Examples of spontaneous endothermic reactions and processes
An endothermic process absorbs heat from its surroundings, so the surroundings get colder. Most processes that happen on their own release heat, but the ones below are spontaneous at room temperature and normal pressure even though they absorb heat, because they greatly increase entropy (disorder). The list includes chemical reactions as well as physical processes such as melting, evaporating and dissolving, and each item says which it is.
- Ice melting at room temperature — a physical change that absorbs about 6 kJ of heat per mole of ice.
- Water evaporating from a puddle or your skin — a physical change that absorbs heat, which is why sweating cools you down.
- Dry ice subliming — a physical change in which solid carbon dioxide absorbs heat and turns directly into gas.
- Ammonium nitrate dissolving in water — a dissolving process that makes the water cold enough to use in instant cold packs.
- Urea dissolving in water — a dissolving process used in some instant cold packs instead of ammonium nitrate.
- Ammonium chloride dissolving in water — a dissolving process that makes the water temperature drop noticeably.
- Potassium chloride dissolving in water — a dissolving process that leaves the solution colder than the water was.
- Barium hydroxide octahydrate and ammonium chloride — a chemical reaction between two solids that releases ammonia and can get cold enough to freeze water.
- Baking soda and vinegar — a chemical reaction that releases carbon dioxide gas while the mixture cools slightly.
- Baking soda and citric acid — a chemical reaction that gives the fizz in bath bombs and the cool feeling of sherbet powder.
Why can an endothermic process be spontaneous?
Whether a process happens on its own depends on the Gibbs free energy change, ΔG = ΔH − TΔS. An endothermic process has a positive ΔH, which works against it. But if the increase in entropy (ΔS) is large enough, the TΔS term outweighs ΔH and ΔG becomes negative. That is why ice melts above 0 °C but not below it, and why reactions that release a gas, such as baking soda with vinegar, can absorb heat and still go.
Endothermic reactions that are not spontaneous
Many endothermic reactions need a constant supply of energy and stop when it is removed, such as photosynthesis, which needs light, and the breakdown of limestone into lime, which needs strong heating. They are not included in the list above.