Ask A Scientist Chemistry Archive

name         CraigC
status       other
grade        other
location     AR

Question -   I read the answer to the question about whether or not
mass is converted to energy in a normal chemical  reaction. The answer
was very unsatisfying to my students because there did not seem to be a
consensus. My question is twofold.

In an exothermic chemical reaction is mass converted to energy according
to E=MC^2? If I had a sealed glass container floating isolated in a vacuum
and inside I initiated an exothermic reaction and then let the excess heat
radiate out would the whole thing (container and reaction products) weigh
a minuscule amount less?

If so, what happens in an endothermic reaction?
Does the extra energy needed for the reaction convert to mass in the
reaction products? If I had a sealed glass container floating isolated in
a vacuum and inside I initiated an endothermic reaction and then added
energy in the form of infrared light would the whole thing (container and
reaction products) weigh a minuscule amount more when the reaction was
complete?

This would seem to me to be a very important basic physics question. Has
it ever been answered or is it impossible to answer because the resulting
mass change is too small?
---------------------------------------
The lack of "consensus" is a matter of a difference in "significantly
measurable" vs. "in principle". In principle, in an exothermic reaction
there is a small loss of mass. The reaction CH4 + O2---> CO2+2H2O has a
heat of reaction -890 kJ. This is equivalent to a loss in mass of ~ 10^-11
kg. So it is not "significantly measurable. The factor of c^2 in
Einstein's equation E=mc^2 is a large lever -- a very small loss in mass
results in a very large liberation of energy. Only in nuclear reactions
and/or matter --
anti-matter is the mass change large enough to be conveniently measured. And
this conservation of mass-energy has been confirmed millions of times
(provided of course all factors such as the generation of neutrinos) are
taken into account. In the case of endothermic reactions the addition of
energy in the form of radiation is an addition of a minuscule amount of the
equivalent amount of mass. There are two other "complications" that need to
be recognized. First, it is assumed that the reaction vessel is not moving
at speeds approaching the speed of light, for then relativistic corrections
to the mass would have to be taken into account. Second, it is assumed that
the time and energy scale of the observations, delta time and delta energy,
is not of the order of Plank's constant: h=6.6x10^-34 J*sec. That is the
lower limit of precision that time and energy can be measured as a result of
the Heisenberg uncertainty principle: dt*dE ~ h. At smaller scales the
conservation laws do not apply. You might add another question
(hypothetical). If mass and/or energy is lost/gained in a chemical reaction,
which particles lose/gain the energy? I do not know the answer to that
question. Only in nuclear and high energy "reactions" are the changes large
enough to be accessible experimentally. The citations below give you some
details (probably more than you want).

http://www.newton.dep.anl.gov/askasci/chem03/chem03534.htm

http://www2.yk.psu.edu/~jhb3/cotw06.htm

Treptow, Richard S. J. Chem. Educ. 2005 82 1636

http://en.wikipedia.org/wiki/Antimatter

Vince Calder
===================================================================
First, you must make sure that your students do not confuse "chemical 
reactions" with "nuclear reactions". It is only in nuclear reactions that 
there is a mass discrepancy and where a measurable mass is converted to 
energy and vice-versa. You need to clarify that bonds are not objects, 
they do not have mass, and that exothermic/endothermic chemical reactions 
are the result of the difference in the bond energies of bonds broken and 
bonds formed, not mass conversion.

Secondly, it is useful to do the calculations in class. If you want 
consensus, ask your students to define a lower limit of what they would 
consider a measurable mass loss (a milligram? a thousandth of a 
milligram?) within a normal lab setting. And then do the calculation on 
how much energy that would provide if in fact that energy was completely 
converted to energy. Then show what that energy loss would mean in terms 
of powering a light bulb (or a city!).

Greg (Roberto Gregorius)
====================================================================
Craig,

I looked up the answers that were given to this question originally,
including my own answer. I gave a very simple answer aimed
at a K-12 audience (which is what NEWTON is for), since the original
questioner gave no details as to his/her background or knowledge
other than "other" (this is also true of your current question).

As I look at all three responses, I agree with all of three
responses (including my own) at different levels of sophistication
and I see no substantial disagreement between them.
My own response ("no") was very cut-and-dried because the change of
mass that would be involved is so small as to be
unmeasurable by any apparatus I can conceive of (which is what Vince
Calder very correctly said). Therefore, to me this is a moot question.

