Examples for Exothermic and Endothermic Reactions

During an exothermic reaction more energy is released into the environment than activation energy was required to start the reaction. In contrast, during an endothermic reaction, energy must continuously be absorbed from the environment in order to maintain the reaction.

In this article I would like to give some examples for both of these types of reactions and thus also classify some chemical reactions known from everyday life.

Examples of Exothermic Reactions

First, let's look at several examples for exothermic reactions.

• Fire, Burning and Combustion Reactions: Gasoline or diesel in the car, coal on the grill, in the power plant or in the steam locomotive, wood in the fireplace or the campfire, the flame of a candle, heating with gas or oil - with all of these chemical reactions, in which fire and combustion are involved, energy is released into the environment - the reaction is exothermic.
• Oxygen Gas Sample: Every student knows the so-called "oxyhydrogen gas sample" or "knallgas probe", which is intended to provide the evidence of hydrogen. The thereby occurring reaction (hydrogen and oxygen combine to form water) is exothermic, as can easily be seen from the explosion.
• Setting / Hardening of Gypsum and Cement: If dried gypsum (for example gypsum powder from the hardware store) combines with water, heat is released. The same thing happens when cement respectively concrete sets or hardens.
• Energy Generation in the Cell: Our cells store energy in the form of adenosine triphosphate (ATP). ATP consists of an adenosine molecule and three phosphate groups. When one of these phosphate groups is split off, the "stored" energy is released and the ATP becomes adenosine diphosphate (ADP). This is also an example of an exothermic reaction that continually occurs in our cells.
• Hand Warmers (charcoal warmers) use the oxidation of iron to produce heat for a prolonged period of time through this slow exothermic reaction. The reaction is started when the iron, which is initially packed airtight, is brought into contact with oxygen from the air and thus begins to oxidize.
• Nuclear Reactions: Also the nuclear fusion of hydrogen isotopes in the sun or the neutron-induced nuclear fission of uranium, such as that which takes place in nuclear power plants, is exothermic. Other nuclear fusions can also be endothermic, depending on whether fusing the nuclei costs energy or delivers more energy than it is put in.

Examples of Endothermic Reactions

In addition to the examples for exothermic reactions, here are also some examples for endothermic reactions as well.

• Decomposition of Water (H₂O) into the components oxygen (O) and hydrogen (H) during electrolysis. In order for this reaction to work, we must constantly add external energy in the form of electric current, otherwise the reaction will stop.
• Sherbet / Effervescent Powder in Water: When dissolving effervescent powder (citric acid C₆H₈O₇ + sodium hydrogen carbonate NaHCO₃) in water, energy from the water is required. This causes the water to become colder and it also begins to bubble due to the formation of carbon dioxide (CO₂).
• Limescale Precipitation: The settling of lime on the walls of the kettle, of glasses or the swimming pool is also an endothermic chemical reaction. The more energy is added in the form of heat (switching on the kettle), the better the reaction can proceed.
• Formation of the Ozone Layer: By adding energy in the form of ultraviolet rays from the sun, ozone is created from oxygen in the upper layers of the earth's atmosphere (3O₂→ 2O₃). The high-energy UV-C radiation splits the O₂molecules into individual oxygen atoms, which each immediately combine with another O₂ molecule to form ozone (O₃). The same reaction can also be triggered by the electrical energy from lightning during thunderstorms.
• Photosynthesis: The photosynthesis of plants that constantly occurs in nature also belongs to the endothermic reactions. Apart from artificial light sources, it only works at sunshine using the energy from the sun.

In the article about the difference between exothermic and endothermic chemical reactions you will find further explanations and information on the topic and you can learn more about the differences and similarities between exothermic and endothermic reactions.