Instant cold packs used to treat athletic injuries contain solid NH_4NO_3 and a
ID: 497877 • 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) MH_4NO_3 (aq) Delta H_rxn = + 25, 750 J/mol This reaction is: endothermic exothermic What is the final temperature, in degree C, in a squeezed cold pack that contains 49.9 g of NH_4NO_3 dissolved in 143 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.)Explanation / Answer
moles of NH4NO3 = 49.9 / 80.04 = 0.6234
delta Hrxn = 25,700 J/mol
delta H = - Q / n
25,700 = - Q / 0.6234
Q = - 16022.4 J
Q = m Cp dT
- 16022.4 = 143 x 4.184 x (Tf - 25)
Tf = - 1.78
final temperature = - 1.78 oC