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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)?)

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ntno
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  • 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? – Floris Oct 06 '15 at 19:50
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    Possible duplicate of Conversion of mass to energy in chemical/nuclear reactions – Floris Oct 06 '15 at 19:51
  • 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. – ntno Oct 06 '15 at 19:53
  • @Floris So the observed mass is different because you look at the system as a whole instead of the sum of the parts? ( i.e. mass(gasoline + air) > mass(gasoline) + mass(air) ) – ntno Oct 06 '15 at 20:01
  • If you weigh a sealed container with H2 and O2 molecules in it, and then you allow an exothermal reaction between them to occur, some heat will be generated. Once the system is back at thermal equilibrium with the environment, the mass of the container will be less - by $\Delta m = \frac{E}{c^2}$ where $E$ is the energy of the reaction (the binding energy of the water molecules minus the binding energy of the individual hydrogen and oxygen molecules). – Floris Oct 06 '15 at 21:12
  • @Floris: if a hydrogen atom absorbs a photon, does it get heavier? If it loses that excitation energy, does it get lighter again? – Gert Oct 06 '15 at 21:21
  • @Gert - I believe the surprising answer is "yes". See for example http://physics.stackexchange.com/a/53437/26969 – Floris Oct 06 '15 at 21:24
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    @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! ;-) – Gert Oct 06 '15 at 21:33
  • It's not the reaction that I'm talking about, but the energy of seperate atoms treated as a system (i.e. an unreacted mixture of H2 and O2) versus the mass of the H2 in an empty universe + the mass of the O2 in an empty universe. – ntno Oct 06 '15 at 21:35
  • Oh yes @Gert - this would be a very hard thing to measure, especially in the 18th century. Lavoisier was right "within experimental error". They did chop his head off - but not because he was wrong about this. – Floris Oct 06 '15 at 21:35
  • @DOS4004 - whenever atoms mix (even when they don't react) there will be a finite amount of interaction (Van der Waals force... as they approach each other and collide - no gas is "ideal".) But that is an even smaller effect. The fact that they "would change mass if they reacted" is not taken into account at the time of mixing: it only comes about at the time of the actual reaction. – Floris Oct 06 '15 at 21:38
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    @Floris: "A 100 years for a mind like that to come around and 5 mins to chop his head off!" someone apparently claimed... – Gert Oct 06 '15 at 21:39
  • @Gert - one of many tragedies of the French Revolution. – Floris Oct 06 '15 at 21:42
  • it's not that they would change mass if they reacted, but that separately they have no chemical potential energy, and therefore less mass, than when mixed – ntno Oct 06 '15 at 21:42

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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:

<speculation>

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. 

</speculation>

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

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Floris
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  • Thanks! I realized that even when separated, the energy of the hydrogen or the oxygen remains the same. However, it would be inaccessible. – ntno Oct 06 '15 at 22:43
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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.

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