Uncovering the Source of Heat in a Chemical Reaction

[benzun_1999]

dear reader,
The source of heat in a chemical reaction is the stored energy in an atom. if there is stored energy where is it in the atom?

 

[Jonathan]

The energy is stored in the electrons, I think in the form of electrostatic potential, which varies according to the distance between an electron and the nucleus. But that's probably not accurate, there are probably other variables and it would only apply to lone atoms. I'm not a really chemistry buff.

 

[STAii]

Umm .. as far as i know the changes in heat energy made during chemical reactions are due to changes in potential energy between different atoms (and not the same atom).
You see, each atom in a compound attract the other atom(s) by forces (which causes chemical bounds).
Any chemical reaction normally involves 'breaking' the chemical bounds and 'forming' new bounds.
Breaking chemical bounds means getting the atoms of each molecule away from each other, to do this you must overcome the binding force (the binding force will be in the opposite direction of the movement of atoms while breaking a bound, therefore work will be negative), this needs energy (this energy will be transformed into potential energy between the atoms).
When continuing the reaction (reforming bounds) the atoms will come nearer together (potential energy will be transformed to kinetic energy), and each group of atoms will form a new molecule.
The kinetic energies of the atoms will still be kinetic energy in the molecule, and there will still be potential energy between the atoms forming the single molecule (but of course, it will be less than before reforming the bounds, since the atoms are nearer).
The kinetic energy of this molecule is what we see (macroscopically) as heat.
So, if the energy needed to 'break' the original bounds is more than the sum up of kinetic energies of the molecules made after the reaction, then the reaction absorbs energy.
If the energy needed to 'break' the original bounds is less than the sum up of kinetic energies of the molecules made after the reaction then the reaction radiates energy.

 

[russ_watters]

Pretty much, Staii - and the nature of those bonds is the sharing (covalent bonds) or swapping (ionic bonds) of electrons by atoms.

 

[zoobyshoe]

I would like to respectfully
disagree wih you guys.

In answer to the question of where
an atom stores it's heat, or any
kind of energy, I believe it is
in a higher energy state of the
electrons. If you add energy to
am atom the electrons accept this
energy and use it to jump to
an "orbit" of larger diameter.

When an electron gives up energy
it drops to a lower orbit.

The extra energy is released as
a photon whose frequency falls
somewhere in the electromagnetic
spectrum. In the case of heat
it is in the infrared part ofthe spectrum.

If two atoms need heat to combine
it means they need photons from
the infrared, and these remain
stored in the now higher orbits.

-Zoob

 

[Jonathan]

That's what I said!

 

[Integral]

Originally posted by zoobyshoe
I would like to respectfully
disagree wih you guys.

In answer to the question of where
an atom stores it's heat, or any
kind of energy, I believe it is
in a higher energy state of the
electrons. If you add energy to
am atom the electrons accept this
energy and use it to jump to
an "orbit" of larger diameter.

When an electron gives up energy
it drops to a lower orbit.

The extra energy is released as
a photon whose frequency falls
somewhere in the electromagnetic
spectrum. In the case of heat
it is in the infrared part ofthe spectrum.

If two atoms need heat to combine
it means they need photons from
the infrared, and these remain
stored in the now higher orbits.

-Zoob

While you are free to disagree, it does not make you correct. Infrared wavelenghts generally correspond more to bond length then electron orbitals. Electron orbitals are much higher energy then infrared thus most atoms can neither adsorb nor radiate energy in that band. While some of the outer orbitals do correspond to infrared energies they are a very small fraction of the total atomic energy spectrum. Molecular bonds can be treated as springs connecting atoms, thermal effects (infrared photons) enduce vibration states in this spring the related atomic kinetic energy is related to the theramal energy.

High levels of thermal energy can destroy molecular bonds thus melting or other wise destroying the molecular sturcture.

Once again minimal thermal energy is stored in simple atomic orbitals.

 

[zoobyshoe]

Integral:

Perhaps, then, I have been labor-
ing under another misconception
that you can clear up for me.
While I know that a lot of elements
pair up (O2, H2,etc) I am not
aware that this is common among
metals. Do Fe and Au and Cu atoms
pair up like this, as well? With
nothing to bring my attention to
any notion contrary to this I
guess I had assumed many elements
existed as individual atoms.

I follow your argument about infra
red causing vibration in the bonds
due to those bonds being the cor-
rect length to respond to infra-
red, but I wasn't clear about the
exact origin of the infrared that
is radiated when a chemical re-
action produces heat.

-zoob

 

[Integral]

most metals arrange themselfs in some form of a crystlyn structure. So there is bonds to transmit and store thermal energy. The malable metals such as gold, silver and copper essentiall share many electrons in the outer shells, it is these loosely bonded electons which form the "electron gas" which is the reason for their conductive properties. The core electron shells form the crystle structure.

What is the source of infrared, any mechanical induction of molecular bond vibrarion such as collisions will generate heat. Any chemical reaction which involves molecular bonds will generate heat, all of these are sources of infrared energy.

This is all a matter of the amount of energy involved, infrared is at the low end of the electromagnetic energy specturm, below it is radio waves, above it is visible light. Visible light is produced by atomic shell interactions, infrared by molecular interaction, the wave length of the radiation is an indication of the size of the source, visible light has a wavelength compareable to atomic sizes, infrared is on the order of molecular sizes. Of course this is a pretty rough gauge, and is only roughly true, but it is a valid indicator. This also holds true for radio waves where the antenna are sized acording to the broadcast or received wavelenghts.

 

[zoobyshoe]

Integral,

You said "Any chemical reaction which involves molecular bonds
will generate heat..."

I see how this applies in the case
of infrared energy being applied
from the outside causing the bonds
to vibrate, even to the point of
breaking them down.

I don't have a clear picture of
where the infrared vibration
occurs during chemical regroupings
that release heat.

If it's useful in helping you
explain let's use the example of
Sodium Hydroxide dissolving in
water, a situation there a lot of
heat is developed. If that is not
useful, feel free to pick a per-
spective that will be . Thanks.

Zoob

 

[Integral]

When molecules are formed energy is stored in the bonds (springs streatched or compressed. When in presence of atoms or molecules which allow a lower energy configuration the molecules will move to that configuration. Thus relasing stored energy.

Consider also that the water molecules in your particular example have kinetic energy which are transferred to the KaOH molecules via collision, thus exciting the vibrational states and starting the breakdown of KaOH.

 

[russ_watters]

Originally posted by Integral
most metals arrange themselfs in some form of a crystlyn structure.

Oh yeah, that reminds me - there are other types of bonds. Metallic bonds and hydrogen bonds. Not sure about others.