Question

1. Why is it not possible to reuse a heat pack in term of the chemistry?

2. Compare the specific heat of water to the specific heat of metal (Copper). Which would heat up faster (with less energy required)?

3. Why would metal make a poor ingredient in a heat pack?

4. If there was a delay between measuring the initial temperature of a hot object and its transfer to the calorimeter, how would the heat capacity of the object be affected? How would the calculation of the heat capacity of the calorimeter be affected (too high, too low, or no affect)?

Answer 1

Answer 1

Heat packs are those materials which generates heat when used. Disposable chemical heat packs are those which contains water and salt (such as CaCl2) present in two compartments separated by a barrier. When pressure is applied by hand, the barrier ruptures and salt dissolves in water. The dissolution of calcium chloride is an exothermic reaction and generates heat.

As dissolution is an exothermic and spontaneous reaction, it cannot be reversed to crystallization of salt. So, the heat packs are non reusable.

Answer 2

Specific heat of copper is less than that of water. So, water requires more heat to raise its temperature by 1 degree celsius.

Answer 3

Metals have low specific heat. So, these will not retain heat for longer time and will get cooled faster. Hence, it is not used in heat packs.

Answer 4

If there is delay between observing the temperature of hot object and its transfer to calorimeter, some of the heat would be lost by the object.

So, final equilibrium temperature will be less than actual.

This will lead to lower calculated specific heat of the metal.

Heat capacity of calorimeter is equal to heat absorbed per unit temperature.

As the heat absorbed by calorimeter is less than that used in calculation, then heat capacity of calorimeter will decrease. So, it will be too low.