Final answer:
The chemical reaction in ice packs stops when all the ammonium nitrate has dissolved, as it can no longer absorb heat, ending the endothermic reaction.
Step-by-step explanation:
The chemical reaction inside the ice packs usually stops after 2 hours because all of the solid ammonium nitrate that is needed for the endothermic reaction has been dissolved into the water contained within the pack. This endothermic reaction absorbs heat energy from its surroundings, which includes the water inside the pack as well as from the body when applied to an affected area.
When the ammonium nitrate is completely dissolved, there will be no more of the chemical left to absorb additional heat, hence the reaction ceases, resulting in the pack warming up again.