________ reaction requires energy for breaking down the reactants.

The reaction that requires energy for breaking down the reactants is Decomposition. In a decomposition reaction, a single compound is broken down into two or more simpler substances. This process often requires energy input, which can come from heat, light, or electricity. A single reactant, calcium carbonate (CaCO3), absorbs heat energy and breaks down into two simpler substances: calcium oxide (CaO, also known as quicklime) and carbon dioxide gas (CO2). A decomposition reaction occurs when one complex compound splits into two or more products.