Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fundamental understanding of why energy is released in covalent bond formation

  1. Mar 27, 2010 #1
    Can someone explain to me the fundamental understanding of why, when two hydrogen atoms for example, release energy when they form a covalent bond? I can't seem to comprehend. Here is my reasoning, please see if this makes any sense of I am simply going in the wrong direction, I would really appreciate this. I am an undergraduate student loving the sciences and have tried figuring this out all night, it is frustrating me:

    The kinetic-molecular theory of matter states that all matter consists of tiny particles (everything from atoms, molecules, or ions) which are in constant motion. This means that atoms, molecules, ions, etc. all have kinetic energy associated with them on the microscopic scale known as thermal energy.

    I know bond formation between two hydrogen atoms occurs when two atoms (that are moving very high speeds) are stabilized due to the electrostatic attraction-repulsion stability formed between the electrons and protons of the two atoms. This constitutes a chemical bond.

    So, did the kinetic energy of the atoms that was moving really fast, get converted to heat given off to the surroundings, because the bond formation slowed down the thermal energy of the once separated, fast moving atoms??

    I am also having a hard time understanding chemical potential energy in a covalent bond. I know that chemical bond formation within a molecules, gives rise to chemical potential energy due to electrostatic attraction-repulsion stability formed between the electrons and protons of the two atoms.

    Is potential energy of a chemical system, for example a molecule, visualized as the energy required to counteract the electrostatic force that holds atoms together, i.e the chemical bond?

    So, to break a bond we would need to increase the kinetic energy of the individual atoms so that they overcome the potential energy formed in a covalent bond by the electrostatic interactions that was holding the bond together?

    How do you increase the kinetic energy of an individual atom in order to overcome the potential energy of covalent bond in the molecules? They absorb thermal energy from the surroundings, correct?

    Thus chemical potential energy is converted to kinetic energy in bond breaking? But in bond formation kinetic energy (thermal energy) of the atoms is converted to heat and/or light?
  2. jcsd
  3. Mar 28, 2010 #2


    User Avatar

    Staff: Mentor

    Not necesarilly - after all, molecule after atoms reacted moves on its own, there is no reason why it shouldn't carry original momentum and kinetic energy of reacting atoms.

    I am not sure if I follow your thinking about potential energy. Please remember that separate atoms also had a potential energy - after all, energy released when the bods are created doesn't come from nowhere, it existed in the system.

    They absorb energy either during collisions or from the electromagnetic radiation (be it light, UV, infrared, whatever with a correct energy).

  4. Mar 28, 2010 #3


    User Avatar
    Homework Helper
    Gold Member

    However I think this is a fair statement for an exothermic reaction like between two H atoms you mentioned. In fact in a rare gas such reactions can fail to happen because there is nothing to take away the energy given off. The reaction may therefore need a third molecule or atom to collide within a short time for the excess energy to be transferred to and allow the bond to be formed. Or else the energy may be taken up in for a time in the vibrations/rotations of other bonds in a larger molecule and again be redistributed in collisions, failing which it goes back into the new bond and the molecule re-dissociates, (reaction reverses). In solution on the other hand the energy is readily transferred and this kind of limitation does not occur.

    I think this is partly why as many bare hydrogen atoms exist in some regions of space as do - not just the rarity of the collisions (which would only need to happen once each if they all gave rise to an atom) - but I'd need an astronomer to fill this in.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook