Chemical Potential to Kinetic Energy
Location: South Africa
Date: July 2006
How is chemical potential energy transferred into
kinetic energy? How does this work on the microscopic scale?
There is an unfortunately ambiguous use of the term "chemical
potential" imbedded within your inquiry. In chemical
thermodynamics, the term "chemical potential" has a very
specialized meaning that differs from the use of "potential energy"
in the sense of gravitational potential energy.
In chemical thermodynamics the term "chemical potential energy"
or "chemical potential" refers to the differential change in the
free energy per mol of a component "i", or trying to write this in
the limitations of fonts available here as (dG/dni)T,P, nj where T
and P are the temperature and pressure respectively and "nj" is the
number of moles of all other components "nj" where "i" not equal to
"j". This change in free energy (available work) designates how
much energy is available to do some sort of other work, or change
the chemical system in some way. It is not specific with regard to
what or how that energy is converted. It may be some sort of energy
of motion, kinetic energy, or it may apply to some other form of
energy or chemical change.
The power and the weakness of the thermodynamics is that it does
not specify what and how that change occurs.
Generally, chemical potential is converted to kinetic energy by
having the starting chemicals react to form some product chemicals
with lower potential energy. When this happens, energy is
conserved, so the difference in potential energy is released as
kinetic energy. What this means on a molecular scale is that the
product molecules will move faster and possibly vibrate faster than
the starting molecules. On the macroscopic scale that we can
observe, faster molecular motion (more correctly, higher molecular
kinetic energy) means higher temperature. So, the short answer to
your question is that chemical potential energy is transformed into
the heat of the reaction.
Take the simplest possible case-
two hydrogen atoms drifting together and combining to make an H2 molecule.
When they first meet in space and combine,
there is no immediate way their sudden sticking-together can cause
the joined pair
to gain translational kinetic energy in any one direction.
The most of the net energy gain of the reaction
can temporarily can reside in the molecule's electrons, as an excited state.
Eventually this excitation energy will exit somehow.
This can take a while, for "metastable" excited states.
If it is eventually emitted as a photon or two,
not much of the potential energy gets to become kinetic energy,
at least in the short run.
In the long run,
the photon will be absorbed and scattered as bits of heat energy.
Heat energy is kinetic energy, right?
However if the excited molecule is lucky enough to gently bump
into another molecule or the walls of the container, this could
with a good amount of the energy going into a short pulse of
"pushing apart" force.
This force might be considered electron-electron repulsion suddenly
as the shape of the electron cloud abruptly changes upon de-excitation.
More formally, in the quantum-mechanical perspective,
it could be considered a burst of photons briefly Ping-Ponging back and forth
between the excited molecule and the other molecule it is bumping into.
At any reasonable distance it would seem unlikely for photons to go
between the same two objects over and over,
but if they are "touching": much more frequent round trips are possible,
and the size of the perceived targets is much larger,
so multiple-round-trips can become the thing they naturally do.
The net result would be to smoothly push the two molecules apart
over a time period longer
than the period of any single photon holding all the energy being ejected.
Think of it as a brief bout of electromagnetic arm-wrestling between
two people on ice-skates.
Since the wrestlers do not have chairs to hold them in place,
it is plausible they would get pushed apart.
Though the newly formed excited molecule cannot gain translational energy
immediately upon combining,
it can gain some rotational and/or vibrational energy.
Both of these are considered thermal and kinetic energy,
and also they are easily converted to translational kinetic energy
during the next few times our excited molecule touches any other.
I am not sure how large a percentage of the reaction energy can be
handled this way.
I think it is significant and noticeable but less than a majority.
First, let us understand what is meant by "chemical potential
energy". Imagine two magnets that are stuck to each other (the N and
S poles of the magnet are touching). In order to separate the
magnets, you would have to put in some energy and physically pull
the magnets apart. Note, that you actually had to put in energy into
the system in order to push the magnets farther and farther away
from each other. But where did the energy that you put in go? The
answer is in the concept of potential energy, the energy that you
put in is envisioned, imagined, conceptualized as a "potential" -
the observable physical phenomena being the distance between the two
magnets - which when released can result in the magnets getting back
together again. This is similar to the idea of the distance of a
ball from the center of the Earth, gravity is acting on an object
(ball) and the farther the ball is, the more "potential" it has to
come crashing back to the center of the Earth (where it not for the
matter in between) and converting that potential into kinetic (motion) energy.
With compounds, the potential energy is visualized as the energy
required to counteract the electrostatic force (like in the magnet)
that holds atoms together - we give this electrostatic force a
special name: "chemical bond". So, to pull atoms apart, we would
need to put in energy and increase the potential energy of the
individual atoms. When atoms come together, then we imagine that the
potential energy of the individual atoms must decrease, so energy
must leave the system. You can imagine that atoms that are too full
of potential energy are unstable and in order for them to stabilize
they have to release that potential energy (and form chemical
bonds). To summarize then, breaking a chemical bond, requires energy
to increase the potential energy of the atoms. Forming a bond must
mean that the potential energy of the atoms decreased and this
energy must be released.
So, in a chemical reaction, when the energies needed to break
chemical bonds is less than the energies released in forming bonds,
then there must be an overall release of energy.
Commonly (not always), this released energy is observed as heat, and
heat can (not always) result in the increased motion of particles in
the environment. Increased motion is observed as an increase in
temperature. So when there is a release of potential energy and this
release results in an observed increase in temperature, we say that
potential energy has been converted to kinetic energy.
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Update: June 2012