Instant hot packs contain a solid and a pouch of water. When the pack is squeeze
ID: 550022 • Letter: I
Question
Instant hot packs contain a solid and a pouch of water. When the pack is squeezed, the pouch breaks and the solid dissolves, increasing the temperature because of the exothermic reaction. The following reaction is used to make a hot pack: LiCl(s)Li+(aq)+Cl(aq)H=36.9kJ
What is the final temperature in a squeezed hot pack that contains 23.9 g of LiCl dissolved in 139 mL of water? Assume a specific heat of 4.18 J/(gC)for the solution, an initial temperature of 25.0 C, and no heat transfer between the hot pack and the environment.
Express your answer with the appropriate units.
Explanation / Answer
moles of LiCl = mass / molar mass
= 23.9 / 42.4
= 0.564
H=36.9kJ / mol
H = - Q / n
-36.9 = - Q / 0.564
Q = 20.8 kJ
Q = m Cp dT
20.8 x 10^3 = 139 x 4.18 x dT
dT = 35.8
T2 - T1 = 35.8
T2 = 35.8 + 25
T2 = 60.8 oC
final temperature = 60.8 oC