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)ΔH=−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/(g⋅∘C)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.

Answer 1

moles of LiCl = mass / molar mass

                     = 23.9 / 42.4

                     = 0.564

ΔH=−36.9kJ / mol

ΔH    = - 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