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Subatomically, what causes an atom speed up when it has a chemical bond broken?

  1. Nov 2, 2011 #1

    dlr

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    1. The problem statement, all variables and given/known data

    According to the molecular biology book I am reading, when a chemical bond breaks, the energy in the bond is released in the form of heat.

    which is to say, when a chemical bond breaks the entire atom's speed increases (the atom gains 'heat energy').

    But what is happening at the electron/proton level to make this happen?

    2. Relevant equations



    3. The attempt at a solution

    I am trying to imagine this in my mind, say for a covalent bond. This is the best theory I have been able to come up with.

    1) An electron is physically located between the two nuclei of the atoms it is being shared by.

    2) the electron that is being shared by the two atoms is further away from each of the nuclei in the atoms than it would be from the nuclei in a lone atom, as the protons in the two nuclei repel each other electrically.

    2) When the bond breaks, the nucleus of the remaining atom moves closer to the bonding electron, since it is no longer being electrically repelled by the protons of the other nucleus. ergo, physcial movement of the atom increases (ie, the atom speeds up, ie, experiences heating) when the chemical bond breaks.

    Is this right, or am I completely out in left field?

    And so if this is true, sometimes, statistically, the atom would actually slow down instead of speed up, cause the nucleus would move 180 degrees out from the direction it happens to be travelling.
     
  2. jcsd
  3. Nov 3, 2011 #2
    I think you need to consider why the bond is broken, that is, the bond will not spontaneously break. Say, for example, you have a chlorine molecule, it will not suddenly fly apart into its individual atoms, each gaining energy. That would violate the second law of thermodynamics. Imagine that another chlorine molecule hits the first at high speed and causes the bond to break and the atoms to separate. Here the energy comes from the collision. Or perhaps you hit the chlorine molecule with a photon of appropriate frequency such that the molecule begins oscillating so much, it flies apart. Here the energy is coming from the photon.
    Hope this helps.
     
    Last edited: Nov 3, 2011
  4. Nov 3, 2011 #3
    I don't know too much about chemistry, but physics wise, perhaps it is a conservation of momentum related phenomenon? If one atom is more massive than the over, depending on the angles, the less massive atom would move with a higher magnitude of velocity, right?

    What the person above me said sounds pretty correct. The energy to split the molecule would come from an outside source. Think of two pool balls rolling along (or something like that) and then the cue ball come flying at them and hits them both with an equal amount of force, energy should be conserved here pretty much, so the two pool balls sytem (which will break apart) will also gain energy and they will move faster.
     
  5. Nov 3, 2011 #4

    dlr

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    I'm thinking of the case when one molecule hits another at high speed and causes a bond to break. Sometimes when a molecule breaks apart into two pieces the reaction is exothermic (gives off heat--causes the resultants to speed up), and sometimes it is endothermic (absorbs heat-- causes the resultants to slow down).

    What I am trying to understand is what physically happens, down at the subatomic/atomic level to make this happen.

    There is a transformation going on from "chemical energy" to "kinetic energy", but how?
     
  6. Nov 3, 2011 #5
    I'll take the easier situation first, an endothermic reaction,where we add energy. This is as simple as heating up the container that the reactants are in, increasing the average speed of the molecules. Some of the molecules are moving slower than the average and some faster. If the molecules are [itex]CL_{2}[/itex] for example, I will consider two molecules that have sufficient energy(speed) to break the bonds of one of the chlorine molecules into two chlorine atoms.
    The chlorine atoms in [itex]CL_{2}[/itex] sit in a "potential well", a well of potential energy. One must add energy to one of the atoms(or both) to break the bond, similar to hitting a ball hard enough on the moon so that it will escape from its surface and keep going. The ball will slow down as it continues its escape from the moon. When one molecule hits the other, it's like the bat hitting the ball. After the collision, the total kinetic energy is less; the incoming chlorine molecule has transferred some of its energy(it now moves slower)to the other molecule, an atom in the other molecule initially moves fast enough to separate from its partner, slowing as the separation continues. If one were to calculate the kinetic energies before and after there would be a net loss, the difference being found in the increased potential energy of the separated atoms(bond dissociation energy) and probably vibrational energy of the intact molecule. Less kinetic energy means a lower temperature, the two molecule system has cooled overall. If we wish to have additional molecules collide(react) we need to keep adding energy to keep the speed of the molecules high enough to have the energy to break the chlorine molecule bonds, hence, an endothermic reaction.
    The exothermic reaction is a bit more complicated but it is largely the reverse of the exothermic reaction and I will try to get to it in the near future if no one else does.
    Hope this is helping.
     
  7. Nov 3, 2011 #6

    dlr

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    Yes, but, I really want to talk about it at the level of protons and electrons.

    Say, a fast moving chlorine molecule hits a 2nd chlorine molecule. Or rather it's electromagnetic field rams into the electromagnetic field of the 2nd molecule hard enough to ... what?

    Does molecule 1 get close enough to molecule 2 to push away some of molecule 2's electrons to the other end of molecule 2, turning it temporarily into polar molecule?

    Or rather, wouldn't the electrons of both molecules be pushed away from each other at the 'point of impact' by their electromagnetic fields, making both molecules temporarily somewhat polar...

    So then, would the protons of molecule 1 closest to the place of impact be close enough to, at least temporarily, grab onto the electrons of molecule 2, (and vice versa for molecule 2's protons and molecule 1's electrons), ie, a very, very temporary bond between molecule 1 and 2?

    But then what happens?
     
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