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Explain exceptions to Conservation of Matter to me?

by maximiliano
Tags: conservation, exceptions, explain, matter
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Feb20-14, 08:32 PM
Drakkith's Avatar
P: 11,887
Quote Quote by maximiliano View Post
So.....both strong force and weak force have mass?
Careful with your terminology. Forces don't have mass, objects and systems of objects do.

Then, would it be appropriate to say that energy (exclude "dark energy" for the moment) has mass....and when sufficiently concentrated in space and time, manifests itself as what we consider, at present, to be matter?
Not really. Energy does have mass but the "concentration" of energy has little to do with matter.
Feb20-14, 09:15 PM
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Quote Quote by Agrasin View Post
And, just wondering, is the heat of enthalpy (-891 kJ/mol in the case of the combustion of methane) = (missing mass)c^2?
One mole of methane has a mass of about 16 grams. Two moles of oxygen have a mass of about 64 grams. So 80 grams of reactants go into this reaction that yields less than 1000 kJ.

Plug 1000kJ into ##E=mc^2## and you come up with a mass deficit of .01 micrograms.

So, yes there is missing mass. But it's small enough so that we can forgive past generations of chemists for not noticing it and forgive current generations of chemistry teachers for telling us calculate as if mass is conserved in chemical reactions.

This might be a good time for a link to Asimov's classic essay:
Feb20-14, 10:04 PM
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Quote Quote by Agrasin View Post
You're right. I didn't think that through.

So in chemical/ nuclear reactions, where does mass disappear from? Can the neutrons and protons simply lose mass? And then gain it back in reverse reactions?

And, just wondering, is the heat of enthalpy (-891 kJ/mol in the case of the combustion of methane) = (missing mass)c^2?
The answer is not that the mass of the neutrons/protons changes. The answer is that the mass of a composite object is NOT the sum of the masses of its constituents. Energy lost (or gained) as a consequence of interactions between the constituents must be taken into account.
Feb20-14, 11:13 PM
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Quote Quote by Agrasin View Post
...meaning the product molecules are less massive than the reactant molecules. This is obviously odd because both sides have the same atoms.
You are neglecting the fact that although all the same sub atomic particles can be in two different molecules the arrangement of the electrons can be held with more or less energy. Check out how a plant uses a photon to shift electrons around, and how the body uses glucose and expels carbon dioxide and water. Both containing the same atoms with the sub atomic particles arranged differently. C6H12O6 = 6 x H2O plus 6 x C

In fact to make it even simpler take two Hydrogen atoms H plus H, each contains a single proton and a single electron, when they get close together they form a Hydrogen molecule, H2 which contains the same two protons and the electrons enclose both protons. The molecule has less mass than the two atoms because the electrons are in a lower energy configuration.
Feb21-14, 05:27 PM
P: 887
The mass of a particle is the mass of a (hypothetical) bare particle and the mass of the fields. You can't just count up all the constituent particles and add up the masses, since the fields overlap and interact with each other. That might be good enough for chemistry, but it isn't exact.

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