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Why combustion is exothermic?

  1. May 26, 2015 #1
    hello!

    what is the atomic mechanism of combustion that makes it so exothermic?

    what makes O2 so reactive with hydrocarbons for example and why is the reaction exothermic?

    thanks
     
  2. jcsd
  3. May 26, 2015 #2

    Drakkith

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    Staff: Mentor

    Do you know anything about chemistry? The answer can be fairly in depth depending on what level of detail you want to get into.
     
  4. May 26, 2015 #3
    yes, I know first years uni chemistry
    yes, I am looking for the deepest I can get, or a little less, that's ok
    at least something, because I can't find something online at all!
    I think it has to do with oxygen roots that are highly reactive
    but why are they so reactive?
     
  5. May 26, 2015 #4
    In the end it's all a question of energy. During combustion, the molecules' atoms of two substances get rearranged into new combinations. Whether a process is exothermic or endothermic depends on whether the final product is energetically lower or not. CO2 is energetically a lower state than O2 or hydrocarbons respectively. So, once you overcome the energy that keeps the O2 molecules together (and the hydrocarbons too), they can recombine into CO2 for example. Because the final energy state is lower, the energy difference ends up as heat. Which means, the reaction is exothermic.
     
  6. May 26, 2015 #5
    Oxygen has a high electronegativity so it likes to steal away the bonds from carbon. The highest electronegativities are on the top right of the periodic table to the left of the noble gases. That's because a full shell is very stable. Any additional electrons over a full shell have to go to a higher energy shell, which means these electrons take less energy to release.
     
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