Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 509030 • 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: NH_4NO_3(s) rightarrow NH^+_4 (aq) + NO^-_3 (aq) In order to measure the change in enthalpy for this reaction, 1.25 g of ammonium nitrate is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 degree C and final temperature (after the solid dissolves) is 21.9 degree C. Calculate the change in enthalpy for the reaction in kJ.Explanation / Answer
Mass of solution = 25.0 mL = 25.0 g.
Specific heat of solution = 4.184 J/g.oC
Initial temperature = 25.8 0C
Final temperature = 21.9 0C
Therefore,
q = m * s * (t2 - t1)
q = 25.0 * 4.184 * (21.9 - 25.8 )
q = - 407.9 J
Mass of ammonium nitrate = 1.25 g.
No.of moles of ammonium nitrate = mass / molar mass = 1.25 / 80.043 = 0.0156 mol
therefore,
change in enthalpy, delta H = q / n = 407.9 / 0.0156 = 26147 J/mol = 26.1 kJ/mol
26.1 kJ of heat has been absorbed.