Exothermic process

An exothermic process is a thermodynamic transformation or chemical reaction in which energy is released from a system into its surroundings. This release usually occurs in the form of heat, but may also appear as light, electrical energy or sound. The term, introduced in the nineteenth century by the French chemist Marcellin Berthelot, stands in contrast to an endothermic process, where energy is absorbed from the surroundings. Exothermic reactions are fundamental in physical and chemical sciences because they involve the conversion of stored chemical energy into more dispersive forms, often driving spontaneous processes in nature and technology.

Characteristics and Thermodynamic Basis

In an exothermic reaction, the total standard enthalpy change of the system is negative. This indicates that the chemical bonds formed in the products are energetically more stable than those broken in the reactants, resulting in a net release of energy. When the process takes place at constant pressure, the heat evolved equals the enthalpy change. At constant volume, however, the heat corresponds to the change in internal energy, as no work is done by volume expansion.
Within an adiabatic system, where no heat is exchanged with the surroundings, an exothermic reaction raises the temperature of the system itself. The heat released in such reactions may be expressed in electromagnetic energy or as kinetic energy, which manifests as thermal motion. Electronic transitions and bond rearrangements often emit light, which may then be absorbed by neighbouring molecules and translated into chaotic molecular motion.
A defining characteristic of exothermic reactions is that the activation energy required to initiate them is lower than the amount of energy ultimately released. As a result, the reaction yields a net output of energy, contributing to their tendency to proceed spontaneously under suitable conditions.

Exothermic and Endothermic Reactions

Chemical reactions and physical transformations can generally be categorised into exothermic and endothermic types.

• Exothermic reactions: These release heat to the surroundings. Typical examples include combustion, condensation and nuclear fission. Many industrial and natural processes rely on such reactions to supply large amounts of usable energy.
• Endothermic reactions: These absorb energy from the surroundings. Examples include the dissolution reactions in instant cold packs and the photosynthetic process in plants, in which solar energy is captured and stored in chemical form. The stored energy can later be released through the exothermic combustion of sugars.

Mechanisms of Energy Release

In exothermic reactions, energy is released because the products form stronger or more stable bonds than those present in the reactants. The difference in bond energies appears as heat or other forms of energy.

• Bond formation releases energy.
• Bond breaking requires energy input.

When the net effect of these two processes is the release of energy, the reaction is exothermic. The energy may appear as:

• Heat (seen as temperature rise)
• Light (sparks, flames or flashes)
• Sound (explosive reactions)
• Electricity (as in electrochemical cells)

Examples of Exothermic Processes

Many exothermic reactions occur both in nature and in technological applications. Common examples include:

• Combustion of fuels such as wood, coal and petroleum.
• The thermite reaction, producing molten iron.
• Reactions of alkali metals and other highly electropositive elements with water.
• Mixing of strong acids and bases.
• Dehydration of carbohydrates by concentrated sulphuric acid.
• Setting of cement, concrete and some polymer resins such as epoxy.
• Oxidation reactions of metals, including those occurring in stellar cores.
• Reaction of zinc with hydrochloric acid.
• Cellular respiration, where glucose is metabolised to release energy.

These reactions may differ in scale and intensity, but all display the core property of releasing energy to the environment.

Implications for Chemical Reactions

Exothermic reactions tend to be more spontaneous than endothermic reactions, although spontaneity also depends on entropy changes. The heat released in an exothermic transformation may be included as a product in a thermochemical equation, highlighting its role in driving the reaction forward.
Because of their spontaneity and capacity to release large quantities of energy, exothermic reactions play vital roles in industrial chemistry, energy generation, materials synthesis and biological metabolism. Understanding the energetics of such reactions enables scientists and engineers to design safer, more efficient processes across a range of applications.