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In every chemical reaction, established molecular bonds are cleaved while new bonds are forged. Common examples include combustion, reduction, and precipitation. The rearrangement of atoms yields entirely new compounds. While mere contact between reactants can suffice, most reactions necessitate an external trigger—typically heat—to initiate bond cleavage. The outcome depends on the interplay of molecular forces, energy states, and environmental conditions.
Chemical reactions make and break the bonds between molecules, producing new materials. They can occur spontaneously or need an energy input. Breaking bonds absorbs energy; forming bonds releases energy, so the overall reaction may be endothermic or exothermic.
The foundation of chemistry is the breaking (decomposition) and formation (synthesis) of bonds. Decomposition is endothermic because stable bonds must be overcome by energy input. Conversely, synthesis is exothermic; atoms reach a lower‑energy, more stable configuration, releasing energy. The net energy change of a reaction depends on the balance between bond cleavage and bond formation.
For instance, heating mercury(II) oxide (√HgO) causes it to decompose into mercury metal and oxygen gas, absorbing heat in the process.
Endothermic reactions require an energy input to break bonds and initiate the process. They rarely occur spontaneously. An example is the thermal decomposition of mercury(II) oxide, which demands heat to proceed. More complex endothermic reactions can draw heat from their surroundings; the solid-state reaction between barium hydroxide and ammonium chloride at room temperature produces barium chloride and ammonia while cooling the mixture, absorbing heat from the container and ambient air.
Exothermic reactions release heat and are frequently self‑sustaining. Synthesis reactions, such as the vigorous reaction of sodium with water, generate sodium hydroxide and hydrogen gas while producing enough heat to ignite the hydrogen. Combustion of hydrocarbons—wood, gasoline, or fuel oil—needs an initial spark or flame to break a few bonds. Once the new, energetically favorable bonds form, the released heat keeps the reaction going, producing carbon dioxide and water vapor.
Industrial and commercial processes often exploit exothermic, self‑sustaining reactions. Their efficiency and the work they deliver hinge on the specific reactants and the bonds that break and reform during the reaction.
