What do we mean by the 'enthalpy' of a chemical bond? How does it help us to know whether a reaction will be endothermic (take in heat energy from the environment) or exothermic (release heat energy to the environment)?
The enthalpy of a chemical reaction relates to the energy used to break all bonds that are made, and the energy released from all the bonds made. We can think of it as "a bond is a debt": we take energy out of the system when we form a bond, and to break a bond must pay back the energy, like paying off a debt.
If more energy is required to break all the bonds in the reactants than is released when the new bonds in the products are formed, the reaction will be endothermic, and vice versa.