Sciencemadness Discussion Board

Fundamental understanding of why energy is released in covalent bond formation

dolimitless - 27-3-2010 at 20:44

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 once 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 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?

JohnWW - 27-3-2010 at 21:08

It is because of the lower energy level of two spin-paired electrons occupying a single bonding molecular orbital, which constitute a covalent bond, compared to single atoms and free radicals with unpaired electrons alone in orbitals (which are much less stable and more highly reactive, due to their higher energy levels.). There are, however, a few exceptions in which this spin-pairing is not so energetically favored: complex organic free-radical molecules where steric factors and resonance delocalization of unpaired electrons preclude unpaired electrons from pairing; and high-spin transition metal atoms and their cations in which the energy-lowering obtainable by pairing of electrons in "d" and "f" orbitals (distinct from those in the outermost "s" and "p" orbitals) is less than the ligand field stabilization energy conferred by bonding to ligands.

In the pairing of two atoms or free radicals with unpaired electrons to form a covalent bond in a molecular orbital, in an exothermic chemical reaction, a quantum of light of a wavelength L corresponding, by the Planck equation E = hf = hc/L, to the lowering of the energy-level of the electrons, is emitted. This can be detected and measured with various spectrometers, depending on the wavelength.

[Edited on 28-3-10 by JohnWW]

dolimitless - 27-3-2010 at 23:12

That didn't help me at all :(

Can someone please refer to the questions above and see if you can answer them for me? Thanks so much!

Vogelzang - 28-3-2010 at 05:07

Chemists use Gibbs free energy (G) for chemical reactions. Not all chemical reactions are exothermic.

Gibbs free energy is defined as: G(p,T) = U + pV − TS
which is the same as: G(p,T) = H − TS

H = ethalpy
T = temperature in Kelvin
S = entropy

Pauli's exclusion principle says only two electrons can occupy one electron orbital with both having opposite spin. Hydrogen has one electron orbital and all atoms want full outer orbital(s) if possible. Atoms combine with other atoms to donate or share electrons with other atoms producing molecules that have either covalent or ionic bonds. As Aristotle said thousands of years ago, the reason something falls is because it belongs on the ground. The modern explanation is that things naturally go from a high potential to a lower potential in spontaneous reactions or processes. Gibbs free energy per mole is chemical potential.

[Edited on 28-3-2010 by Vogelzang]

Vogelzang - 28-3-2010 at 05:22

Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals.

http://en.wikipedia.org/wiki/Electron_shell

bdgackle - 28-3-2010 at 05:46

The double pair is "more stable" than the single pair... so you'd have to put energy IN to break it down. Logically, you get that energy BACK when you put it back together. That's sort of circular, though, isn't it?

The visualization you have there is clever, but unfortunately, you aren't going to be able to comprehend this one applying your intuition about classical mechanics at the atomic scale. You're talking about quantum behavior here, and while there is a well defined set of equations to define WHAT happens, no one has ever really come up with a good understanding/story of WHY things happen the way that they have with Newtonian physics and relativity. Technically, that little electron doesn't even exist at a definite point -- google "wave particle duality"... this will not help you understand, but might help you begin to understand why you DON'T understand... if that makes sense.

There are a certain number of phenomena at these size scales that have no analog in the world of everyday experience, and hence human intuition deals rather poorly with them. The end result is that science frequently has to confine itself to describing WHAT happens, without a clear picture of WHY. There is even a school of philosophy called "Logical Positivism" that asserts that anything you cannot directly measure (this includes why questions) HAS no reality. Scientists fall along a spectrum in their adherence to this idea -- Einstein felt deeply that there was a fundamental order and reality behind observations, Steven Hawking would be an example of a fairly strict logical positivist.

If you haven’t looked at these ideas at all before, I’d recommend the book “The Search For Schrödinger’s Cat” as a very light and friendly introduction to these ideas. Doesn’t cover electron orbital energies specifically, but has a lot of ideas that a mind working like yours seems to might enjoy.

watson.fawkes - 28-3-2010 at 06:53

Quote: Originally posted by dolimitless  
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.
This understanding is the core of your difficulty, because you're describing an ionic bond here, not a covalent one. Now there's no sharp boundary between the two, but the description you've got accurate enough in the ionic case. The covalent case, as others have pointed out, is essentially a quantum phenomenon.