Question

why there is an energy change in a chemical reaction

Answer 1

Because in chemical reaction electron transfer takes place

Answer 2

here we can say that as we know energy can neither be created or nor be destroyed bt it can be transferred to one form to other
here also in chemical reaction same thing happens..
energy just get transferred
because otherwise reaction won't happen and also transfer of electrons takes place..
hope this is pretty clear to u frnd
be happy always..