Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 795656 • 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 process, 1.25 g of NH4NO3 is dissolved in enough water to make 25.0mL of solution. The initial temperature is 25.8 Celcius and the final temperature (after the solid dissolves) is 21.9 Celcius.
Calculate the change in enthalpy for the reaction in kJ/mol. (use 1.0g/mL as the density of the solution and 4.18 J/g*Celcius as the specific heat capacity.)
Explanation / Answer
Moles of NH4NO3 = mass/molar mass of NH4NO3
= 1.25/80.0435 = 0.01562 mol
Mass of solution = volume x density
= 25.0 x 1.0 = 25.0 g
Heat absorbed by reaction = heat released by solution
= mass x specific heat x temperature change of solution
= 25.0 x 4.18 x (25.8 - 21.9) = 407.55 J
Enthalpy change = heat absorbed/moles of NH4NO3
= 407.55/0.01562
= 2.61 x 10^4 J/mol = 26.1 kJ/mol (or approximately 26 kJ/mol)