Question

During soccer practice one of your teammates sprains her ankle. You take an "instant cold pack" from the first aid kit to use on her ankle. It is not at all cold when you take it out. The instructions on the cold pack tell you to punch the pack so that you break open a sake of chemicals inside it. When you do that, you notice that the cold pack rapidly becomes quite cold. What happened?

Select one of these answers:

a. The cold pack forcefully expelled heat into its surroundings, which lowered its temperature

b. By punching the pack you gave the chemicals inside the energy they needed to permit the cooling down

c. Thermal energy was used up in forming chemical bonds, which made the cold pack colder

d. A chemical reaction produced cold as one of its products, which cooled down the cold pack

e. More than one of these

f. None of these

Answer 1

When the seal inside an instant cold pack is broken, an endothermic reaction occurs, absorbing heat from the environment and causing the pack to become cold. This heat absorption is due to the endothermic dissolution of ammonium nitrate in water. The correct answer is option c: Thermal energy was used up in forming chemical bonds, which made the cold pack colder.

Step-by-step explanation:

When you punch the instant cold pack, you break a seal that separates two substances inside the pack: a chemical (usually ammonium nitrate) and water. Once the seal is broken, the ammonium nitrate dissolves in the water. This is an endothermic reaction, meaning it absorbs heat from its surroundings rather than expelling it.

This absorption of heat is what causes the cold pack to become cold. Therefore, the chemicals do not need energy to cool down; instead, they absorb thermal energy from the environment, and in doing so, they decrease in temperature. The key here is the heat of solution, which is the amount of heat energy absorbed or released when a substance dissolves in a solvent.

In the case of ammonium nitrate, the process is endothermic, as described by the reaction: NH₄NO₃(s) + 25.7 kJ → NH⁴⁺ (aq) + NO₃⁺ (aq). The ΔH soln being positive indicates that heat is absorbed during the dissolution process, and as heat is absorbed from the injured person's body and the surrounding environment, the area in contact with the cold pack cools down.

Based on this explanation, the correct answer to your question is c. Thermal energy was used up in forming chemical bonds, which made the cold pack colder.