Chemical reactions involve a change in energy as atoms rearrange to form new substances. When a reaction is classified as exothermic, the total energy released during the process is greater than the total energy absorbed. This net energy difference is expelled from the reacting system, most commonly as heat, but sometimes appearing as light or sound.
Defining Exothermic Reactions and Energy Transfer
The release of energy in an exothermic reaction is tied to the breaking and forming of chemical bonds. To begin any reaction, activation energy must be supplied to break the existing bonds within the starting materials, called reactants.
Once old bonds are broken, new, more stable chemical bonds are formed to create the resulting substances, or products. The formation of these new bonds releases energy back into the environment. In an exothermic process, the energy released when forming product bonds significantly exceeds the energy required to break reactant bonds.
This imbalance results in a net output of energy, which is why the surroundings feel hot. The products of an exothermic reaction possess a lower amount of chemical potential energy than the original reactants.
The Contrast: Understanding Endothermic Reactions
While exothermic reactions release energy, endothermic reactions operate by absorbing energy from their surroundings. This absorption causes a noticeable cooling effect in the immediate environment.
In an endothermic process, the energy required to break reactant bonds is greater than the energy released when new product bonds are formed. The system must continuously pull in energy from the environment to sustain the reaction. The resulting products therefore have a higher chemical potential energy than the reactants.
A common example is the reaction inside an instant cold pack. When the chemicals mix, they absorb heat from the surroundings, making the pack feel cold.
Common Exothermic Reactions in Daily Life
Exothermic reactions are responsible for many familiar occurrences that provide light, heat, and power.
Combustion
One of the most common examples is combustion, the rapid reaction of a fuel source with oxygen. Burning natural gas or wood involves the conversion of stored chemical energy into substantial amounts of heat and light.
Neutralization Reactions
A different type of energy release occurs in neutralization reactions, such as when an acid and a base are mixed. When hydrochloric acid and sodium hydroxide react, they form salt and water, accompanied by a distinct increase in temperature.
Oxidation (Rusting)
Another widespread, though slower, exothermic reaction is the rusting of iron, known chemically as oxidation. Iron metal reacts with oxygen and water to form iron oxide, or rust. This process releases a small amount of heat, but it happens so slowly that the temperature change is imperceptible. The controlled oxidation of iron is used in instant hand warmers, where iron filings generate a slow, steady release of warmth.