View Full Version : OT: chemistry question

12-07-2010, 09:46 PM

I'm posting a question about chemistry because I know many of you have in-depth knowledge of science.

When carbon and oxygen combine to produce CO2, the resulting molecule has higher speed than the previous compounds. After all, the reaction is exothermic which means that the resulting compound has to have a higher kinetic energy (i.e. hotter). Note this question is really a question in particle physics, since I'm trying to understand what's happening with a single instance of this reaction.

1. What's the mechanism by which the extra kinetic energy is obtained? Since all chemical reactions are interaction of electromagnetic force, something must get pulled or pushed by the nucleus or the electrons.

2. Is there a conservation of momentum in this reaction?

3. Is there net loss of mass in this reaction? (yes, I'm referring to very very very small loss).

Bruce Griffing
12-07-2010, 10:53 PM
3: loss of mass - yes, but as you suggest it is very tiny

2: momentum is conserved

1: The initial state you suggest is not easy to achieve because single carbon atoms are difficult to produce - especially a carbon atom that is not ionized. That said, if you could produce the suggested initial condition, the reaction would follow the rules of quantum mechanics. The electrons are the actors in the play, and they must have an initial and final state allowed by quantum mechanics, as well as a place to put any energy difference between the states. There are places to put the energy. The molecule can radiate EM energy, or vibrate internally or both. It cannot take off at high speed in violation of #2. All of this said, the results would depend on the details. The Carbon would have to be moving slowly toward the Oxygen molecule. Someone could do the calculation, but it would depend in detail on the actual velocity and the geometry of the intersecting paths as well as the energetic state of the electrons on both the carbon and oxygens.

12-07-2010, 11:02 PM
In an exothermic reaction when the molecular reaction proceeds to form a new compound the atoms involved end up in a lower energy state. This results in a release of binding energy in electromagnetic form. It may be very wide spectrum from the far infrared to the extreme ultraviolet. It appears as both heat and light in most combustion processes. The binding energy released is responsible for initiating further reactions in the surrounding molecules by giving them sufficient energy to overcome the electromagnetic repulsion of the electron cloud of the atoms.

When they combine some of the binding energy is released. That release does result in a reduction of mass although not a reduction of the number of fundamental particles present. The reduction is in accordance with e=mc^2. It is extremely tiny and is not a factor in any real world calculations of the chemistry of combustion processes.

12-08-2010, 12:49 AM
Thanks for the reply. The answers that you provided is consistent with what I was thinking.

I'm still puzzled over how such a reaction increases the temperature. I understand how this works as a macro phenomina (i.e. energy/mass relationship), however in order to increase temperature, you have to make the atoms/molecules move faster, which means the reaction must have some how caused the atoms/moecules to accelerate.

I'm wondering whether this type of particle dynamics is well understood?

I was trying Google this topic, but I didn't know which keyword to use.

Bruce Griffing
12-08-2010, 01:28 AM
Two things to consider from my previous post. When EM radiation is emitted, it carries momentum. The emitting object recoils with the opposite but equal momentum vector. Internal vibrations can be relaxed in a collision with another molecule, also resulting in both molecules recoiling with extra velocity. Similar collisions occur with the walls of a vessel - which themselves may have been heated by EM radiation. If you have a deep interest in this subject you will need to study both statistical mechanics and quantum mechanics. The questions you are asking are not easily answered, even then.

12-08-2010, 01:38 AM
First of all, never mind the e=mc2 stuff. It's a small effect, and isn't needed to understand this problem. What other posters have said is true.
However, there are two key concepts missing:
Thermal equilibrium and multi-particle interactions.

After the atoms combine to form the new molecule, there is excess energy around. There are different ways that a molecule can hold energy: It can jiggle, rotate, move faster (not in this case), have electrons at higher energy levels, and then shoot off photons when those electrons relax, etc. However, as Bruce said, it doesn't move "faster" due to conservation of momentum.

Due to quantum mechanics and thermodynamics, for systems at equilibrium, the energy eventually spreads out into a preferred distribution of modes. For example, at a given temperature, a certain fraction of energy is in the molecule moving faster, another fraction is in the molecule jiggling, and another fraction is in excited electrons that like to shoot out photons.

In general, the further apart the energy levels of a particular mode, the less energy will be in that mode at low temperatures. For example, the kinetic energy modes have very small energy spacing because a molecule can be moving "slowly", or it can move "just a little tiny bit faster". On the other hand, electron energy levels have wide energy spacings, so at low temperatures, there isn't much energy in this mode. That's why a balloon of helium doesn't go around emitting light. Now in this case, your CO2 molecule is not at equilibrium because conservation of momentum says it can't be moving fast. How does it reach equilibrium?

The key point is that there are other molecules around as well. Molecules can give energy to each other by ramming , shooting photons, etc. When they interact with each other, energy can be transferred from one mode to another. For example, two slow moving molecules that are jiggling violently can bump into each other and shoot off in opposite directions, at a faster speed than before, but still conserve momentum. It's like when two people punch each other at the same time, and they both go flying.

After enough interactions, the cloud of CO2 is at equilibrium, and there is more energy than before in the "kinetic energy" mode, so on average, all molecules are moving faster than before.

The short version is that you need interactions with other molecules to transfer energy from the "internally excited" modes to the "moving faster" modes.

Edit: OK, Bruce beat me to it.

Forrest Addy
12-08-2010, 02:37 AM
Back when went to school "phlogiston" entered the alchemy picture somewhere. In biology we learned about "spontaneous generation" and astronomy Aristotle ruled the day. Sooooo what happened? Advancements? I'm still waiting for the soup cans and string to improve so I can call long distance.