Final answer:
A decomposition reaction can be either endothermic or exothermic. It depends on whether more energy is absorbed during bond-breaking than is released during bond-forming, which is indicated by the enthalpy change (ΔH). The decomposition of calcium carbonate is an example of an endothermic reaction.
Step-by-step explanation:
Decomposition reactions can be either endothermic or exothermic depending on the nature of the substance involved and the bonds being broken and formed. An endothermic reaction absorbs energy from the surroundings as the reaction occurs. For instance, the decomposition of calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2) is an endothermic process. During this reaction, 177.8 kJ of heat is absorbed from the surroundings to break the bonds in CaCO3.
It's important to remember that bond-breaking is intrinsically endothermic; energy is required to break the bonds between atoms. Conversely, bond-forming is exothermic as energy is released when new bonds are formed. Therefore, whether a decomposition reaction is endothermic or exothermic depends on whether more energy is absorbed during bond-breaking than is released during bond-forming.
The enthalpy change (ΔH) for a process provides insight into the thermal nature of the reaction. If ΔH is positive, as in the decomposition of dinitrogen tetroxide (N2O4 into 2NO2, with ΔH = +57.20 kJ, the reaction is endothermic - it absorbs heat. However, if ΔH is negative, the reaction would be exothermic, releasing heat to the surroundings.