An exothermic reaction is a chemical process that releases energy into its surroundings, typically as heat or light. This energy release causes the temperature of the immediate environment to increase. The opposite is an endothermic reaction, which absorbs energy from the surroundings, causing a drop in temperature.
How Energy is Released in Chemical Reactions
All chemical reactions involve two distinct energy steps: breaking existing bonds in the starting materials and forming new bonds in the resulting products. Energy must be absorbed to break the chemical bonds holding the initial reactants together. New bonds are formed as the atoms rearrange themselves into the final product molecules, and this bond formation always releases energy. For a reaction to be classified as exothermic, the energy released when the new, stable bonds are formed must be greater than the energy absorbed to break the old bonds. The net difference in energy is then expelled into the surroundings as heat.
This overall energy change is quantified by enthalpy, which represents the total heat content of a system. Exothermic reactions are characterized by a negative enthalpy change, indicating that the products possess less stored chemical energy than the original reactants. The excess energy is what we feel as warmth or see as light.
Common Exothermic Reactions Seen Daily
One of the most common examples of an exothermic reaction is combustion, the rapid reaction between a substance and oxygen. When natural gas, wood, or gasoline burns, the fuel rapidly combines with atmospheric oxygen to produce carbon dioxide and water vapor. This process releases significant heat and light, which is why combustion is used to power engines and heat homes.
Another exothermic process is the neutralization reaction between a strong acid and a strong base. When mixing hydrochloric acid with sodium hydroxide, the hydrogen ions (\(\text{H}^+\)) from the acid combine with the hydroxide ions (\(\text{OH}^-\)) from the base to form water (\(\text{H}_2\text{O}\)). The energy released from the formation of these stable water bonds causes the resulting solution to become noticeably warmer. For a strong acid-base reaction, the energy released is consistently around \(57.3 \text{ kilojoules}\) for every mole of water formed.
Explosions represent exothermic reactions that occur at high speeds. In the case of black powder (a mixture of potassium nitrate, charcoal, and sulfur), the reaction is self-sustaining because the potassium nitrate provides its own oxygen. The rapid, confined burning releases significant heat, which instantly converts the solid components into a large volume of gas, primarily nitrogen and carbon dioxide. This sudden expansion of superheated gases creates the blast wave.
Slower and Specialized Exothermic Processes
Not all exothermic reactions are fast; some occur so slowly that the heat release is nearly imperceptible. The rusting of iron, or the oxidation of iron metal in the presence of water and oxygen, is a prime example of a slow exothermic process. The formation of stable iron oxide (rust) releases energy, but because the reaction takes place over days, months, or years, the minuscule amount of heat produced simply dissipates into the environment.
This same chemical mechanism is harnessed in commercial hand warmers to provide sustained heat. These small packets contain powdered iron, which provides an increased surface area for the oxidation reaction. The iron powder is mixed with salt (a catalyst) and activated charcoal, which helps distribute the heat. When the packet is exposed to air, the rapid and controlled oxidation of the iron generates heat for several hours, often reaching up to \(180^\circ \text{F}\) or \(82^\circ \text{C}\).
An industrial example of a slower exothermic process is the setting of cement, known as the hydration of Portland cement. When water is mixed with cement powder, it initiates chemical reactions that form a solid matrix. This hydration process releases substantial heat, known as the heat of hydration, which can average around \(120 \text{ calories}\) per gram of cement. This internal heat generation is so intense in large-scale applications that engineers must sometimes use cooling systems to prevent the concrete from cracking as it cures.