All that said: your present question (about an exothermic reaction in a
glass container floating in vacuum) presupposes that all of the energy
generated by the exothermic reaction is lost to the environment. In
reality all of the energy generated by the reaction in your example
will have to go into the glass walls of the container because
"true" vacuum does not conduct heat at all. Now, if the glass
gets hot enough it may start to glow and give off some energy
in the form of radiation. I suppose that would
very slowly decrease the mass, but without actually putting it
on a balance to weigh it, who knows?
However, a caveat: quantum mechanics limits what
frequencies of energy can be liberated this way,
and not all of the energy that could be liberated as heat will be
converted into radiation. Some will remain as increased kinetic energy
of the molecules in the glass. Basically, the system will not evolve
in the manner you have suggested and give off all of the energy
that is liberated during the reaction to the surroundings.
In fact, keeping the system isolated in this fashion will inhibit
the progress of an exothermic reaction by keeping it physically
unable to release the heat it must generate in order to it to
proceed.

Now, what if we set the glass container on a scale in an actual
room? Well, some heat will go into the glass walls of the container,
some will go into the balance used to weigh the apparatus
during (or after) the reaction...some will even go back into the
system itself if the room is warm enough!

The second law does not permit us to predict how much energy will go
into each of these heat sinks, because the heat flow is an
irreversible/spontaneous process. It therefore becomes
impossible to answer the question of how much the mass would change
because there is no way to predict the dynamics of how the energy will
be disposed of as a result of the process.

I hope this helps....you have not asked a simple question...this
is really beyond the scope of NEWTON...

Dr. Topper
===================================================================
Craig-
     Sorry we did not answer it clearly before.
Thanks for sending us questions so clear that our answers cannot be unclear!

*** In an exothermic chemical reaction is mass converted to energy 
according to E=MC^2

Yes.

***...sealed glass container ... exothermic reaction... excess heat 
radiate out
would the whole thing (container and reaction products) weigh
minuscule amount less?

Exactly So.

  (It would not weigh less until the heat radiated out.
Potential energy of under-satisfied molecular bonds
weighs exactly as much (per joule)
as kinetic energy of thermal motion.
So the reaction itself would not change the weight of the container.
Radiating the heat away would!
Chemical reactions in a perfectly silvery thermal bottle
would then never change the mass of the container and contents!)

*** If so, what happens in an endothermic reaction?

Pretty much the reverse.
The reaction makes it cold, so energy is now allowed by laws of thermodynamics
to flow inwards from the "normal" (warm) surroundings,
and when it finally does so, the container weighs more.
Of course, for this to happen requires that the surroundings to be well 
above absolute zero.
Real deep space has a radiation temperature of 2.7Kelvin.
If a container was in thermal equilibrium with that, it would be too cold 
to allow most chemical reactions to happen.
So it seems you are imagining a Thermos bottle in a vacuum chamber whose 
opaque walls are at room temperature.
The vacuum you specify has the effect of leaving only one path (radiation) 
for energy transport between the world and the container.


*** Does the extra energy needed for the reaction convert to mass in the 
reaction products?

Yes, but I think the right words, the meaningful distinction, is {matter 
vs. energy}, rather than {mass vs. energy}.
Matter is a static, (un-moving), usually rather concentrated state of 
energy, and
Energy itself has mass at all times, regardless of what form it is in.
Try to adopt that concept, instead of deciding when it is matter and when 
it is energy.

(It bothers my mind a little trying to imagine how being weakly married to 
neighbors makes atoms heavier,
and being tightly bonded makes them lighter, but that is the polarity of 
the E=mc^2 mass change.
I would rather not assert that this potential energy situation is or is 
not part of the matter in the molecule.
All I have to do is remember it is more energy, so it has more mass.
I suppose, since potential energy is un-moving, it is plausible to call it 
part of the matter.)

*** If...sealed glass container...endothermic reaction....then 
added...infrared light
would the whole thing ... weigh more ...?

Yes, exactly.

The mass picked up by the container would be exactly the mass previously owned
by the free-flying IR (infra-red) photons that were absorbed in the container.
There you have it: conservation of mass and energy, simultaneously.
Which is less surprising if you believe that energy has mass
and matter is merely a "bound" state of some more energy.


*** ...is it impossible to answer...?

Regrettably, we cannot at present satisfy our skepticism by measuring mass 
change after chemical reactions.
But it is considered so consistent with everything we have been able to 
measure and think through,
and its effects are so impossible to perceive anyway,
that we adopt it as true and wrap our thinking around it.
It is pretty much an Occam's razor situation:
We have no demonstrative proof of this, but we do of larger energies than 
chemical,
and of many related situations, so it has become simpler to believe it, 
until proven noticeably wrong.

Someday we will come across a way to do the experiment, I bet.
Then we will see if the universe teaches us another curious exception to 
our "common sense",
or whether the theory was boringly right all along.
My guess, E=mc^2 will stand fine, but some other subtle fundamental thing 
might be noticed
in the effort to do this extremely sensitive measurement.

Jim Swenson
====================================================================