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Name: Bob W.
Status: educator
Grade: 9-12
Location: MT
Country: N/A
Date: 6/4/2005

If there is a chemical reaction that releases heat (an exothermic reaction) converts mass to energy. Does it hold true, that during a chemical reaction that absorbs energy (endothermic reaction) will add mass to the reactants?

The equivalence of energy and mass expressed by the famous equation E=mc^2 applies to chemical reactions too. However, if you do the calculation, you see that the mass "defect" is so small compared to a typical energy change of a chemical reaction as to be negligible of any practical consequence.

Vince Calder


Thanks for your question. The energy liberated by an exothermic reaction is not converted to mass. Rather, the energy is released to the environment in the form of heat. All chemical reactions obey the conservation of mass principle.

Dr. Topper

No. The energy from a chemical reaction is not coming from a conversion of mass but from a rearrangement of the atoms. The atoms all have those electrons all around them, repelling all the other electrons in all the other atoms. Ideally they would want to get far apart. But if they move too far from one, they bump up against another. So they jostle around, looking for a minimum, like a bunch of ping pong balls being blown from below, they settle into a low energy state. Then somebody does a chemical reaction and things are re-arranged, either to a higher energy state (but still a place where they can remain, otherwise the chemical reaction would fall back to its reactants), or lower energy state. Not much relativity theory in all this.

Steve Ross

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