Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 986679 • 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
For solution, mass = volume * density = 25.0 mL * 1.0 g/mL = 25 g
s = 4.18 J/g-0C, T = 21.9 - 25.8 = -3.9 0C
For the reaction,
H = -(energy needed for the reaction) = -(msT) = -(25 * 4.18 * (-3.9)) J = 0.41 kJ