Talk:Le Chatelier's principle

Why is Le Chatelier's principle true?

WHY is Le Chatelier's principle true? What are the physical reasons for it? Is it a 2nd law of thermodynamics thing?

—The preceding unsigned comment was added by 67.99.238.198 (talk) 01:54, 12 January 2007 (UTC).

It's related to kinetics, if my understanding is correct. (Perhaps a professional who has more experience could check this, to make sure that it's accurate?) There is actually no "drive" or "force" that pushes an equilibrium in a certain direction, it's just that when one reactant has an increase in concentration, it collides with its fellow reactants more often, producing a higher rate of reaction in the forward direction than the product side is reacting in the reverse direction. Eventually, the increase of products means that their collisions increase too, causing the reverse-reaction rate to increase as well. Once the new forward and reverse rates have become equal, i.e., Q = K, the equilibrium has been reached again. I hope this was helpful. 70.149.178.146 03:37, 9 March 2007 (UTC)

My knowledge of this is not as good as it should be, but it is in my opinion that when you look at it in terms delta G, you realize that since delta G is neither positive nor negative, the reaction will no longer become spontaneous in either direction. If the reaction is not spontaneous in either direction, the reaction will stop moving in either direction. So maybe that will help. Q E11even 04:19, 14 March 2007 (UTC)

The formalism is the law of mass action. Q = products / reactants. Given an infinite amount of time, Q will approach the value K, which is the position in equillibrium. When you change the concentrations of things, you change Q - then the reaction responds by trying to change it back to K. If you change things such as reaction temperature, you change K - so again the reaction proceeds so as to make Q = K.

If you need to understand why this formalism is a good model for a _true reaction_, then you should check out "the law of mass action" and review an introductory, college-level chemistry text. It's probably a little too involved to describe here. 151.151.73.170 (talk) 15:52, 26 June 2008 (UTC)

The Principle is quite useful and rewarding, but it is actually a tautology, so there is no derivation possible! If we roughly divide the processes and reactions in chemistry and related areas (thus I include change of state, which is as much physics as chemistry) into stable and unstable processes, we see that stable processes by definition respond according to the Principle, because, for example, adding a reaction product drives the reaction so as to consume it. If we consider an unstable system, such as nitroglycerine subject to sudden shock, or a nuclear chain reaction, the increasing presence of the products (heat in the first case, neutrons in the second) speeds the process and by no means reverses it! Thus, the Principle is really, in a sense, a way to characterize stable systems. Polvadera (talk) 15:14, 21 July 2008 (UTC)

I agree strongly with the last comment. The article states the principle rather vaguely, and does not emphasize the fact that it is really a kind of definition rather than some kind of mystical overarching self-correction principle of Nature.

The article should be sharpened and de-mystified, so that one does not not have to come to the Talk Page to get the real lowdown. It can fool you for awhile,even if you do quantitative thinking all the time; perhaps the authors of the article should even be commended for re-evoking the awe and majesty of this 19th century equilibrium principle. ;-) Though perhaps Leibniz also had something to say in his time.

In defense of the principle:

(1) As we all know, the equilibria that we actually see are the stable ones, since the unstable ones don't last long. Even nitroglycerin has its little domain of stability, as is amply illustrated in stories and anecdotes.

(2) In many simple systems that we like to study, the entropy or free energy is actually globally convex (or concave), so that there really is only one equilibrium and it is stable. Equivalently, the (corrective) force is monotone in the displacement, concentration or whatever. For example, this is true in simple chemical reactions. You can also observe it (the concavity) in entropy expressions such as the Sackur-Tetrode equation.

When you do more complicated things, the global convexity goes away.

The point then is not that Le Chatelier's principle is universal, but rather how to recognize where it is valid and get a feel for its functioning.

I found G. Lebon, D. Jou, J. Casas-Vázquez, Understanding Non-equilibrium Thermodynamics, Section 1.6: Stability of Equilibrium States, pp 24-29 to be useful in this respect. In particular, they characterize stability mathematically by the Hessian of the entropy, as one might expect. And indeed, it is second law of thermodynamics, because you have

dS/dt ≥ 0

until you reach (stable) equilibrium, then you get

dS = 0 (meaning D_X S = 0 for every directional derivative X)

and

d^S ≤ 0 (meaning the Hessian of S is nonpositive in every pure direction X)

reflecting the fact that if there were a positive direction, the system would have found it and S would continue to increase. This illustrates point (1), that we only see stable equilibria; but it does not explain point (2), why so many simple systems have only a stable equilibrium. 178.38.82.124 (talk) 23:42, 29 March 2015 (UTC)

Re point (2), why so many simple systems have only a stable equilibrium: is it jus-cos a picture with multiple equilibria is more complicated / complex, so it needs a fairly complex system to exhibit such a picture?

Broadly, I see the Principle as very much a definitional rather than observational / predictive thing. It seems to be pretty much a definition of what we mean by a stable equilibrium (for a system with so many degrees of freedom: there are at least four - concentration(s), temperature, pressure and volume). I’m always very hesitant about any attempt to explain it in terms of statistical mechanics, reaction kinetics or whatever; or even (as above) from the mathematics of equilibrium constants: not because such explanations would be wrong, but because they miss the point - like an elaborate mathematical analysis of a mechanical situation, with distance-integrals of varying forces, when a three-line argument from energy-conservation gives far better insight.

It reminds me of Ohm’s “Law”: generally regarded as an empirical law, but treated in practical work as a definition. So, for example, when increasing voltage is applied to a filament, and current starts increasing rather slowly as the filament begins to glow, we don’t distress ourselves with the discovery tht Ohm’s Law has been disproved (or has been found to operate only at temperatures in a limited range). We recalculate the resistance - using Ohm’s Law as a definition to base the calculation on, not as an (inaccurate) prediction. Similarly, any departure from Le Châtelier’s Principle is regarded as showing tht the equilibrium is not stable: not as a case disproving the Principle.

If anyone can point me to a source for this, available on the internet, I’ll put it in the article. Or, if they have a suitable source tht says it, and they’re willing to write the citation, I’ll translate the above into encyclospeak and they can put it in the article.

- SquisherDa (talk) 01:01, 13 February 2019 (UTC)

A reliable source is I. Prigogine, R. Defay, translated by D.H. Everett (1954). Chemical Thermodynamics, Longmans, Green and Co Ltd, London, especially Chapter XVII, pp. 262–269. It's not an easy read.Chjoaygame (talk)

Thanks, Chjoaygame! I see what you mean about “not an easy read” - judging by the fact that AbeBooks want about £550 for a first edition!

What I would need, really, is a website I could read. Or, I could work with someone who has a printed text.

- SquisherDa (talk) 17:30, 14 February 2019 (UTC)

Though the physical acessibility of the text is not too easy, that's not what I meant. I meant that the intellectual content is not easy. Things are not too simple. For example, for a closed system at controlled fixed temperature, the text says "if the pressure and volume of a system are perturbed and the pressure is then maintained constant at the perturbed value, the reaction will proceed in a direction such that the volume will continue to change in the same direction as the initial perturbation."

Another reliable source is mentioned above: G. Lebon, D. Jou, J. Casas-Vázquez, Understanding Non-equilibrium Thermodynamics, Section 1.6: Stability of Equilibrium States, pp 24-29. I have not seen another source that treats the Le Chatelier—Braun Principle as thoroughly as does Prigogine & Defay. Because Chapter XVII relies on previous chapters for background, by itself Chapter XVII is not easily intelligible. To study it, one probably needs a copy of the book. Do you have access to a library from which you could borrow a copy of the book? If not, perhaps you can access the book some other way?Chjoaygame (talk) 08:18, 15 February 2019 (UTC)

Thanks again, Chjoaygame. Both books sound well worth being aware of. But my guess is any printed book dealing with this would take quite a while to get anything useful out of - even just to figure how to productively navigate around it - and from what you say about Ch.XVII and its dependency on previous chapters I’m fairly sure that would apply to Prigogine. And time is the thing I really mustn’t promise too freely.

So what I probably need is a modern website, comprising pages designed to be read separately, free-standing where possible and linked as hypertext where not.

That would not be an easy thing to do well; and probably a lot of people and colleges etc have tried, given the subject’s importance.

Can you suggest one? Can anyone point me to a good site?

