Chemical reactions involve a transfer of energy, which scientists use to categorize them based on how heat flows. Reactants rearrange their atoms to form new products. Energy is always exchanged between the reaction (the system) and everything else (the surroundings). Understanding this exchange predicts whether a process will heat or cool the environment.
Defining Energy Flow in Chemical Reactions
The classification of chemical reactions into two major types—exothermic and endothermic—depends on whether the system releases or absorbs thermal energy from its surroundings. Exothermic reactions are those that release energy, usually as heat, causing the temperature of the surrounding environment to rise. A simple example of an exothermic process is the chemical reaction inside a disposable hand warmer, which warms up as the chemicals inside react.
Conversely, endothermic reactions absorb thermal energy from their surroundings as they occur. This absorption of heat causes the temperature of the immediate surroundings to drop, resulting in a cooling effect. The reaction that takes place when a chemical cold pack is activated is a common example of an endothermic process.
The Basic Chemistry of Combustion
Combustion, commonly known as burning, is a rapid chemical reaction between a fuel and an oxidizing agent, usually oxygen gas from the air. The fuel is typically a hydrocarbon containing carbon and hydrogen, such as methane or gasoline. This transformation is a form of oxidation where the fuel atoms aggressively combine with oxygen atoms.
The process requires three components: a fuel source, an oxidizing agent, and an initial heat source to begin the reaction, often visualized as the “fire triangle.” When combustion is complete, the carbon atoms in the fuel convert into carbon dioxide gas, and the hydrogen atoms form water vapor. The overall reaction produces these new, stable compounds.
Why Combustion Reactions Result in Net Energy Release
Combustion reactions are overwhelmingly classified as exothermic because they result in a net release of energy to the surroundings. The energy change in any chemical reaction is determined by the balance between the energy needed to break the bonds in the reactants and the energy released when new bonds form in the products. Breaking the existing chemical bonds in the fuel and the oxygen molecules requires an input of energy.
However, the energy released when the stable bonds of the products—carbon dioxide and water—are formed is significantly greater than the energy required to break the reactant bonds. This difference means that more energy is released by the system than was initially absorbed. The excess energy is expelled into the environment, primarily as heat and light, which is why a fire feels hot and glows.
A small amount of energy, called the activation energy, is needed to overcome an initial barrier and start the reaction, such as the heat from a match or a spark. Once the reaction begins, the heat released by the forming products continuously provides the activation energy for the remaining reactant molecules. This feedback loop makes the combustion self-sustaining and confirms its classification as a highly exothermic process.