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Enthalpy: endo and exo

Discussion in 'DAT Discussions' started by osimsDDS, Jun 14, 2008.

  1. osimsDDS

    osimsDDS 5+ Year Member

    Jun 8, 2007
    Is this correct:

    Breaking of bonds is an endothermic reaction where energy is required ( in the form of heat ) so heat is absorbed by the reaction.

    Making bonds doesnt require energy so therefore it is exothermic and heat is given off from the reaction to the surroundings...from the system to the surroundings...

    thanks...this stuff always messes with me so i wanted to get it straight!
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  3. pistolpete007

    pistolpete007 2+ Year Member

    May 18, 2008
    ya i like to think of ice when dealing with this

    when ice is melted bonds break....and when this happens heat is absorbed so its endothermic

    however when ice is made bonds are it is exothermic bec heat is being released

    htats how i keep it straight if this is wrong im gonna go crazzyyy
  4. lphiewok

    lphiewok ACFAS Member 7+ Year Member

    Aug 19, 2007
    Fort Polk, LA
    sorry - i might have put wrong information
    Last edited: Jun 14, 2008
  5. DCRedskinsRule

    DCRedskinsRule 7+ Year Member

    May 27, 2008
    Yea osims, you're correct and all the suggestions are real helpful. It took me a while to get it straight too until my Inorganic prof explained it to me in real simple terms. When you form a bond, it is stable, more so than the two molecules that were flying around individually. So when something is more stable, it is at a lower energy state than when it was not bonded.

    Thus, to form a bond, which is a stable and lower energy state than before, it must give off energy and is therefore exothermic. It's not so much that making bonds doesn't require energy, it's more the energy state before and after making the bond.

    I know that sounds confusing...but for some strange reason it made sense to me and I didn't forget it after that. But the ice thing works :)
  6. pistolpete007

    pistolpete007 2+ Year Member

    May 18, 2008
    another thing u should note is that........what happens to enthalpy and entropy when going from solid to gas or liquid to solid....

    the way i have it down is knowing the phase diagram i know its stupid but it works for me....I am POSITIVELY sure when looking at the phase diagram it is going from S to L to G so both delta H and S is POSITIVE

    however if it is going OPPOSITE G to L to S delta H and S is OPPOSITE of POSITIVE so both delth H and S is neg

    hope that helps
  7. bumpski20

    bumpski20 2+ Year Member

    Sep 2, 2007
    the way i remember that endothermic reactions result in positive enthalpy change is when ever i see enthalpy or change in H i say to myself "END on a POSITIVE note"

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