How is energy conserved in an endothermic reaction?
The absorbed energy is used to create bonds; transforming it from thermal energy to chemical energy.
An endothermic reaction is one that pulls thermal energy from its surroundings to continue the reaction.
As energy is pulled into the reaction it is used to form bonds between reactants to form products. Through this process energy is neither created nor is it destroyed, it is simply converted into another form (Chemical Potential Energy in this case).