Doubts about potential energy, bonding and chemical reactions

  1. Hello everybody,

    First, sorry for the bad english, it is not my language.
    I have a few theoretical problems and I would be very thankful if you helped me.


    I see the graphic about covalent bonds and potential energy a lot of times and they always say that potential energy decreases at the formation of the molecule >>> ex:

    In my textbook it says: '' the attractive forces between nuclei of the atoms and their electrons tend to approximate the two atoms and lower the potential energy of their system''

    But my doubt is: If two atoms form a molecule, their kinetic energy should decrease and the potential energy rise due to the bond that ''stores'' that energy? Then they go to a state of less kinetic energy (more stable). When they say the potential energy decreases they are ignoring the fact that what's important is the absolute value and in fact it's increasing like the graphs of velocity, that even if goes to negative It's increasing but at an another direction (analogy).The minus sign only shows that is potential.
    I'm confusing it all or not? Please help ^^

    2º Doubt:

    In a exothermic reaction, the energy released from the system comes from the difference of temperature, comes from bonds forming or from both?
  2. jcsd
  3. mfb

    Staff: Mentor

    The binding energy is set free (and heats the material, for example), the molecule has a lower energy than the individual atoms.
    This is independent of their kinetic energy.
    I don't understand what you mean.
    It comes from the chemical reactions - the formation, change or destruction of bonds (depending on the type of reaction).
  4. DrClaude

    Staff: Mentor

    Welcome to PF!

    To form a molecule from two atoms, you need something to carry away the extra energy. Usually, this is through a collision with another body. Otherwise, conservation of energy tells you that the two atoms will fly apart.

    Generally speaking, a chemical reaction will involve many bond formation and breaking. If it is exothermic, it means that the overall process released energy (the potential energy decreased), and the extra energy was transformed into kinetic or vibrational energy, and this increased the temperature.
  5. Thank you for the help!
    Im going to answer both in this message, It's easier.

    What I don't understand is: if at the beginning potential energy associated with the interaction of the two atoms was zero (no interaction), how can it decrease when they get closer?
    Then, I thought could be like the electrons around the nucleus because their potential energy is zero outside, but when they are pulled by the nucleus and enter, their potential energy increases, but is negative. So, if more negative, more potential energy because what matter is the absolute value, or I'm wrong with this too? This was what I was trying to say with the velocity analogy.

    mfb said that the binding energy went free, but before that, in the sistem of the two atoms that energy that went free was kinetic or potential? ---Causing the decrease of one of them--

    If it is independent of their kinetic energy, their kinetic energy stays the same? Then, how it releases the energy? If it is by heat, their kinetic energy rises immediatly after the bonding?

    So, you are saying if a bond is formed the potential gets lower. But if I want to break the bond of any kind, I have always to increase the kinetic to separate them? And decreasing it to bond them?
  6. mfb

    Staff: Mentor

    The scale of potential energy is arbitrary. It is convenient to set it to zero for large ("infinite") separation of the atoms, but it is not required. Only differences in potential energy are relevant. Negative potential energies are no problem, and the sign of it matters.

    Can be anything, but I don't think a description in terms of multiple "steps" (=what you are trying to get here) gives a useful model here.


    Kinetic energy is a bad number anyway - it includes the common motion of both reaction partners, which does not influence the reaction at all.
  7. 1º So, imagine we have a system of two atoms. The potential energy of the system is the sum of potential energy of each atom.
    Their kinetic energy they have is irrelevant for the bond, right? So, when they bond, the total potential energy decreases and that energy is converted in kinetic energy? >> their kinetic energy increases?

    Now, if we think in other system, where occurs an exothermic reaction. We transfer the ativation energy to break the bonds and what happens is that kinetic increases and the colision have enough strenght/energy to break the bond and the potential increases too?
  8. mfb

    Staff: Mentor

    Potential energy is a property of the whole system, or at least a property of each pair of particles.
    Their kinetic energy considered separately yes, their relative motion no.
    The kinetic or potential energy of something nearby (might be the atoms, but does not have to be) increases, or light is emitted, or combinations of those 3.
    (1) was an example of an exothermic reaction already.
    Which collision?
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