At a guess, it’ll be a fairly mathematical treatment. If so, the main challenge for me, working on our article, will be to retain the precision while dropping the math[s]. I suspect Hessians (above, per 178.38.82.124) will come into it - but I don’t (yet!) know what they are(!)

I rather think the multidimensional nature of the thing is essential to it (and I’d think that’s the key feature tht economics shares with chemistry). It’s hard to interpret the Principle in application to a simple (physical) system like a golf-ball sitting in a soup bowl - where radial displacement and height are about the only variables; and the stable equilibrium is a single point. Whereas if u replace the soup bowl with something rather bigger, and the golf ball with a motorbike, speed gets added to the list and the space of possible stable equilibria blossoms out to a continuum . . where I still can’t quite see the Principle applying yet; but if u add energy to the list maybe it starts to . .

- SquisherDa (talk) 17:49, 15 February 2019 (UTC)

I think one should bear in mind that the Le Chatelier principle is about thermodynamic systems, not about a wide range of partly analogous physical systems, such as golf balls in bowls, nor about economic systems.Chjoaygame (talk) 11:05, 16 February 2019 (UTC)

Definitely. But I think it’s correct tht the Principle follows from mathematical features of the equilibrium function - continuities of equilibrium concentrations and of their rates of change in response to changes of the independent variables, non-linearities and so forth. (I suspect this is what Hessians are about.) And it’s probably fairly easy to contrive physical systems which illustrate the issues with better vividity and are very close equivalents in model-theoretic terms. I’d expect, too, tht some economic systems also model the relevant axioms and so are bound to exhibit the same behaviours. But that’s all rather secondary, and the ‘expository agenda’ would have to be all about thermodynamics.

The thing is, have we got a source I can turn to? both to avoid too much heavy lifting in working out how to present all this, and to avoid the problems of original research.

- SquisherDa (talk) 00:01, 17 February 2019 (UTC)

I don't know about economics, but I would find it amazing that the principle could apply to any realistic economic model. Thermodynamics has thermodynamic equilibrium, various conservation laws, and the second law, none of which have fair analogies in real economics.

As for reliable sources for the article, I'm sorry I can't do better than suggest you try harder to get access to Prigogine & Defay. The topic is not easy to present in Wikipedia. The heart of it is the second law.Chjoaygame (talk) 06:02, 17 February 2019 (UTC)

You might well be right about *realistic* economic models!  :-) SquisherDa (talk) 23:18, 17 February 2019 (UTC)

Is that spelling correct?

I thought it was spelt "Le Chatelier", without the circumflex. I have a copy of the original paper by Henri Le Chatelier (Comptes Rendus de l'Académie Française 99, 786-9 (1884)) and there's no accent there. Neither is there one in an article I read on him in Chemistry in Britain, nor in "Modern Physical Chemistry" by Liptrot et al, nor in the classic "Physical Chemistry" texts of Moore or Atkins, nor in Encyclopædia Britannica. In fact I don't think I've seen the accent anywhere other than on Wikipedia.--CSM 18:30, 17 April 2006 (UTC)

I agree. Even Wikipedia's own Henri Louis Le Chatelier Article does not use the circumflex. I suggest a title change. --anomymous user 2:01, 24 April 2006 (EST)

I just want to add something... im a 15 year old studying this for my GCSE's but it still amazes me how some people can be so pedantic! The person who added the origional article (whoever he or she may be) deserves respect for thier work... not people critising about the spelling of a forign word.

Hello... its me again. Just wanted to prove a point.. ORIGIONAL, CRITISISING and FOREIGN.

I think you just proved his point.134.88.164.209 01:55, 6 March 2007 (UTC)

I have seen it spelt with a circumflex in some Chemistry textbooks eg. Chemistry:The Central Science, Brown, Lemay, Burstein. NikolaiHo^☎️ 04:26, 23 November 2017 (UTC)

See next section (â / a).

And yes, this can seem a bizarre distraction if you sought the article to read about one of the founding early insights of The Central Science. But Wikipedia aims to be reliable: including (especially?) on details.

To many readers, French is not a foreign language; nor Le Chatelier a foreigner.

