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    Revisiting Le Chatelier's Principle
    By Enrico Uva | February 8th 2012 09:24 AM | 6 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    Le Chatelier's Principle is a neat concept, but too often it's expressed in a not so wonderful way. Here is a  textbook definition from a popular college textbook, fifth edition:
    A change in any of the factors that determine the equilibrium conditions of a system will cause the system in such a manner as to reduce or counteract the effect of the change.
    With such a definition, students often imagine the system as almost having a consciousness or a purpose. Wikipedia's definition also leaves something to be desired:
    If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure, the equilibrium will shift in order to minimize that change.
    They go on to use an example, of course giving the correct prediction of the disturbance, but they also mystify what is actually going on.

    2 NO2(g)  ⇌ N2O4(g)

    If one increases the pressure of the reactants (2 NO2(g) ) the reaction will tend to move towards the products to decrease the pressure of the reaction.

    Maybe it's not just the paucity of pretty women or (handsome guys) in freshman chemistry classes that keeps the classes so small!

    An equilibrium is established in a closed system when the forward reaction rate equals that of the reverse reaction. To predict how a system in equilibrium will react to a disturbance, we could view the forward and reverse reactions as if they were in competition with one another. The prevailing reaction will be the one that benefits more from the disturbance. If one reaction is being more hampered than the other, obviously the one facing the obstacle will not win out.

    Returning to the nitrogen dioxide reaction,  2 NO2(g)  ⇌ N2O4(g) , if we increase the pressure of the system by decreasing the volume, the concentration of every reactant will increase. But the forward reaction in this case will benefit more from the increased frequency of molecular collisions. Why? If compression increases the concentration of each reactant  by a factor of x then, since the forward rate can be expressed as k[NO22 , the rate will increase by a factor of x2. But the reverse rate, which equals k[N2O4(g)] will only increase by a factor of x. So increasing the pressure creates more N2O4 not because of a compensation mechanism but because compression increases the forward rate more than the reverse rate.

    In the equilibrium Cr2O72-(aq) + H2O(l) 2 H+(aq) + 2 CrO42-(aq)
    the orange dichromate(Cr2O72-(aq) ) reacts with water to create acid(H+(aq) )and yellow chromate(CrO42-(aq)). Meanwhile at the same rate, acid and chromate combine to regenerate dichromate and water. So to the naked eye, it looks like nothing is going on because the work of the forward reaction is being offset by that of the reverse reaction. We could have the stable orange color of dichromate dominating.

    Although the ions are all mixed in a soup and don't have respective sides, it's a bit like if I dump snow on my neighbor's driveway, and he dumps snow from his driveway onto mine at the same rate. A plane's passengers flying over us will simply see an unchanging white driveway.

    But if we disturb the equilibrium by adding some base, it eliminates some H+(aq) .  Reducing the concentration of H+(aq)thus lowers the reverse rate. But since the forward reaction is unaffected by the presence of base, it proceeds at its regular rate. So what we see is the yellowish color of chromate dominating. There's no system trying to "reorder things" or "offset anything". It's simply the case of a hampered reverse reaction that does not keep up and maintain the original equilibrium. If I suddenly had trouble picking up the snow while my neighbor proceeded as normal, the snow would suddenly pile up on my side. The plane passengers would see the asphalt on the neighbor's driveway.


    Comments

    rholley
    Alas, this late, I am far too tired to make any sensible comment about this now.

    However, that snow-shovelling reminds me of a joke told me by one of our Jordanian students, some years ago:

    Two neighbouring villages were locked in a dispute over water usage of the lake that lay between them.  There had been some rough stuff, so they decided to get together and see if they could come to an agreement.

    After hours of argument, one of the village heads got up and said:

    “Let’s draw a line across the lake from here . . . . . to here”, so dividing the lake in two.

    They all agreed, and the meeting broke up.

    Once the party from the other village had departed, his own people turned on him and said:

    “How could you give in so easily?  There’s a lot more of us than them, and we could easily have taken a much bigger portion!”

    “Don’t worry,” he replied, “there’s a full moon tonight.  Once it’s up, we’ll go with our buckets and take all the water from their half, and pour it into ours!”
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    vongehr
    Yea, the "equilibrium moving to the other side" ?!? I have seen many formulations of this and similar principles in Chemistry/Thermodynamics/..., and I never understood what the point was in the first place. I mean, it is all completely clear until they come with these principles that do nothing but add confusion. This seems to have something to do with the difference between a Physicist's brain and a Chemist's, ha ha, sorry.  :-|
    UvaE
    So does that imply that I have a physicist's brain ?  :)

    Actually as you realize, the division between chemistry and physics is purely academic. Physicists tended to look down more on chemists when the divisions between the fields seemed sharp.

    I'm reminded of the famous story...
    when Pauli's wife left him for a chemist, he said, "Had she taken a bullfighter I would have understood but an ordinary chemist.

    Apparently that sent him into a deep depression! But of course, ironically, his work not only gives us insight into stellar evolution(more physics than chem) but also into the nature of electrons and their unique quantum states(major basis for periodic table and bonding; more chem than physics)
    vongehr
    Ha ha - great story - didn't know that one. I guess you have a physics brain. Somehow, chemistry/engineering types can just take such rules and apply them and do it I guess correctly most of the time, while the physicist sees such rules and goes immediately "wait a minute, but what if ..." and then does not know anymore whether to apply it that way or the other way around.
    Nice. I would add that Le Chatelier-Braum 'principle' is a theorem that can be derived from chemical stability theory which also applies to open systems. This 'principle' is sometimes named the «Le Chatelier-Braun theorem of moderation»

    Reactions go Away from Increase (AI), and Toward a Decrease (TD)
    Vowels, consonants...makes it real easy to remember which way to go....