* Exothermic reactions are reactions where heat is released to the surroundings. This means the products have lower energy than the reactants.

* Decomposition reactions break down a larger molecule into smaller ones. This can be exothermic if the bonds being broken release more energy than is required to break them.

* Synthesis reactions combine smaller molecules into a larger one. This can be exothermic if the bonds formed release more energy than is required to break the bonds in the reactants.

Examples:

* Exothermic Decomposition: The combustion of methane (CH4) breaks it down into carbon dioxide (CO2) and water (H2O), releasing heat.

* Exothermic Synthesis: The reaction of sodium (Na) with chlorine (Cl2) to form sodium chloride (NaCl) releases heat.

In short, the type of reaction (decomposition or synthesis) doesn't determine whether it's exothermic or endothermic. The energy change during bond breaking and formation determines whether heat is released or absorbed.