- SquisherDa (talk) 13:30, 13 February 2019 (UTC)

â / a

Chemistry (8e, Raymond Chang, McGraw-Hill) spells it Le Châtelier. Dmbrown00 04:50, 27 January 2007 (UTC)

Chemistry the Central Science (9e, Theodore Brown, H. LeMay Jr., Bruce Bursten, and Julia Burdge, Pearson Education Inc.) also spells it Le Châtelier. Fluffybun 17:11, 4 March 2007 (UTC)

Hey is that the standard text book or the AP one? I've the AP :) Arc88

This seems to be a recurring hypercorrection. The question was checked out pretty thoroughly on the Talk page for the article on the man himself.

- SquisherDa (talk) 13:21, 13 February 2019 (UTC)

Effect of Adding an Inert Gas

Could someone please verify whether the edit on 3 January 2008 concerning the effect of adding an inert gas was accurate?

Of the first 20 results obtained when searching for '"le Chatelier's principle" "inert gas"' on Google, 12 state that adding an inert gas has no effect on equilibrium when volume is held constant and only 5 state that equilibrium is affected, of which 4 quote Wikipedia and the fifth is Yahoo Answers.

I thought that equilibrium was only affected by the partial pressure of each gaseous substance, which is unaffected by adding an inert gas, rather than the total pressure of the system? Vqors (talk) 09:20, 14 February 2008 (UTC)

You are correct. I tracked down a legit source and made the changes. I also changed the "changes in pressure" section, since that, too, implied a change in total pressure would shift the equilibrium. This is one of those misconceptions that I can't stand. Brittlandk (talk) 08:23, 23 February 2008 (UTC)

Let us come to a consensus on this issue before I roll back the change. I am in agreement with the above that when an inert gas is added at fixed volume, the equilibrium is unaffected (as stated in my original post) because the concentrations of the other gases are still the same. However, adding an inert gas at constant pressure and allowing the volume to increase (ex. inside a balloon), would in fact result in a decrease in the concentrations of all other gases involved, thus resulting in a shift.

For example, let's consider CO + 2 H2 <--> CH3OH in a balloon. If we add enough helium to double the volume of our balloon (still at same pressure), the concentrations of CO, H2 and methane would all be halved.

Yes, this agrees with my original source (and others).

Since the decreases in concentrations would affect the left side more than on the right, a shift would occur to the left to "fill the void". Please verify your sources and advise. Brent Woods 22:44, 1 March 2008 (UTC)

In summary, I agree that adding an inert gas at constant volume (thus increasing total pressure) will not result in a shift because the partial pressures are unchanged. However, if the addition of the inert gas causes a volume increase, the partial pressures of all gases involved are decreased, thus causing a shift towards the side with more moles. Brent Woods 15:56, 2 March 2008 (UTC)

Yes, I see what you are saying. It is important to make the distinction whether the change in total pressure is allowed to change the volume of the system. Thank you for clarifying this.Brittlandk (talk) 04:38, 12 March 2008 (UTC)

...so effectively it depends whether it is an open or closed system. Open system, outside a balloon, would not affect the pp of each reactant/product, so make no difference, but a closed system would affect pp of a reactant/product and so would change equilibria. yeah??...--82.9.21.247 (talk) 19:22, 15 April 2008 (UTC)

The system must be closed. Equilibrium calculations always assume a closed system. I believe you are confusing closed/open systems with the possibility that a closed system could have either a fixed or changing volume. A tied-off balloon would have a decrease in volume if internal pressure is decreased, or an increase in volume if internal pressure is increased. This would cause a shift in the equilibrium. If the volume of the system is fixed, like a sealed glass or metal vessel, the volume of the container does not change with a change in internal pressure (such as adding an inert gas,) so the partial pressures of the gases within the system do not change and there will be no shift in the equilibrium. Both systems are closed. Brittlandk (talk) 21:01, 15 April 2008 (UTC)

Ok, but what will happen if the reaction changes pressure ? For instance N2 + 3 H2 <--> 2 NH3 ? From what I have learned, in this case increasing system pressure causes equilibrium to shift towards NH3 (thus lowering system pressure, counteracting its increase)? —Preceding unsigned comment added by 81.219.200.242 (talk) 22:54, 6 January 2010 (UTC)

Pressure changes the equilibrium

According to the article, pressure changes Kc. How can this be correct? —Preceding unsigned comment added by 99.237.39.148 (talk) 22:06, 4 May 2009 (UTC)

Response reactions

Response reactions is a new article which seems to be something on this topic but I don't know what it's on about... Can anyone help? Rd232 ^talk 13:48, 22 September 2009 (UTC)

Economy?

