Instant cold packs used to treat athletic injuries contain solid NH_4NO_3 and a
ID: 497833 • 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: This reaction is: What is the final temperature. in C, in n squeezed cold pack that contains 32 1 g of NH_4NO_3 dissolved in 1.50 times 10^2 g of water? (Assume that the solution has the same specific heat as that of water. an initial temperature of 25 0 C. and no heat transfer between the cold pack and the environment.)Explanation / Answer
The cold pack absorbs heat, dH for the reaction is positive.
(a) Thus the reaction is : endothermic
(b) using,
q = -dH = mCpdT
with,
dH = 25,700 J/mol
moles of NH4NO3 = 32.1 g/80.043 g/mol = 0.40 mols
dH = 25,700 x 0.40 mol = 10306.6 J
m = (1.50 x 10^2 + 32.1) g
Cp = specific heat of water
dT = Tf - 25
Tf = final temperature
thus,
-10306.6 = (1.50 x 10^2 + 32.1) x 4.184 x (Tf - 25)
Tf = 11.47 oC