Thread: Change of mass in a chemical reaction

2H + O -> H2O + energy

The mass of H2O is lower than the masses of the original components. Although extremelly low, chemical reactions have associated mass changes. No 'material' particles are created/destroyed, or emitted/captured in normal chemical reactions, but energy is (in the general case) radiated/absorved. This energy has an equivalent mass, and this is the change of mass related to chemical reactions (according to E=mc2). Since the mass change is very low, I think it is not possible to directly measure it, but I could be wrong.

Has somebody measured this change?

There aren't any analytical instruments sensitive enough to measure such a small mass change.

As far as I know, the fact above wasn't proved. It would bring quantum physics to ruins...

Originally Posted by Sindrato

As far as I know, the fact above wasn't proved. It would bring quantum physics to ruins...

The OP is correct. There should be a small mass change associated with changes in chemical bond energies. This doesn't ruin quantum physics. It's no different than the mass defect in atoms from nuclear binding energies.

Why do people feel the need to post this sort of bullshit when they obviously don't know anything about the topic?

Originally Posted by Scifor Refugee

Originally Posted by Sindrato

As far as I know, the fact above wasn't proved. It would bring quantum physics to ruins...

The OP is correct. There should be a small mass change associated with changes in chemical bond energies. This doesn't ruin quantum physics. It's no different than the mass defect in atoms from nuclear binding energies.

Why do people feel the need to post this sort of bullshit when they obviously don't know anything about the topic?

Thanks Scifor Refugee. Indeed, I have gotten the same naive answer in several places.

People do no seem to be aware that the effect is exactly the same, regardless of whether we are talking about one kind of energy or another. Mass is energy is mass. Any change in mass (without material particles being emitted / captured) implies an energy emission / absorbtion. Any energy emission / absorbtion implies a mass change.

Energy can not be classified according to the process generating it: energy comming from a nuclear reaction has the same nature as energy comming from a chemical reaction. The same law governs them, the same effects are related to it. We are usually taught about the mass-energy relation in the context of nuclear reactions, or in the context of big accelerations, just because in those situations the effects are big enough to be considered. But that does not mean that other processes do not show the same effects; they are just much weaker.

Indeed it is we, for simplification purposes, that classify things and say: this is a chemical reaction, this is a nuclear reaction, this is a macroscopic process, etc ...

Let's think carefully about it: what is the difference between an atom and a molecule? It is not a "quality" difference, but a "quantity": an atom is a system of particles which, to be decomposed, requires several orders of magnitude more energy than a molecule. A molecule is a system of particles which, to be decomposed, requires several orders of magnitude more energy than a macroscopic mixture.

So the difference is not fundamental; it just happens to be quite useful to have the concept of an atom, because an atom is usually immutable (except radioactive atoms, of course) when we are using normal energy levels. Even this is not completelly true: atoms are unstable on the long term, and they tend to decompose (this can take thousands of million of years, of course). Even more elemental particles (protons, for example) are not completelly stable.

So, every energy emission / absorbtion implies mass change. Even more "strange" energy absorbtions imply mass change: a compressed spring has absorved energy, and is thus more massive than a relaxed spring. Of course, that mass change is so small that it is probably impossible to measure with our current technology.

The same probably applies to the mass change related to chemical reactions, but I am not sure, because a chemical reaction can absorve / radiate quite a big amount of energy. That is why I ask.

When I was in grad school we actually had a question about this in one of my classes. It was something about having to calculate how strong of a magnetic field you would need to detect the chemical bond mass defect using Fourier transform ion cyclotron resonance, which is probably the most sensitive tool out there for measuring the mass of molecules. I don't remember the answer, but it was something ridiculous.

I wonder how much energy one would need to concentrate in a small space to produce a "measurable" amount of matter?

Originally Posted by Michael Teale

I wonder how much energy one would need to concentrate in a small space to produce a "measurable" amount of matter?

Um, E = mc².

Originally Posted by Michael Teale

I wonder how much energy one would need to concentrate in a small space to produce a "measurable" amount of matter?

Depends what you call measurable, I suppose.

I'm fairly sure we can directly measure the mass of a single electron, which is about 10^-30 kg. This is equivalent to roughly 10^-13 joules.

So, the answer would seem to be: not much.

Enthalpy of formation H2O(liquid) = 286kJ/mol = 4.751x10^-19J

Energy~mass relation:
E=mc²

Mass lost
m=Ec^-2 = 4.751x10^-19J / (3x10⁸m/s)²

m lost= 5.28x10^-36g = 3.18x10^-12 amu

Considering that the total mass of H₂O is approximately 18 amu, the change in mass is extremely small (10^-12)
this mass difference is about 10000 times smaller than the mass of an electron

it would mean hf=mc^2 ..... or at least a relationship between the two

Originally Posted by fiveworlds

it would mean hf=mc^2 ..... or at least a relationship between the two

Huh?
Since E=hf and also E=mc² then hf=mc² is already true and there is already a relationship.

maybe you should revise the photoelectric effect

Did you notice the letter "E" in each equation?
What does the "E" stand for?
Clue: it means the same in both cases.

Originally Posted by fiveworlds

maybe you should revise the photoelectric effect

Why?

Originally Posted by fiveworlds

maybe you should revise the photoelectric effect

Editing a post after it's been replied to is less than honest.
But: the photoelectric effect is because of the energy (notice that word) of the photons. It doesn't matter about the quantity at all. E.g. no matter how bright/ intense a red light light is materials will not react. But, even with a faint blue light they will.
All due to the energy (there's that word again!) of the photons.

Maybe a basic primer would help you.

work function=1/2mv^2+hfo hVmax=hfo

So, basically, no actual argument then.

na i just havent learnt this in a while