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

Effect of accel/velocity on chemical reactions

  1. Dec 2, 2007 #1
    A simple question really. Does acceleration affect the rate of a chemicall reaction. I.e if I was to put some test (chemical) experiment aboard a suitable craft and accelerate it to a high velocity, would the reaction occur at the same rate as it would at rest - ignoring relativistic effects.

    If not what would be the mechnism for the change?
  2. jcsd
  3. Dec 2, 2007 #2


    User Avatar

    I thought there was no way to discern between acceleration and the presence of a strong gravitational field nearby? That would make it opposite but equivalent to asking "How would a reaction work if we took away gravity".
  4. Dec 2, 2007 #3
    Except for the fact that an experiment performed on say the top of a table on the earth isn't experiencing a change in velocity, gravity or no gravity . .
    Last edited: Dec 2, 2007
  5. Dec 2, 2007 #4


    User Avatar
    Science Advisor

    In chemistry, the velocity of particles is called "temperature"
  6. Dec 2, 2007 #5
    Ok, but not sure how that relates to my question.
  7. Dec 3, 2007 #6


    User Avatar
    Science Advisor

    You asked if velocity affects the reaction. I said velocity is called temperature. That means the answer is yes.

    On a space ship, the velocity of the reaction mixture relative to the ship is zero, so that doesn't really matter. Sort of like when you're riding in a car, your speed relative to the car is zero, so it feels the same as sitting on the porch. Acceleration would give no effect if the reaction mixture is in free fall because it would not affect the temperature (particle velocity). If the acceleration creates forces inside the reaction mixture, then yes that does affect the reaction.
    Last edited: Dec 3, 2007
  8. Dec 4, 2007 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is an incredibly misleading thing to say! Velocity of particles (whatever that means) is not called "temperature" in chemistry, or anything else. The temperature is directly related to the entropy of the system, and is approximated by a number proportional to the mean square velocity of the system in its center of mass frame.
  9. Dec 4, 2007 #8
    Never heard about equivalence principle, isnt'it?
  10. Dec 4, 2007 #9


    User Avatar
    Science Advisor

    "On the microscopic scale, temperature is defined as simply the average energy of microscopic motions of a single particle in the system per degree of freedom."

    When it says energy, it means kinetic energy. There is a direct relation between temperature and particle velocity when comparing systems that are made of the same thing.

    thermodynamic temperature
    "The thermodynamic temperature of any bulk quantity of a substance (a statistically significant quantity of particles) is directly proportional to the average—or “mean”—kinetic energy of a specific kind of particle motion known as translational motion. These simple movements in the three x, y, and z–axis dimensions of space means the particles move in the three spatial degrees of freedom. This particular form of kinetic energy is sometimes referred to as kinetic temperature. Translational motion is but one form of heat energy and is what gives gases not only their temperature, but also their pressure and the vast majority of their volume. This relationship between the temperature, pressure, and volume of gases is established by the ideal gas law’s formula pV = nRT and is embodied in the gas laws."

    If you have two glasses of pure water, one hot and one cold, you can say with absolute certainty that the hot water has faster moving particles than the cold water.
  11. Dec 4, 2007 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Shawn, you should know better than to be so sloppy as to say "velocity of particles is temperature". Since the particles all don't have the same velocity, do you take the arithmetic mean, the geometric mean, the root mean square, the most probable...which one?

    In any case, equation (8) from your first wiki link has the rigorous definition of temperature (which I mentioned in the first half of my second sentence above), and the two quoted parts are exactly what I said in the second half of that sentence.

    To attempt an answer of the OP's question: Empirically, we see that a varying acceleration does affect kinetics (at the very least, in reactions that are rate limited by diffusion) as is often seen by shaking a reaction beaker. The effect of a uniform acceleration is more tricky. There can be different regimes and different factors to consider depending on the phase of the system and the reaction mechanism. There might be an interesting transition during acceleration when you go past the speed of sound in the system. Much below this speed, there may be little happening, as the system equilibrates faster than the box moves. Above this speed, the system may be in a significantly non-equilibrium state. For a gas phase system, this will probably show up as a pressure gradient, which will affect the kinetics through the partial pressures in the rate equation. Also, a second order effect is that the work done on the system by the external force might induce a change in internal energy (hence, temperature).
    Last edited: Dec 4, 2007
  12. Dec 8, 2007 #11
    Here's what i'm thinking.. if the chemicals are in a solution state, we're basically dealing with a fluid over here.

    When accelerated in a centrifuge, we can get very high centripetal acceleration by increasing the radius. Doing so will cause a psuedoforce to act on the liquid, acting outwards to the surface. As such, it will increase the pressure of the liquid as the area of the holder [let's say the test tube] will remain the same, but the force will be increased.

    If accelerated linearly in a circular track, let's say, we have a tangential acceleration and a radial acceleration. Both of them will cause an increase in pressure, however the pressure increase will be a function of space coordinates.

    as far as velocity of the mixture is concerned.. it would make no difference as the frame remains inertial [practically speaking] and no psuedoforce applies.

    even for your craft.. when projected against gravity.. it'll cause an increase in pressure due to a psuedoforce acting on it.

    This is just a way of thinking of what would happen if the mixture is accelerated.. because it would be ridiculous to increase pressure like this. A piston-chamber system would be much effective :D
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Effect of accel/velocity on chemical reactions
  1. Chemical reaction (Replies: 1)

  2. Chemical reactions (Replies: 7)