Ask A Scientist Chemistry Archive

  
name         Ron S.
status       educator
age          30s

Question -   I am a science teacher and we have been looking at the
law of conservation of mass. We did an experiment in which we weighed
the reactants (Sodium chloride, Sodium carbonate and water) and then we
weighed the products. This was all done in a sealed bag (hopefully). The
problem is that every one of my students had a loss of mass. In my
trials I had a loss of mass of 0.6 grams every time. Question: Does this
mean that the Baggie was not acting as a closed system and we loss mass?
Or does the exothermic energy have mass and that's were we lost the
mass. I don't believe that energy has mass but another teacher here
believes that to be true.

Now I have tested an endothermic reaction and if the second hypothesis is
correct the weight of the products should be more than the reactants
because it takes in heat energy but it too lost mass.
-----------------
Ron,

You are correct, your Baggie is likely not being a closed system.   I'm
presuming that your class is being very rigorous about the additions,  i.e.
not allowing the reactants to come into contact within the Baggie until it
is already sealed, otherwise if you're generating gas that can be lost very
quickly.
If your Baggie was truly a closed system, then I suspect something more
pedestrian, like not properly taring the balance, or the balance being
miscalibrated.
The last possibility I can think of is that if there is enough exothermic
energy, the Baggie essentially becomes a hot air balloon (it may not be
enough to lift the Baggie, but could result in a decrease in
weight).  Although its mass is not truly changed, its buoyancy relative to
ambient air may have.  Assuming you have no leaks, once it is cooled down
the weight (not the same as mass) should come back up.

Don Yee
====================================================
Ron,

NO mass is lost in a chemical reaction. Yes, energy can come from mass
(E=mc^2), but (1) that is in nuclear reactions, and (2) the energies that you
produce are so small that the mass loss -if any- would be so infinitesimally
tiny (c is after all 3x10^8 - and it is squared). The energy that you
observe in the reaction is really due to the difference in the internal
energy (or Gibb's Free Energy) between reactant and product.

I am a bit confused on what reaction you are doing NaCl + Na2CO3 + H2O to my
mind does nothing. I'm going to guess that you meant HCl instead of NaCl and
that the expected products are NaCl, H2O and CO2. If my guess is correct,
then there might be a problem in your experimental set-up. I would suggest
using a reaction wherein the reactants and products are either solids,
liquids and aqueous solutions - no gases.

Greg (Roberto Gregorius)
=====================================================
Strictly speaking, mass is not a conserved quantity. Mass and energy
are related by the famous equation: E=mc^2. However, because the relating
quantity, c^2 is so large for purposes of "chemistry" it is conserved for
all practical purposes. The conversion is 1 cal = 4.184 joules. The mass
of a single proton (1.6726x10^-27 kg) ~ 1.5x10^-10 joules, so clearly you
could never observe a violation of that approximately conserved quantity.
I don't think you can trust that a "Baggie" is going to be "gas-tight".
  In about 1918 mathematician Emmy Noether proved that a physical variable
is conserved, if and only if, there is a continuous symmetry associated with
that variable. No less  than Albert Einstein praised her contribution to
physics as "penetrating mathematical thinking". So of the common physical
variables: 'energy', with the associated continuous symmetry 'time';
'momentum' with the associated continuous symmetry 'position'; and 'angular
momentum' with the associated continuous symmetry 'rotation' are strictly
conserved. Mass is not.
     Born in 1882, her contributions to both the physics and mathematics
(abstract algebra) are all the more remarkable, being a woman in pre-Hitler
Germany. She moved to the U.S. in the 1930's and taught at Bryn Mawr
University until her untimely death in 1935. She is considered the most
prestigious woman mathematician of all time. It is a statement to how far we
have yet to go, that few recognize her name, a name that ranks right up
there with Gauss, Cauchy, Riemann, and Hilbert (her mentor).

Vince Calder
=====================================================
You do not loose mass by loosing energy.  If you only turned a tiny
amount of matter into energy, E=mc2, then you would have quite a bang!

So maybe other errors.  I do not see much of a reaction here, but maybe
you are loosing some CO2 from the carbonate.  Maybe the scale is not
working well. Maybe the Baggie is getting filled with gas, getting
buoyant in the air.  0.6 grams is a lot if you are using a fancy
electronic balance, they are good to much finer level.  But if you are
using a cheap scale, then maybe you just have this as error.

Steve Ross
=====================================================
Normally when we weigh objects we neglect the buoyant effect of air because
of the small effect it has.  However, if your products include a gas, such
as carbon dioxide, it will significantly increase the volume of what you are
weighing.  In this case you have to account for the weight of the air
displaced by the volume of the products.  Air weighs a little over a gram
per liter so if you produced half a liter of gas this might account for the
mis-match in masses.

Greg Bradburn
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