The process of breaking chemical bonds actually requires an input of energy. The overall energy change in a chemical reaction is a complex interplay of both bond breaking and bond formation.
The Energy Cost of Breaking Bonds
Chemical bonds represent a stable, lower-energy state for atoms. To disrupt this arrangement, energy must be supplied. This is similar to lifting a weight against gravity, which requires effort to move it to a less stable position.
The energy required to break a specific chemical bond is known as bond energy or bond dissociation energy. For instance, breaking a bond between two hydrogen atoms requires a specific amount of energy, while breaking a double bond between two oxygen atoms requires more energy because it is a stronger bond. This process is always endothermic, meaning it absorbs energy from its surroundings. Energy can be supplied in various forms, such as heat, light, or electrical energy, to overcome the attractive forces holding the atoms together.
The Energy Gain from Forming Bonds
Conversely, when new chemical bonds form, energy is released into the surroundings. Atoms achieve a more stable, lower-energy state when they come together to form bonds. This can be compared to a weight falling to a lower position, where its potential energy is released.
The energy released during bond formation is precisely the same amount of energy required to break that bond. This bond-forming process is always exothermic, meaning it releases energy, often in the form of heat or light. This principle highlights that stability is associated with lower energy states, which atoms achieve through bond formation.
Understanding Overall Reaction Energy
Most chemical reactions involve both breaking existing bonds in reactant molecules and forming new bonds in product molecules. The net energy change of a chemical reaction depends on the balance between energy absorbed to break bonds and energy released when new bonds form. This determines whether a reaction is classified as exothermic or endothermic.
In an exothermic reaction, more energy is released during new bond formation than is absorbed to break old ones. This results in a net release of energy to the surroundings, such as in combustion. For example, methane combustion releases 890.4 kilojoules of heat energy per mole.
Conversely, an endothermic reaction occurs when more energy is absorbed to break bonds than is released when new bonds form. This type of reaction absorbs energy from the surroundings, causing a temperature decrease, like in instant cold packs or photosynthesis.