3 ms·

2H₂ + O₂ --> 2H₂O + Q 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

2H₂ + O₂ --> 2H₂O + Q 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).

MarcScott 12y ago
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.
- sp332 12y ago
  The energy Q is in the molecules. It is released during the chemical reaction.
  - MarcScott 12y ago
    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.
- dietrichepp 12y ago
  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.
fluidcruft 12y ago
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?
- dragonwriter 12y ago
  Its part of the system, but its not mass anymore but another form of energy. Mass is lost, energy is conserved.
  - fluidcruft 12y ago
    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.

MarcScott 12y ago

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.

sp332 12y ago
The energy Q is in the molecules. It is released during the chemical reaction.
- MarcScott 12y ago
  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.
dietrichepp 12y ago
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.

fluidcruft 12y ago

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?

dragonwriter 12y ago
Its part of the system, but its not mass anymore but another form of energy. Mass is lost, energy is conserved.
- fluidcruft 12y ago
  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.