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Does chemical energy contribute to the mass of an object? I don't mean the bond energy, but the possible energy that could be released (i.e. Does an atom of oxygen and a molecule of hydrogen (H2) have more mass together (even when not bonded) than the sum of the masses separately (assuming each one is considered in empty space)?)


marked as duplicate by Floris, John Duffield, ACuriousMind, Bill N, Qmechanic Oct 6 '15 at 23:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ The answer "yes" is too short to count as a valid answer on this site. What are you looking for, really? This link perhaps? How is "the energy that could be released" really different than the bond energy? $\endgroup$ – Floris Oct 6 '15 at 19:50
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    $\begingroup$ Possible duplicate of Conversion of mass to energy in chemical/nuclear reactions $\endgroup$ – Floris Oct 6 '15 at 19:51
  • $\begingroup$ But let's say the oxygen atom is in an empty universe with no H2, it would have no "chemical potential energy" as there is no possible lower energy state. $\endgroup$ – DOS4004 Oct 6 '15 at 19:53
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    $\begingroup$ @Floris: that does settle the question then. 'Chemical energy' is no more than a inaccurate term for bound energies of molecular orbitals. The mass lost in chemical reactions is almost imperceptibly small though, so Lavoisier was almost, almost, almost (repeat many times), almost right! ;-) $\endgroup$ – Gert Oct 6 '15 at 21:33
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    $\begingroup$ @Floris: "A 100 years for a mind like that to come around and 5 mins to chop his head off!" someone apparently claimed... $\endgroup$ – Gert Oct 6 '15 at 21:39

I believe I now understand the question - and my earlier comment and the link for a potential duplicate does not apply.

If I'm right, you are actually asking this question:

If you take two containers - one with hydrogen, and one with oxygen - and you weigh them separately; then allow the gases to mix, will the mass of the mixture be different?

If the atoms react, the chemical energy releases will result in a corresponding loss of mass for the combined system; see for example this earlier question or the answer to another related question.

If the atoms did not react, there is no change in the chemical energy; but interestingly, there is a change in the entropy of the system. And entropy is closely related to energy. Speculating here - my thermodynamics is very very rusty:


In fact, if you have a perfectly reversible engine, you can do work by changing the amount of entropy: $\Delta U = T\Delta S$. This means that the small change in entropy as you mix your gases will result in a small change in energy, and this should produce a change in mass.


I'm willing to be proven wrong on this last point. As I said - thermodynamics rusty.

  • $\begingroup$ Thanks! I realized that even when separated, the energy of the hydrogen or the oxygen remains the same. However, it would be inaccessible. $\endgroup$ – DOS4004 Oct 6 '15 at 22:43

One example of saying "yes" is that the mass of a single atom is different when the same atom is bonded by chemical reaction to another atom. In this case, a little bit of mass is converted to chemical energy for bonding atoms in the compound.

  • $\begingroup$ i'm talking about unbonded atoms considered together $\endgroup$ – DOS4004 Oct 6 '15 at 20:12
  • $\begingroup$ I should reference to this post. $\endgroup$ – X Qi Oct 6 '15 at 20:28

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