The energy
released or absorbed in a chemical reaction can be traced to the
difference in chemical (not nuclear) binding energies of atoms
and molecules on the two sides of a reaction Since, strictly
speaking, chemical binding energy also gives a negative
contribution (mass defect) to the total mass of an atom or molecule,
we can equally well say that the difference in the total mass of
atoms or molecules, on the two sides of the chemical reaction
gets converted into energy or vice-versa However, the mass
defects involved in a chemical reaction are almost a million times
smaller than those in a nuclear reaction This is the reason for
the general impression, (which is incorrect) that mass-energy
interconversion does not take place in a chemical reaction