Instant cold packs used to treat athletic injuries contain solid NH_4NO_3 and a
ID: 957824 • 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_4 NO_3 (aq) This reaction is: endothermic exothermic What is the final temperature, in ^degree C, in a squeezed cold pack that contains 44.2 g of NH_4NO_3 dissolved in 146 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
a) the reaction is endothermic
because delta H reaction is positive .
b)
moles of NH4NO3 = 44.2 / 80 = 0.5525
delta H = 25,700 J/mol
heat energy Q = n x delta H = 0.5525 x 25,700 = -14199.25 J
Q = m Cp dT
-14199.25 = 146 x 4.18 x (Tf - 25)
Tf = - 1.73
final temperature = - 1.73 oC