A(n) chemical reaction releases energy, whereas a(n) reaction requires an input of energy.

There are two types of chemical reaction based on the direction of heat or energy. 

If the energy or heat is absorbed by the system of reactants in order to form the specific products then, the reaction is called endothermic. 

However, if the energy is released by the reactants in the process of producing the products then, the reaction is exothermic.

Hence, for this item, the first blank should be filled with exothermic and the second blank should be filled with endothermic. 

There are sometimes given chemical reactions that do not involve the transfer of heat from the system or from the system.