Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Chemical Potential to Kinetic Energy
Name: Vuyelwa
Status: Unknown
Grade: N/A
Location: South Africa
Country: N/A
Date: July 2006

Question:
How is chemical potential energy transferred into kinetic energy? How does this work on the microscopic scale?



Replies:
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.

Vince Calder



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.

Richard Barrans


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 catalyze de-excitation, 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 grown stronger 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 back-and-forth 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.

Jim Swenson


Vuyelwa,

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.

Roberto Gregorius


Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory