Final answer:
Joe's hand warmer likely uses a chemical reaction involving sodium acetate crystallization or the oxidation of iron, both of which are exothermic reactions producing heat energy used to warm the hands.
Step-by-step explanation:
Joe's hand warmer produces heat through a chemical reaction. When he squeezes the packet, it indicates that the hand warmer likely contains a supersaturated solution of NaC2H3O2 (sodium acetate) and a metal disc. The disc, when flexed, initiates a crystallization process which is exothermic. This means that as the sodium acetate shifts from a supersaturated solution to a solid form, it releases heat energy that warms the hands.
Additionally, some hand warmers contain iron, water, and salt. The salt acts as a catalyst in the oxidation of iron, which is an exothermic reaction that also produces heat. This occurs when iron inside the hand warmer is exposed to oxygen in the air, represented by the chemical equation 2Fe(s) + 3/2O₂(g) → Fe2O3(s).
The design of the hand warmers with components like cellulose, vermiculite, and activated carbon helps distribute the generated heat more evenly, increasing the effectiveness of the hand warmer. Some hand warmers are reusable; they can be reheated to dissolve the sodium acetate again, making them ready for another use.