Some chemical reactions consume energy, and others release energy, usually as heat or light. Exergonic reactions include the combustion of gasoline, because a molecule in the gasoline, such as octane, contains more energy than the water and carbon dioxide molecules that are released after burning the gasoline. A tree's use of photosynthesis to assemble its bark from carbon dioxide and water is endergonic.
Endergonic reactions are frequently found in biological organisms, because the organism needs to assemble complex molecules such as fats and amino acids, according to Johnson County Community College. Although these reactions use up energy, the organism has the ability to use other types of molecules, such as sugars, as fuel. Endergonic reactions can never occur without a power source.
Exergonic reactions usually still require some energy to start, even though the reaction will release energy once it is complete. This extra energy is the activation energy, which a molecule temporarily stores before releasing the activation energy and some additional energy. Charcoal requires a source of energy, such as a match, before it ignites, even though the charcoal releases much more energy once it starts burning.
An endergonic reaction is also known as a reversible reaction. Burning a log reverses the reaction that was used to produce the log, breaking the carbohydrates in the log apart and releasing carbon and water, with the addition of a small amount of heat. It's more difficult to reverse the exergonic reaction, burning the log, because the tree needs to collect much more energy from the sun to assemble the log. According to the University of Nebraska, Lincoln, reversibility depends on how much additional energy it would take to perform the reverse reaction, not whether the reverse reaction is possible.
Energy Hill Diagram
An energy hill diagram provides a visual display that shows whether a reaction is exergonic or endergonic. The diagram includes two axes, time at the bottom and the total energy of the chemical solution on the side. For an exergonic reaction, the amount of energy rises until the solution has enough activation energy, and then it falls. For an exergonic reaction, once the solution has enough activation energy, it may either continue to rise, or drop to a lower level that is still higher than the initial energy of the original molecules.