Any chemical reaction involves the breaking of molecular bonds and the possible formation of new bonds. A process that breaks bonds is one that releases energy, and scientists refer to it as an exergonic process. On the other hand, the formation of new bonds requires an input of energy, and scientists refer to such a process as endergonic. Energy can be released or absorbed in many forms, including light, electricity and heat. When energy is released as heat, the process is exothermic, and when heat is absorbed, the process is endothermic. An endothermic reaction is one which results in a net decrease in temperature because it absorbs heat from the surroundings and stores the energy in the bonds formed in the reaction.
TL;DR (Too Long; Didn't Read)
Endothermic reactions absorb energy from the surroundings and lower the temperature. They are a type of endogenic reaction. In biology, anabolic processes are examples of endothermic reactions.
Equation for Endothermic Reactions
The general equation for endothermic reactions is:
Reactants + Heat Energy --> Products
A reaction may involve multiple processes, and some of these may release heat, but as long as a reaction involves a net reduction in temperature, the reaction is endothermic. It's possible for this to happen because a chemical reaction always proceeds in a way that increases entropy. By contrast, exothermic reactions are those which release heat. Oxidation reactions are usually exothermic such as a burning log.
Formation of Nitric Oxide: An example of an endothermic reaction that occurs every day in Earth's atmosphere is the combination of molecular oxygen with molecular nitrogen to form nitric oxide. Chemists know exactly how much heat energy it takes for this reaction to occur. The balanced equation for this reaction is:
O2 + N2 + 180.5 KJ --> 2 NO
In words, it requires 180.5 kilojoules of energy to make this reaction happen, and it's a good thing, too. Otherwise, all the oxygen in the atmosphere would have been used up long ago. The heat energy for this reaction often comes from automobile exhaust.
Endothermic Processes Are Not All Reactions
An example of an endothermic process with which everyone knows is sweating, the process by which the body produces water on the skin as a cooling strategy. It works because water absorbs energy when it changes state from a liquid to a gas. This is an endothermic process, but it isn't a reaction, because a reaction always involves the destruction or formation of chemical bonds. On the other hand, squeezing an instant-cold ice pack does produce an endothermic reaction. A chemical in the pack reacts with water to absorb energy and freezes the water into ice.
Examples from Biology
During photosynthesis, plants absorb carbon dioxide from the atmosphere and turn it into glucose and oxygen. The process requires energy in the form of sunlight and is more endogenic than it is endothermic. The equation for the reaction is:
6CO2 (carbon dioxide) + 6H2O (water) + sunlight --> C6H12O6 (glucose) + O2 (oxygen)
A number of endothermic reactions are important for the metabolism of mammals and humans. Many of these occur inside cells, and when they do, scientists call them anabolic reactions, as opposed to catabolic reactions, which release energy. Some of these reactions include:
- Amino acids joining together to form peptides.
- Small sugar molecules joining to form disaccharides.
- Glycerol reacting with fatty acids to make lipids.