Which type of reaction captures energy in the bonds of reactants?

Synthesis reactions are chemical processes where two or more reactants combine to form a more complex product. During this process, energy is absorbed and stored in the chemical bonds of the newly formed compound, making it a process that captures energy. This occurs because, in order to form new bonds in the product, energy must be supplied to overcome the energy barrier of bond formation, resulting in stored energy within the bonds of the product.

In contrast, decomposition reactions involve the breakdown of a compound into simpler substances, typically releasing energy in the form of heat or light as bonds are broken, which does not capture energy in the same way. Physical state changes, such as melting or boiling, primarily involve changes in energy states associated with physical forms rather than chemical bonds and do not inherently involve energy capture in bond formation. Exothermic reactions specifically refer to those that release energy during the reaction, highlighting a process that is the opposite of energy capture. Thus, synthesis reactions are clearly the correct choice as they are associated with the capturing and storing of energy in chemical bonds.