Exothermic process

An exothermic process is a thermodynamic reaction that releases energy from a system to its surroundings. This energy movement is a fundamental part of how the physical world functions. While energy is most often released as heat, it can also take several other forms. It might appear as light, such as a spark, a flame, or a bright flash. It can also be released as electricity, like the power stored in a battery. In some cases, energy is released as sound, such as the noise heard during a hydrogen explosion.

To understand the mechanism, we must look at the energy stored within chemical bonds. In an exothermic chemical reaction, the chemical bond energy is converted into thermal energy, which we call heat. This process relies on the relationship between activation energy and released energy. Activation energy is the specific amount of energy required to start a reaction. In an exothermic process, this starting energy is less than the total energy subsequently released. This creates a net release of energy into the environment.

Energy can also be released through the movement of electrons. When electrons transition from one quantum energy level to another, they cause light to be released. This light is equivalent in energy to some of the stabilization energy of the chemical reaction, known as bond energy. This released light can then be absorbed by other molecules in a solution. This absorption causes molecular translations and rotations. This movement is what creates the classical scientific understanding of heat.

There are different ways to measure these energy changes depending on the system. In a closed system, the released energy is expressed through specific thermodynamic changes. If the transformation occurs at constant pressure without exchanging electrical energy, the heat is equal to the enthalpy change. The IUPAC defines an exothermic reaction as one where the overall standard enthalpy change, or ΔH°, is negative. If the transformation happens at a constant volume, the heat equals the change in internal energy according to the first law of thermodynamics. In an adiabatic system, which does not exchange heat with its surroundings, an exothermic process will result in an increase in the system's temperature.

Scientists distinguish these processes from endothermic processes. An endothermic process is the exact opposite, as it absorbs energy from its surroundings. This is often driven by a favorable increase in entropy within the system. For example, a first aid cold pack is endothermic because it requires calories from its surroundings to work. This causes the pouch and the area around it to cool down. Another example is photosynthesis. In this process, plants absorb radiant energy from the sun to convert carbon dioxide and water into sugar and oxygen. This is an otherwise non-spontaneous process.

History shows us that these concepts were carefully defined by researchers. The term "exothermic" was coined by the 19th-century French chemist Marcellin Berthelot. His work helped categorize how energy moves during transformations. Today, we see many distinct types of exothermic reactions in nature and technology. Fuel combustion, such as burning wood, coal, or oil, is a primary example. The thermite reaction is another, which can produce molten iron and bright sparks.

Other examples include the setting of cement and concrete, or the setting of epoxy resin during polymerization.

Exothermic processes also occur on a massive scale in physics. Nuclear fission, which is the splitting of heavy elements, is used in nuclear power plants to release large amounts of energy. On a much larger scale, nuclear fusion occurs in stellar cores and in hydrogen bombs. Even biological processes like respiration are exothermic, as cells break down glucose to release energy. These reactions are generally more spontaneous than endothermic ones. Understanding these energy transfers helps scientists study everything from chemistry to the life cycles of stars.