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(at the most fundamental level) why does energy release upon chemical reactions?

  1. Mar 5, 2012 #1
    Please don't be bothered by my question, i know it sounds real elementary and it could easily be researched, but after doing so, i haven't been able to find anything on how or why the natural phenomenon of energy release takes place as chemicals react with one another. If you think about it, this is probably a more physics related phenomenon as I've learned of most of this energy difference takes place at a quantom level.

    Let me add that I am currently specialized in automotive technology and right now im pursuing my major in mechanical engineering. So i try to train my brain to think about these concepts while i go through my "gen-ed" college courses, because the teachers seem to have no time to do so themselves.. (they are trained to get you the material that will be covered in the exams.. sadly)

    I've spend a chapter on quantom physics (in a college course, but i haven't had time to grasp the real significance/connection of it..) so i understand the the quantom leaps and energy transfer upon light emission and so on. I wasnt sure if it was quantom physics related.. more as nuclear unstable elements are.

    Let me provide an example, butane: c4h10 reacts with oxygen: o2 to produce h2o...
    During that re-composition, a hella bunch of energy is released upon ignition of two molecules that seemed previously dormant in our atmosphere, but this phenomenon isn't explained in my basic chemistry course... I guess so far my observation of this example would be the fact that hydrogen is a good source of energy because it is so simple and concentrated. (correct me if I'm wrong)
    -another example is the relation between pbo2 and pb when sulfuric acid is between them (acting as a strong electrolyte), i understand the strong chemical reaction that occurs. But what i don't understand is how such a reaction causes a difference in potential energy across the two plates- creating a voltage and flow of electrons. I know its gotta happen at a subatomic particle level, again leading this to more of a physical observation.

    If someone could further explain this origin of energy that is transformed, maybe its just one of the natural occurrences just as you press your finger on a spring, energy is now stored potentially, then released upon letting off the spring.. its gotta be the same as loading a battery full of electrolyte, so that a controlled chemical reaction can occur between the two dissimilar metals, releasing energy in the form of something else.

    Anyone care to enlighten me, and pardon me for my in depth analysis, I think at this point i just need to be redirected in my thinking strategy.
    Last edited: Mar 5, 2012
  2. jcsd
  3. Mar 5, 2012 #2
    263 views and no response. Come on guys
  4. Mar 5, 2012 #3
    It's not at the most fundamental level, but..
    There exist an optimum level of bonding between two atoms, similar to the noble gas' rule, if they are pushed further together they become strained, likewise, the further apart they get the more strained. It's all a consequence of the electronic configuration and the noble gas' rule which I don't know the intricate mechanics of, I'm afraid.

    When the bond is no longer 'strained' (falls into optimum position), the force acting against the strain gets released and acts on the molecule instead, thus becomes "energy"

    Hydrogen is a very energetic atom/molecule because it's so light and the bonding occurs in the first shell: It's only the electrons in the farthest shell that atoms interact with eachother chemically. so all protons/neutrons/electrons in the inner shells are sort of 'wasted' mass/deadweight (not true if you talk per mole, but whatever)

    The inner shells also generally have a bigger strain when it's not noble-gas-rule-configured than the outer shells.
  5. Mar 5, 2012 #4
    The formation of bonds stores energy. It's worth noting first off, that not all reactions give off energy. Those that do are referred to as exothermic reactions. The energy stored in the chemical bonds is released as heat or light. The inverse are endothermic reactions. Those take energy from the environment. An example of this would be cold packs.
  6. Mar 5, 2012 #5


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    Atoms bond together to form molecules in general because the process of forming a chemical bond makes the atoms more stable. If we consider the energy to two isolated hydrogen atoms versus the hydrogen atoms bonded together in a hydrogen molecule (H2), the hydrogen molecule has a much lower potential energy than the isolated hydrogen atoms. Therefore, when the hydrogen atoms come together to form a hydrogen atom, the potential energy of the system decreases and the excess potential energy is released as heat.

    Now, not all chemical bonds are equally stable. As it turns out, H-O and C-O bonds will in general have much lower potential energies than C-C, C-H, and O-O bonds. Therefore, when you convert the C-C and C-H bonds butane (C4H10) into C-O and H-O bonds in carbon dioxide and water, you are taking bonds with higher potential energies and converting them into bonds with lower potential energies. The lower potential energies of the bonds in CO2 and H2O explains why burning butane releases heat.

    This general principle helps explain other observations as well, for example, why fats store more energy than carbohydrates. Fats are composed mostly of C-C and C-H bonds whereas carbohydrates have C-C, C-H, C-O and O-H bonds in roughly equal proportion. Because carbohydrates already have many low energy C-O and O-H bonds, they will release less energy when burnt compared to fats, which consist almost entirely of the high energy C-C and C-H bonds.

    To understand which bonds have lower energies than others requires some knowledge of quantum mechanics, as quantum mechanics governs the behavior of the electrons in chemical bonds.
  7. Mar 5, 2012 #6


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    Although many textbooks (usually biology texts) make this statement, I think this is an incorrect way of thinking about chemical bonding. Bonded species almost always have a lower potential energy than the unbound radicals that form the molecule. Generally, we do not think of energy being "stored" in states with low potential energies.

    The energy from chemical does not come from breaking "high energy bonds" and releasing the stored potential energy. Rather, the high energy bonds do require some energy to be broken, but much more energy is released when the broken pieces of the molecule form much stronger bonds in the products. Therefore, the replacement of weak, high-energy bonds with much stronger, low-energy bonds in the products drives the exothermic reactions to release energy.
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