Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 903105 • 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.)
Explanation / Answer
mass of water, m= density * volume = 1 g/mL*25 mL = 25 g
Heat absorbed,
Q= m*C*(Ti-Tf)
= 25*4.19*(25.8 - 21.9)
= 408.525 J
Since water's Temperature decreases, heat is abdorbed in the reactiomn.
So,
delta H will be positive
number of moles of NH4NO3 = Mass/molar mass
=1.25/80
Heat absorbed by 1.25/80 mol = 408.525 J
so,
Heat absorbed by 1 mol = 408.525 * 80/1.25 = 26146 J = 26.146 KJ
Answer: +26.146 KJ/mol