Today in class we had a tricky time coming up with good examples of endothermic reactions. But here’s a great video demonstration:
In this case, the reaction absorbs so much heat from its surroundings that it is able to freeze a bit of water!
Another example of a reaction that absorbs energy from its surroundings is photosynthesis, in which plants convert carbon dioxide to sugars with the aid of light. Technically, photosynthesis would be considered an endergonic reaction, not quite endothermic, since the energy input is in the form of light rather than thermal energy.
Several students mentioned cold packs in class as examples of endothermic reactions as well. Many of these work by becoming cold when a small container of ammonium chloride is broken and allowed to dissolve in a larger bag of water. In this case, the heat from the surroundings goes into breaking the lattice energy of solid ammonium chloride as it is solvated by water. We can write this as:
NH4Cl(s) → NH4+(aq) + Cl–(aq)
Heat packs, on the other hand, contain exothermic reactions! A supersaturated solution of sodium acetate will crystallize very rapidly if you “click” a small piece of metal, and the crystallization of sodium acetate gives off a lot of heat:
Na+(aq) + C2H3O2–(aq) → NaC2H3O2(s)