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Free radical Halogenation for heat

  1. Jul 19, 2011 #1
    Hi all

    I am an Mechanical Engineering major and I had a quick chemistry question. This may be crazy since I only had 2 semesters of College Chem

    If I had a tank of just bromine gas. Now my understanding is that these diatomic bonds can be broken by simply exposing the gas to sunlight. If I were to bombard the tank with radiation and light. To break up all the bonds (hypothetically) and then subsequently shielded the gas and allowed the bonds to reform would generate heat and at what rate.

    I read that the bonding for diatomic bromine is 192KJ per mole. So its possible for me to get that energy out as heat correct?

    Then there is the question of the rate of reformation. How long would this take? what information would I need to study such a reaction? Is this Pchem?

    Any help that can be provided is much apprecaited. Thanks in advance.

  2. jcsd
  3. Aug 10, 2011 #2
    Three hundred views and not even a suggestion in regards to where to start!?!?
  4. Aug 10, 2011 #3
    A point of clarification - halogenation generally refers to the substitution of hydrogens by halogens. Given that you just want to dissociate bromine molecules and let it recombine, it's probably the wrong search phrase. You'd have better success looking up "bromine photochemistry" or "bromine photodissociation." And yes, chemical kinetics & thermodynamics - and even some basic photochemistry, depending on the course and instructor - are topics taught in physical chemistry courses.

    The laws of thermodynamics most assuredly still hold, as I'm sure you know. You start off with a container of bromine molecules, add some energy, wait for it to equilibrate with its surroundings, and end up with a container of bromine molecules. You are not going to get something for nothing.
  5. Aug 10, 2011 #4
    Hi Mike thanks for your responsce

    I am well aware of thermo and the resulting constraints. The goal is to take that radiation (solar or UV) and have it converted to heat via the exothermic reaction. I need help figureing out how to calculate this.

    Can anyone assist?
  6. Aug 10, 2011 #5
    FYI - bromine is a volatile liquid even at standard pressure/temperature. Forgot to mention that earlier.

    I suppose my comment about "getting something for nothing" didn't quite make the impression I had hoped - take a look at this:


    Examine the sketch of an exothermic reaction, where they plot potential energy versus the reaction coordinate.

    Your reactants and products are exactly the same in this scenario you've outlined for us here (a container of bromine molecules at the start, a container of bromine molecules a the end). There's no heat being evolved in going from the start to finish.

    If you want to figure that cleaving the bond (with a bond dissociation strength of 192 kJ/mol, according to your first post) results in two bromine atom radicals, then given that radical recombinations tend to be very rapid with (generally) minimal activation energy barriers, you'll get most of that back. But that's only half the story.

    If you look at something like the combustion of methane with this same sort of analysis, you can see very clearly that it is an exothermic reaction. Your scenario, to be honest, seems like an exercise in futility.
  7. Aug 11, 2011 #6
    Hi Mike

    I understand your point and I have already discussed this matter with someone with much more chemistry knowledge than I and he did give me a note of warning that this is inefficent and suggested go with a solar collector approach but for my needs I do not need to be efficent.

    My reactants and product are the same. Just pure bromine, it is simply a vehicle to transfer light to thermal energy. That is all, i am going to have this device well insulated to trap this heat.

    I am not expecting this to be the equivalent to burning methane or anything of the like. I if I can eek out a some thermal energy and trap it I am happy.

    Yeah i am aware that bromine is horrible and reacts with everything, but at this point just looking to see if the overall concepts will work

    So to anybody willing to help

    How can I go about quantifying the heat released by pulsing UV light at pure diatomic bromine gas ? I have seem some papers that give reaction constants but each constant seems very specific and experimentally derived. I would really like to go forward with this regradless of the practicality of it all.

    Thanks in advance
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