Question

1. What is the difference between the heat of reaction, Qrxn, and the enthalpy of reaction, Hrxn? (

2. In a coffee-cup calorimeter, 1.40 g of NH4NO3 is mixed with 71.0 g of water at an initial temperature of 25.00 oC. After dissolution of the salt, the final temperature of the calorimeter content is 23.39 oC. Assuming the solution has a heat capacity (Cp) of 4.18 J/ oC•g and assuming no heat loss to the calorimeter (surroundings), calculate the enthalpy change for the dissolution of NH4NO3 in units of kJ/mol. (MW(NH4NO3) = 80.05 g/mol)

3. How do cold packs and hot packs work? What kind of thermodynamic process is involved in a hot pack?

4. Who uses hot packs and cold packs? What kind of process is involved in a cold pack?

5. What is the chemical that is used in hot packs that use a super cooled fluid? Why is that chemical a good choice for use to manufacture a hot pack?

Answer 1

1. No difference between the heat of reaction and enthalpy of reaction, both are same. By definition, enthalpy is the heat content.

2. As the final temperature is lower than the initial temperature; it is an endothermic reaction.

Heat absorbed = mass of water * heat capacity of solution* change in temperature

                        = (71.0 g)(4.18 J/^oC.g)(23.39 ^oC - 25.00 ^oC)

                         = -477.82 J

                         = -0.47782 kJ

Moles of NH4NO3 = mass / molar mass

                               = 1.40 g / (80.04 g/mol)

                               = 0.0175 mol

Hence, enthalpy change of the recation, Hrxn = 0.47782 kJ / 0.0175 mol =   27.3 kJ/mol

3. A cold pack contains a chemical and water, when seal is opened and shake the pack, the chemical in the pack mixes dissolve in the ware taking heat from the water the cools the pack. Heat absorption takes place in cold pack.

A hot pack contains a chemical and water, on shaking, the chemical dissolve in water and release heat, warms up the pack. Heat evolved in hot pack.

4. Athletes frequently use the cold packs and hot packs. In cold pack, heat absorbed from the water in the pack and cools the pack

5. Sodium acetate one of the chemical uses in hot packs. This solid can absorb water undergo hydration; this hydration is a exothermic process.