Endothermic reactions are chemical reactions which absorb energy from the environment. The products of the reaction store the energy absorbed in the form of chemical bonds, according to Reference.com.
When plants convert carbon dioxide and water into glucose through photosynthesis, the reaction requires light to provide the necessary energy. A plant requires 15 megajoules of energy to produce 1 kilogram of glucose. When animals consume glucose, they release this energy through an exothermic reaction called respiration.
Some reactions require energy in the form of heat. These reactions absorb heat from their surroundings, and in doing so cause their containers to become cold. The addition of barium hydroxide octahydrate to ammonium thiocyanate is one such reaction. When these two solids are mixed in a flask and set in a puddle of water on a wooden block, the reaction absorbs enough heat to cause the water to freeze the flask to the block.
Instant cold packs from the pharmacy work using exothermic reactions. These packs include a plastic bag filled with water and smaller bag filled with ammonium nitrate, the ingredient in fertilizer. Hitting the outside bag breaks the inner one, allowing the ammonium nitrate to react with water and lower the temperature of the water. Most of these bags have enough material to lower their temperature to approximately 34 degrees Fahrenheit and keep it there for 15 minutes, according to HowStuffWorks.