Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 573054 • 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)NH4+(aq)+NO3(aq) In order to measure the enthalpy change for this reaction, 1.40 g of NH4NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.5 C and the final temperature (after the solid dissolves) is 19.5 C. Calculate the change in enthalpy for the reaction in kJmol1. (Use 1.0gmL1 as the density of the solution and 4.184Jg1C1 as the specific heat capacity.)
Explanation / Answer
moles of NH4NO3 = mass / molar mass = 1.40 g / 80 g/mol = 0.0175 mol
volume of the solution = 25 mL
density of the solution =1.0 g/mL
mass of the solution = density x volume = 25 g
T1 = initial temperature = 25.5 C
T2 = final temperature = 19.5 C
temperature change = dT = T2 - T1 = 19.5 - 25.5 = -6 C
Specific heat (S) = 4.184Jg1C1
qrxn = m x S x dT = 25 x 4.184 x -6 = -627.6 J
Enthalpy of the reaction = -qrxn / n = 627.6 J / 0.0175 mol = 35.86 kJ/mol <<<<<<---------(ANSWER)