Why does this article on chemistry have a section on economy? While it may well be true, it has nothing to do with Le Chatelier, or chemical equilibrium. Would it not be better to move this out to a separate page, or refer to it in the "See also" links? —Preceding unsigned comment added by Furrfu (talk • contribs) 12:58, 9 January 2010 (UTC)

Too far afield

I find the section entitled "Status as a Physical Law" to be misleading. The problem seems to be these sentences. I made some changes regarding (a).

(a) Le Chatelier's principle also states that when there is an external constraint on a system, a behavioural shift in the system occurs so as to annul the effect of that change.

(b) Where a shock initially induces positive feedback (such as thermal runaway), the new equilibrium can be far from the old one, and can take a long time to reach. In some dynamic systems, the end-state cannot be determined from the shock.

(c) The principle ... applies, in general, to thermodynamically closed and isolated systems in nature, since the second law of thermodynamics ensures that the disequilibrium caused by an instantaneous shock must have a finite half-life.[1]

Le Chatelier's principle applies to chemical reactions that start in equilibrium, and adjust to a new equilibrium in an orderly way (not too far from linear) after a change in the amount of one participant. If the change is small enough, the adjustment will lie within the linear regime. If the change is somewhat larger -- and this can be quite large in many systems, for example, chemical solutions with not too many participants and monotone power-law type relations of quantities -- the adjustment will at least share the direction of the adjustment in the linear regime. In both cases, Le Chatelier's principle states that the system adjust so as to partially cancel the change in the amount of the externally changed participant.

Several remarks:

(1) The shift does not annul the change, it only opposes it (partially annuls it). I would suggest removing sentence (a) -- in any case it mostly repeats the preceding sentence in the main text. In fact I did this.

(2) If the change (the "shock") is large enough, then we are way out of the linear regime, and naturally all bets are off -- thermal runaway being a good example. At this point, Le Chatelier's principle stops applying. Perhaps this should be said directly. As the text stands, I find the two sentences in (b) to be true, but distracting (a red herring), because no conclusion is drawn from them -- the correct conclusion being that we are out of bounds for Le Chatelier. And what does chaos have to do with it? Sure Le Chatelier's principle does not apply then!

(3) In any case, Le Chatelier's principle only says that the externally changed participant goes part of the way back to its old value. The other participant actually depart from their old values -- in fact, they absorb part of the change. So Le Chatelier's principle does not restore or return to the old equilibrium, but finds a new one (more or less nearby), with different numbers. "Opposition" only applies to the quantity of the externally changed participant. This means that (c) is misleading or wrong. In fact, it is seems to be generalizing the principle too much.

I made changes for (1), but I have trouble understanding the intent of the rest of the paragraph. It seems to me to be motivated by overarching philosophical drives that conflate together different kinds of dynamical phenomena to form overbroad principles. So I hesitate to make changes.

84.226.178.205 (talk) 12:10, 19 October 2014 (UTC)

Citations needed

@Chjoaygame, Wragge, TuxLibNit, Jaguar2k, ChristopherKingChemist, LouriePieterse, and Thomasmeeks:

I'm concerned about this article. On the one hand, it does have 13 references, but there are significant statements of fact that are not supported by a reference. The section on chemistry is over 1400 words with a single reference.

My chemistry knowledge is too weak to be a meaningful contributor to the science, but if someone's interested in working on this and has subject matter expertise, I will be happy to help. S Philbrick(Talk) 16:11, 16 July 2022 (UTC)

Le Chatelier's principle is a special version of the second law

Le Chatelier's principle belongs to thermodynamics, which is a branch of physics. It is a mistake to classify as belonging to physiology. Perhaps a forgivable mistake, but still a mistake. Because the wording is loose and qualitative, people try to appropriate it to areas beyond thermodynamics, but they are mistaken to do so. So the article's classification needs to be corrected.Chjoaygame (talk) 17:22, 9 February 2024 (UTC)