Le Chatelier's Principle

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quartrmster007
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Le Chatelier's Principle

Postby quartrmster007 » Mon Feb 02, 2009 3:19 pm UTC

Ok, so today in Chemistry, we started discussing Le Chatelier's Principle. When we got to the discussion on changes in volume and pressure, a couple of my friends and I began to wonder if the principle holds true for extreme ranges; ie, an increase in pressure (due to the volume decrease) so much that the reaction shifts almost completely (all products except for the chance reactant particles) to the product side of the equation, and (the converse, my theory) that a decrease in pressure (volume increase) so much that it completely shifts to the reactant side.

What I'm wondering is twofold- a, are we correct in reasoning, and b, is there a point such that the volume has increased so much that any further change is irrelevant? (If the equilibrium state is trying to preserve the pressure by increasing the number of moles of a gas, would there be a point where it would become futile, in a sense?)

Going further, if one has a reaction that produces intermediate products and the volume increases (shifting equilibrium to the left), would the final product virtually cease to be formed (except for the law of probability stating that there will be at least several individual particles of that product) and the intermediate products would sort of take over as a de-facto final product?
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danpilon54
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Re: Le Chatelier's Principle

Postby danpilon54 » Mon Feb 02, 2009 3:26 pm UTC

As a general rule, all simple principles break down at extreme temperatures/pressures etc. I am no chemist so I am not sure what this principle is, but rest assured at extremely high pressures it will not work correctly. (I am talking near infinite pressure about which current science cannot even speak).
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BlackSails
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Re: Le Chatelier's Principle

Postby BlackSails » Mon Feb 02, 2009 3:59 pm UTC

Im pretty sure it holds at really high pressures. Maybe not at ultra high pressures, like the interiors of neutron stars.

However, pressure would not necessarily shift to the product side. It will shift it to the side with less gas.

My biophysics textbook derives le chatelier's principle from consideration of the chemical potential of a system, so its not just pulled out of a hat.

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quartrmster007
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Re: Le Chatelier's Principle

Postby quartrmster007 » Mon Feb 02, 2009 4:03 pm UTC

ok, that's what our teacher said.

I'm sorry- the OP is a bit misleading. The equation used in class (as well as this example) is N2+3H2>/<2NH3
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Re: Le Chatelier's Principle

Postby IHOPancake » Tue Feb 03, 2009 5:54 am UTC

You'll probably go over this later in the year, but there is a quantity known as the Gibb's free energy that can be used to determine in what direction the reaction will occur. Because of the way it works, the math will never work out that there is only products or only reactants (divide by zero or ln(0) errors), but eventually you will run up against the granularity of matter and be forced to conclude that a concentration of 10-55 moles/liter in 1 liter of solution is equivalent to 0 particles.

If I had to take a guess it would fall apart once quantum mechanical effects can no longer be ignored.
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Rentsy
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Re: Le Chatelier's Principle

Postby Rentsy » Tue Feb 03, 2009 6:32 am UTC

It isn't mathy. It's just theory. Very vague, but also very general.

I'm going to argue against what other people are saying and say that it is always true, assuming chemical reactions are still able to occur. That assumption rules out things like neutron stars or black holes.

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Re: Le Chatelier's Principle

Postby Omega_ » Tue Feb 03, 2009 3:18 pm UTC

BlackSails and IHOPancake gave you what you needed to evaluate your logic. Le Chatelier's principle is a consequence of the underlying statistical nature of chemical matter (which is essentially what yields the chemical potential terms). If you want a mind-blowing philoshophical physics read, look for Amalie (or Emmy) Noether's work on symmetries and conservation. Gibbs free energy quantifies the tendency for an equilibrium reaction towards the left (reactants) or right (products) side of the equation. You cannot have a concentration so dilute that it requres you to split atoms in order to achieve that concentration. So, as long as classical equilibrium thermodynamics (chemical potential, Gibbs free energy, reasonable concentratios) is valid, Le Chatelier's principle should be useful.

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meat.paste
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Re: Le Chatelier's Principle

Postby meat.paste » Tue Feb 03, 2009 7:25 pm UTC

This principle does work at extreme temperatures and pressures. Even something like a neutron star has a reaction (e- + p -> n) that can be expressed in terms of an equilibrium (yes, I'm ignoring the neutrino). Lower the pressure on the neutron star (by removing mass) and the reaction will shift towards making electrons and protons (with a half life of 12 minutes). The same is true of other equilibria. At the low pressures in low earth orbit, the equilibrium shifts from diatomic oxygen to the monatomic variety. At very high pressures, reactions shift in a way to counteract the pressure. Le Chatelier works for any equilibrium I know of.

Reactions that go to "completion" will have very large values of the equilibrium constant and are not usually thought about in terms of equilibrium. However, the OP is correct that a reaction will have some trace amount of reactant left over. However, that amount is not meaningful if it involves less than one molecule per ocean. Just because the math says it is there doesn't mean that it is. You have to be aware of the physical limitations of the universe :)
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