Chapter 9, Problem 70E

Instant cold packs 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 endothermic reaction:
${\text{NH}}_{\text{4}}{\text{NO}}_{\text{3}}\left(\text{s}\right)\to {\text{NH}}_4^{+}\left(\text{aq}\right)+{\text{NO}}_3^{-}\left(\text{aq}\right)$
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 kJ. (Use 1.0 g/mL as the density of the solution and 4.18 J/g • °C as the specific heat capacity.)