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The Physics of Le Chatelier's Principle

  1. Sep 8, 2010 #1
    We recently started learning about equilibria and Le Chatelier's principle at school. One of my classmates asked the teacher the following question: "I know this might sound stupid, and I understand Le Chatelier's principle in that any change made to a system in equilibrium will be undone as much as possible by the system, but why does this happen?". My teacher was at a loss for words, and I, arrogantly and confidently, said "It's obviously to do with physical laws." After I was required to explain my statement, things didn't seem so easy any longer.

    I understand the physics & thermodynamics behind changes such as temperature (I know that an increased temperature favors the endothermic reaction more than the backwards reaction, but bonus point for anyone who could explain why!), product/reactant concentration, and pressure, but it seems that Le Chatelier's principle is something that should be taken for granted in a univrse such as ours - my first guess as to why this might be would be to do with the conservation of energy, but this is just a guess or an intuition and I would be extremely grateful to anyone who could explain if and why Le Chatelier's principle is a must and a given to a world such as the one we live in.

    Thank you!
  2. jcsd
  3. Sep 8, 2010 #2

    Ben Niehoff

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    A distinction needs to be made between stable equilibrium and unstable equilibrium.

    Stable equilibrium is like a ball sitting at the bottom of a valley. If you push the ball slightly, it will tend to roll back to where it started.

    Unstable equilibrium is like a ball sitting at the top of a hill. If you push the ball slightly, it will tend to roll away!

    Stable equilibria exist when there is some kind of restoring force (in this case, gravity).
  4. Sep 10, 2010 #3
    Thats a good question,
    Intuitively I would think the following:

    The equillibrium constant (and the law of mass action) is derived from minimisation of Gibbs free energy arguments. This means that the ammount of products and reactants that exist at equillibrium are determined by minimising the Gibbs Free Energy. When we change our ammount of product or reactant we change our equillibrium constant, and therefore our system is at a higher Gibbs Free Energy.

    LeChatelier's principle is nature getting the system back to the ratio of products and reactants (which set by the equillibrium constant) has minimum gibbs free energy. LeChateliers principle occurs because nature is always driving systems to their state of minimum free energy.

    Therefore I think that Le Chatelier's principle is just the manifestation of the system being driven to minimise its Gibbs free energy.

    EDIT: I just realised you may be in highschool and therefore have no idea what Free energy is - if so do you know what Entropy is ?

    If you are in high school - Here is something to think about - when you have a chemical reaction and you measure the ratio of products to reactants, and you determine the equillibrium constant, under the same temperature and pressure if you repeat the experiment why do you get the same equillibrium constant ?
    Last edited: Sep 10, 2010
  5. Sep 16, 2010 #4
    Hey. So sorry for replying so late - been loaded with work. I do not in fact know anything about free energy, and I've only heard of ''entropy'' as mysterious chaos-related chemistry that we'll only get to towards the end of this year. As for your question - I'd assume that this is down to kinetics/thermodynamics - IE, as in Maxwell-Boltzmann curves, the total amount of reactant particles will stay the same and the energy you put into the system will be distributed amongst these particles in more or less the same way (not completely the same way due to chance) as any other time you might have observed the reaction in said conditions. Is that about right?
  6. Sep 18, 2010 #5
    Are you familiar with the fluctuation dissipation theorem?

    This theorem tells us that the reaction to a macroscopic system to an external force is related to the nature of fluctuations in that system. The fluctuations tend to restore equilibrium, since the system tends towards equilibrium over time.

    An external force applied to the system is counter balanced by frictional forces in the system. These frictional forces are the same forces which during random fluctuations, tend to restore the system to equilibrium, and so when an external force is applied to the system, the reaction forces will tend to restore equilibrium.

    I hope this is clear, there is an excellent discussion of this in anderson, "basic notions of condensed matter physics", which by the way is one of my favorite physics books I have ever read.
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