[Miffymycat]

What's the underlying principle which is behind Le Chatelier's principle that equilibria shift to oppose external changes? Is it the Second Law of Thermodynamics? If so, why? Or is it some other principle? Or just an observed fact we can't explain!? I've quoted this Law for years without realising I dont understand it!

[ramboacid]

A chemical system wants to reach the minimum free energy possible, and thus the reaction will take place until the free energies of the reactants and products are minimized. The relative concentrations of reactants and products when the free energy is minimized are the equilibrium concentrations, which can be used to calculate the equilibrium constant.

Here's a graph to illustrate what I mean:
http://www.chem1.com/acad/webtext/thermeq/TE-images/dGdGo_2.png
Note that the graph is of Gibb's free energy vs the reaction coordinate, not ΔG vs the reaction coordinate.

Locations 1 and 2 on the graph are the Gibb's free energies of reactants and products, respectively. Location 3 is the Gibb's free energy of the system at equilibrium. At equilibrium, when the Gibb's free energy is minimized, the ΔG value moving in either direction is positive (moving away from equilibrium increases the G of the system). As we know that positive ΔG defines nonspontaneity, the system won't move out of equilibrium if we don't mess with the system. If we start anywhere else on the graph, the process of moving towards equilibrium always results in a negative ΔG value, which means moving towards equilibrium is spontaneous. Moving away from the equilibrium concentrations has a positive ΔG value and thus doesn't happen.

Adding reactants or products moves the system out of equilibrium, and so the reaction takes place until equilibrium is reached again.

An equation relating Gibb's free energy and equilibrium is
ΔG = ΔG° + RTln(Q)
where R is the gas constant 8.3145 J/mol*K, T is temperature in Kelvin, and Q is the reaction quotient.

At equilibrium, ΔG = 0 and Q=K_eq, and so we may substitute and rearrange to get
ΔG° = -RTln(K_eq)

Now we have a clear way to relate the equilibrium condition with the overall change in the Gibb's free energy of the system. LeChatelier's principle is just a simple way to say that the system will try to reduce its free energy and thereby attain equilibrium.

[Kemi]

Le Chatelier's principle sums up knowledge of chemical equilibria. I suppose there is no underlying principle as Le Chatelier's principle does not always seem to work.

Consider the reaction CS₂(g) + 4 H₂(g) ⇌ CH₄(g) + 2 H₂S(g). What will happen if you add some CS₂(g) at constant temperature and pressure? Le Chatelier's principle says the equilibrium will shift towards the products, but actually a shift to the reactants is also possible because the reaction quotient depends on the total volume.

You can find more criticism of Le Chatelier's principle in this article: http://pubs.acs.org/doi/abs/10.1021/ed086p514.

[juanrga]

What's the underlying principle which is behind Le Chatelier's principle that equilibria shift to oppose external changes? Is it the Second Law of Thermodynamics? If so, why? Or is it some other principle? Or just an observed fact we can't explain!? I've quoted this Law for years without realising I dont understand it!

The LeChatelier principle is really the LeChatelier-Braun theorem of moderation. It is actually a theorem, not a principle. This theorem can be derived from the second law of thermodynamics, because the second law of thermodynamics defines both the stability and the response to perturbations of an equilibrium system.

[juanrga]

Le Chatelier's principle sums up knowledge of chemical equilibria. I suppose there is no underlying principle as Le Chatelier's principle does not always seem to work.

Consider the reaction CS₂(g) + 4 H₂(g) ⇌ CH₄(g) + 2 H₂S(g). What will happen if you add some CS₂(g) at constant temperature and pressure? Le Chatelier's principle says the equilibrium will shift towards the products, but actually a shift to the reactants is also possible because the reaction quotient depends on the total volume.

You can find more criticism of Le Chatelier's principle in this article: http://pubs.acs.org/doi/abs/10.1021/ed086p514.

The LeChatelier principle is really the LeChatelier-Braun theorem of moderation and this theorem is well-proven.

What happens is that some people misunderstand the 'principle' and/or apply it outside of its field of validity.

The Cheung article that you cite above is a beautiful example of what I am saying. It is not the 'principle' which is wrong, but the teachers who do not understand it. A response to the above article is given here http://pubs.acs.org/doi/full/10.1021/ed800164k

This is an excerpt:

Cheung is distressed by the results of a study of secondary teachers in which the majority of teachers incorrectly analyze questions about perturbations to certain equilibrium systems. These errors are a consequence of the teachers applying the Le Châtelier principle (LCP) instead of proceeding by a more rigorous mathematical equilibrium analysis.

[juanrga]

Le Chatelier's principle sums up knowledge of chemical equilibria. I suppose there is no underlying principle as Le Chatelier's principle does not always seem to work.

Consider the reaction CS₂(g) + 4 H₂(g) ⇌ CH₄(g) + 2 H₂S(g). What will happen if you add some CS₂(g) at constant temperature and pressure? Le Chatelier's principle says the equilibrium will shift towards the products, but actually a shift to the reactants is also possible because the reaction quotient depends on the total volume.

You can find more criticism of Le Chatelier's principle in this article: http://pubs.acs.org/doi/abs/10.1021/ed086p514.

The LeChatelier principle is really the LeChatelier-Braun theorem of moderation and this theorem is well-proven.

What happens is that some people misunderstand the 'principle' and/or apply it outside of its field of validity.

The Cheung article that you cite above is a beautiful example of what I am saying. It is not the 'principle' which is wrong, but the teachers who do not understand it. A response to the above article is given here http://pubs.acs.org/doi/abs/10.1021/ed800164k

This is an excerpt:

Cheung is distressed by the results of a study of secondary teachers in which the majority of teachers incorrectly analyze questions about perturbations to certain equilibrium systems. These errors are a consequence of the teachers applying the Le Châtelier principle (LCP) instead of proceeding by a more rigorous mathematical equilibrium analysis. In my opinion the examples given are highly selective (even contrived), deal exclusively with gas phase phenomena, and are unrepresentative of either typical or important types of equilibria. One example used by the author involves the addition of an inert gas to an equilibrium mixture of gaseous reactants and products. With different combinations of reactant and product, and under special circumstances of an allowed change in volume, the author proves that the equilibrium could shift to products, to reactants, or not shift at all. This is in contrast to one of the most significant generalizations of the Le Châtelier principle that the addition of a species that is not in the equilibrium equation (or expression) does not affect the equilibrium.