Not technically correct. A small amount of mass is lost or gained in chemical reactions, just like when gravitational potential energy is gained or lost.
If you walk up a flight of stairs, you will be heavier at the top. If an electron changes orbital, its mass changes as well, making it heavier or lighter.
Let's calculate it. A 90 kilogram man walks up a flight of stairs, one story (3.3 meters). The expression "m * g * h" gives us the gravitational potential energy of mass "m" ascending height "h". Evaluating the formula we get 2910 joules. So how much heavier has the man become? Given E=mc^2, then m = E/c^2. As you can see, we'll be dividing by c^2, which is a very big number. The resulting increase of mass is 3.24 E-14 kg, or too small to be noticeable, but still very real: A 90 kg man ascending a 3.3 meters gains 0.00000000000003 kilograms of mass.
Chemical potential energy changes result in mass changes as well. These changes in mass are insignificant at the scale of chemical reasons, but are indeed taking place. So while it may be reasonable to simplify a discussion of chemistry by saying that chemical reactions don't change mass, in reality the changes do take place, just at a very small scale. A starting point for further research:
"Whenever any type of energy is removed from a system, the mass associated with the energy is also removed, and the system therefore loses mass. This mass defect in the system may be simply calculated as Δm = ΔE/c^2"
nit: "heavier" refers to weight, not mass. The man will actually be lighter at the top of the stairs due to decreased gravity. (R_earth / (3.3m+R_earth))² ≈ -0.0001 %
Someone else can calculate the decreased buoyancy in thinner air ;)
Q is the released energy. If you weigh the molecules before and after very carefully, you will notice that they lose weight (or mass if you are being pedantic about it). That mass loss corresponds exactly to the Q-value (through E=mc²).
Why is Q not part of the system anymore? Isn't the Q carried by the 2H₂O molecules (or the solution/matrix/whatever)? Doesn't it just represent that the energy is of a different form?
How does bond energy represent mass but kinetic energy does not represent mass? I realize Q is a different symbol in the formula, but that's just notation. The bond energies are implicit in the formula, but that doesn't mean they aren't there.
No mass is lost in this reaction. The mass of the atoms remains the same. The energy that is released is due to the difference in bond energies. To balance the equation you would need to add a Q to the other side, corresponding to the bond energies.
The molecules consist of Hydrogen and Oxygen that have equal atomic masses, either side of the equation, due to the identical numbers of protons, neutrons and electrons.
Any mass gained or lost will be due to changes in bond energies, and the corresponding emission or absorption of photons that change the energy states of the electrons in the bonds.
Putting a Q on the left hand side would be an error. Q indicates that the energy is not bound in the molecular configurations.
If you measure the mass of a quantity of water accurately enough, the mass will be lower than the mass of the equivalent amount of hydrogen and oxygen gas. The difference would be so small that our most sensitive instruments would not be able to detect it.
Likewise, in a nuclear reaction, the only thing changing is the nuclear binding energies of the protons and neutrons. However, since the energy involved is many orders of magnitude higher, our instruments are sensitive enough to detect the difference in mass.
Speaking in generalities yes, but TECHNICALLY stuff that contains more energy is heavier too. But given that most enthalpies are in the kJ/mole range and most moles of things are pretty heavy relative to 9e18 (the speed of light squared) that you might as well round it out and say "no mass is created or destroyed in an ordinary chemical reaction"
