of NH4NO3
= 407.55/0.01562
= 2.61 x 10^4 J/mol = 26.1 kJ/mol