Instant cold packs, often used to ice athletic injuries on the field, contain am
ID: 901644 • 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
solution voluem = 25 ml
solution density = 1.0 g / ml
solution mass = volume x density
= 25 x 1
= 25g
m = 25 g
Cp = 4.184 J / gC
dT = 25.8 -21.9 = 3.9 oC
q = m Cp dT
= 25 x 4.184 x 3.9
= 407.94 J
= 0.408 kJ
moles of NH4NO3 = 1.25 / 80 = 0.0156
heat transfered = 0.408 / 0.0156 = 26.15 kJ /mol