You haven't given the synthesis reaction but in general, In the simplest cases, a synthesis reaction occurs when two different atoms or molecules interact to form a different molecule or compound. Most of the time, when a synthesis reaction occurs, energy is released and the reaction is exothermic. However, an endothermic outcome is also possible. Synthesis reactions are one of the major classes of chemical reactions, which include but are not limited to, single displacement, double displacement, and combustion reactions.