Instant cold packs used to treat athletic injuries contain solid NH_4NO_3 and a
ID: 497990 • Letter: I
Question
Instant cold packs used to treat athletic injuries contain solid NH_4NO_3 and a pouch of water. When the pack is squeezed, the pouch breaks and the solid dissolves, lowering the temperature because of the reaction: NH_4No_3(s) rightarrow_H_2O NH_4NO_3(aq) Delta H_rxn = +25, 700 J/mol (a) This reaction is: endothermic exothermic (b) What is the final temperature, in degree C, in a squeezed cold pack that contains 34.2 g of NH_4NO_3 dissolved in 131 g of water? (Assume that the solution has the same specific heat as that of water, an initial temperature of 25.0 degree C, and no heat transfer between the cold pack and the environment.) Number degree CExplanation / Answer
moles of NH4NO3 = 34.2 / 80.04 = 0.427
delta H = 25700 J / mol
Q = - 25700 J / mol x 0.427 mol
Q = - 10981.3 J
Q = m Cp dT
-10981.3 = 131 x 4.184 x dT
dT = -20.03 oC
T2 - T1 = -20.03 oC
T2 -25 oC = -20.03 oC
T2 = 4.96 oC
final temperature = 4.96 oC