Turning heat change into a product
Because exothermic reactions release heat and endothermic reactions absorb heat, chemists design everyday products around them. A device that must warm something uses an exothermic reaction; a device that must cool something uses an endothermic process.
Hot packs and self-heating products
A reusable hot pack often contains a supersaturated solution of sodium acetate. When a metal disc is clicked, crystallisation begins and heat is released, so the pack warms up. Some self-heating food cans use the exothermic reaction of calcium oxide with water: CaO(s) + H2O(l) → Ca(OH)2(aq). Hand warmers may use the slow oxidation of iron powder, which is also exothermic.
Worked example / 例题
An instant cold pack contains ammonium nitrate and a bag of water. When squeezed, NH4NO3(s) dissolves: NH4NO3(s) + water → NH4+(aq) + NO3−(aq). This dissolving is endothermic, so it absorbs heat from the surroundings and the pack becomes cold enough to treat a sports injury.
Choosing exothermic or endothermic
| Purpose | Heat change | Example |
|---|---|---|
| Warming | exothermic | hand warmer, self-heating can |
| Cooling | endothermic | instant cold pack |
Remember / 记住
- Warming products = exothermic reactions (heat released).
- Cooling products = endothermic reactions (heat absorbed).
- Sodium acetate hot packs are reusable; ammonium nitrate cold packs are single-use.
Understanding whether a reaction gives out or takes in heat lets us match the right chemistry to a practical need, safely and cheaply.