In a chemical cold pack, two substances are kept separate by a divider. When the divider is broken, the substances mix and absorb heat from the surroundings. The chemical cold pack feels cold. Is the reaction exothermic or endothermic? Draw an energy diagram showing the relative energies of the reactants and products in the reaction.
Hand warmers work by creating an exothermic chemical reaction that produces heat when the contents react with air or each other. Most commonly, hand warmers use pouches of fine iron filings that oxidize in the presence of air, water and salt, producing heat in the process.
Iron oxide hand warmers are cheap and easy to produce, but they only work once. Once the hand warmer is exposed to oxygen, the oxidation reaction begins and it cannot be stopped. The reaction continues until all the iron has reacted, at which point the heat dies down. Depending on the amount of iron present in the hand warmer, this may take several hours to complete.
Reusable hand warmers rely on the crystallization of a supersaturated solution to produce heat. These warmers usually contain a sodium acetate solution along with a metal strip. When the warmer is flexed, the strip bends and creates nucleation points for crystals to form. The supersaturated solution crystallizes, producing heat in the process. These warmers typically only last a brief time before the reaction peters out, but they can be recharged by boiling the hand warmer and melting the crystals back into solution.