Chemical reactions involve not only the transformation of substances, but also changes in energy. These energy changes allow us to classify reactions as either exothermic or endothermic, depending on whether they release or absorb energy. Understanding this difference is essential for interpreting how reactions occur and for applying them in real-world settings.

An exothermic reaction releases energy to the surroundings, usually in the form of heat or light. This happens because the products of the reaction are lower in energy than the reactants. As bonds are broken and new ones are formed, the excess energy is given off. Common examples include combustion reactions, like burning wood or gasoline, as well as many oxidation reactions and neutralization reactions between acids and bases. For example:

CH4 + 2O2 -> CO2 + 2H2O + heat

In this reaction, methane reacts with oxygen to form carbon dioxide and water, releasing heat. This type of reaction feels warm or even hot to the touch because it increases the temperature of the surroundings.

In contrast, endothermic reactions absorb energy from the environment. In these reactions, the products are higher in energy than the reactants, meaning that energy must be added to the system for the reaction to proceed. One of the most well-known endothermic reactions is photosynthesis:

6CO2 + 6H2O + energy -> C6H12O6 + 6O2

Here, plants use light energy to convert carbon dioxide and water into glucose and oxygen. Another everyday example is the use of instant cold packs, which rely on endothermic reactions to draw heat away from the skin.

These types of energy changes can be illustrated with energy diagrams. In an exothermic diagram, the energy of the products is lower than that of the reactants, and the difference in height shows the energy released. In an endothermic diagram, the products appear higher on the energy scale, indicating that energy was absorbed.

Chemists measure the energy change in a reaction using a quantity called delta H. This represents the difference in enthalpy between products and reactants. If delta H is negative, the reaction is exothermic. If delta H is positive, the reaction is endothermic. These values are useful for calculating the amount of heat involved and for designing safe and efficient chemical processes.

Both types of reactions are used in everyday applications. Exothermic reactions are found in combustion engines, fireworks, and hand warmers. Endothermic reactions are useful in refrigeration systems and cold compresses. In biology, energy release and absorption are also important. For example, cellular respiration is exothermic, providing energy for cells, while the synthesis of proteins and other molecules is endothermic.

In industrial chemistry, knowing whether a reaction is exothermic or endothermic helps companies manage energy use, control reaction rates, and design equipment that handles heat safely. Controlling energy flow also matters in environmental applications, such as minimizing heat released during waste treatment.

In summary, the classification of reactions as endothermic or exothermic depends on whether energy is absorbed or released. Exothermic reactions feel hot and increase the temperature of their surroundings, while endothermic reactions feel cold and decrease it. By measuring delta H and observing heat flow, scientists can understand how energy moves through chemical systems and use that knowledge to solve practical problems.