Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 976428 • 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.)
Express your answer to two significant figures and include the appropriate units.
Explanation / Answer
n = mol/MW = 1.25/80.05 = 0.015615
total mass = 1.25 + 25 = 26.25
Q = m*C*(Tf-Ti) = 26.25*4.18 * (21.9-25.8) = -427.9275 J
then
Qwin = -Qlost
Q = 427.9275 J
HRxn = Qlos/n = 427.9275 / 0.015615 = 27404.89J/mol = 27.404 kJ/mol