Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: David Cooper on 26/12/2011 22:08:24
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If you turn one O2 and two H2 molecules into water, it's easy to imagine how the excess energy is released through kinetic energy (which might equally be regarded as heat). What happens though if you only have a single oxygen atom and an H2 molecule? Once bound together, the released energy cannot make the resulting H2O molecule shoot off in any direction as there would not be a conservation of momentum of the system. Does the energy get released in some other way such as emission of photons, or do the atoms just vibrate about violently relative to each other within the H2O molecule until it collides with another molecule where it can be converted to kinetic energy for both molecules in opposite directions?
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Does the energy get released in some other way such as emission of photons,
Energy may be released, or it may have to be applied. Making water into hydrogen and oxygen requires a lot of energy. Combining hydrogen and oxygen to make water releases a lot of energy. The energy can take many forms - heat, light, electrical...
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The energy is a combination light, linear velocity and angular velocity. Energy, linear momentum and angular momentum are conserved. Since energy is scalar and momentum is a vector, you can balance both by adjusting the angles. Opposite momentum vectors cancel, but the corresponding energy scalars add as absolute value.
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Am I correct in rephrasing your question this way:
Does an 02 molecule and 2 hydrogen atoms create more energy when combusted than a single Oxygen atom combusted with 2 Hydrogen atoms?
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One of you may have answered my question, but if so I didn't understand it well enough to identify the answer there. I'd better start again:-
When water is formed from hydrogen and oxygen, energy is released. The energy is released as heat and light. (Some of the heat might be infra-red light, but if so that will count as light.)
In the case of one O2 molecule combining with two H2 molecules, we have two H2O molecules being created, but there is an excess of energy now which must be released. Some of this energy will be released as light, while the rest will be released as heat in the form of kinetic energy - the two water molecules can be sent flying away from each other in opposite directions.
Another possible case involves an isolated oxygen atom combining with an H2 molecule. Clearly energy will again need to be released (because O2 molecules would all break up if being on their own took them to a lower energy state). This case is more interesting than the previous one because once our oxygen has combined with the two hydrogen atoms, it cannot take on any kinetic energy from the energy released by the water molecule being formed - if it did, it would change the momentum of the system. So, the question is, does all the energy released from this event have to take the form of light, or is there some other way for kinetic energy to be held inside the stationary water molecule? I can imagine that it could be held in a stationary molecule if the atoms of that molecule moved about more energetically relative to each other. I suppose that wouldn't be very different from how heat is contained in a solid, so I may just have answered my own question. This could then be converted into high-speed movement of the molecule if it came into contact with another almost-stationary molecule, that other molecule taking on half the kinetic energy as they fly apart in opposite directions (ignoring any complications of the two molecules having different masses (which means the kinetic energies could be different, though the momentum of each would be the same).
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Am I correct in rephrasing your question this way:
Does an 02 molecule and 2 hydrogen atoms create more energy when combusted than a single Oxygen atom combusted with 2 Hydrogen atoms?
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I believe the answer is YES, because twice as much water is produced. However, you don't get twice as much energy because atomic oxygen is more reactive than molecular oxygen. It takes energy to break the bond between two oxygen atoms, and they take some of that energy with them until they meet up with the hydrogen.