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Are there coupled reactions in geology?

  1. Oct 6, 2015 #1
    I'm just wondering if all known coupled reactions are biological, or if there are some that geological and could have preceded the emergence of life on earth. In other words, are there some compounds in geochemistry that are much more abundant than you would expect if the reactions that produce them go to equilibrium without some other reaction driving them?

    thank you
  2. jcsd
  3. Oct 6, 2015 #2


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    In particular,
    "From this example, it will be apparent that we can, from a thermodynamic perspective, treat metabolic processes in several ways. We can treat individual reactions as separate systems, or treat a set of coupled reactions (including the complete set representing the metabolism of the organism as a whole) as a single system. The choice is one of convenience, and the important points are that the system should be carefully defined, the reaction equation balanced in conformity with the Law of conservation of mass, and the energy equation balanced in accordance with the First Law of thermodynamics, and the properties of variables of state. "
  4. Oct 7, 2015 #3
    Thank you for the link.

    Perhaps I should have specified catalyzed coupled reaction. But I get the point that there is more than one way to draw the box that includes the "system". I guess there may be, in principle, all sorts of coupled reactions in geology if you completely ignore reaction rate. But in cases where equilibrium takes longer than the age of the earth, it may be impractical to consider that the reactions are effectively coupled. If the total reaction depends on 3 or 4 molecules colliding in a particular orientation with particular energy, the two reactions might as well be taking place in beakers located in different cities.

    So I'm wondering if there are any geological reactions that can be considered coupled in the practical sense--where you actually approach equilibrium in a reasonable time. I have read somewhere that iron pyrite can act as a catalyst, but I don't know what it catalyzes.
  5. Oct 7, 2015 #4


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    All of them.
    "Everything is a catalyst for some reaction, and an inhibitor for some other reaction."
  6. Oct 7, 2015 #5
    Interesting. I am not used to thinking of it that way. Clearly I have a lot to learn :)
  7. Oct 7, 2015 #6


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    "Coupled reactions" strikes me as being a term used more for a very specific class of reactions, the ADP-ATP game, and the like, whereas a literal application of the definition, my posts (nearly all chemical reactions are "coupled reactions"), is semantics.
  8. Oct 8, 2015 #7
    Okay, that does clarify your point for me somewhat. Thank you. Maybe I can clarify my question by putting it this way: are biological reactions, as in metabolism, entirely a thing to themselves--a completely self-contained area of study? Is there nothing in geology that shows a glimmer of a resemblance to them? If we had a time machine and could travel into the past, there must be some time period where we couldn't say where geology ends and biology begins. It would all be blended together. Either you would say that some geochemical processes are oddly lifelike, or that "life" is strangely mineral in character in this period.

    Has the development of life on earth completely erased all traces of its emergence?
  9. Oct 9, 2015 #8


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    Your question still seems extremely fussy to me. Of course there are extremely complex coupled reactions in geology. I am thinking of the separation of magma, where some substances crystallize first, then dissolve again when another one crystallizes out. Or coupled redox reactions depending also on pH when ores are deposited from a solution in contact with other rocks.
  10. Oct 9, 2015 #9
    Can you elaborate? How is it fussy? Maybe I can improve it.

    Excellent! Thank you for those examples. I will do a google search on those. So these reactions where a -ΔG reaction drives a +ΔG reaction?
  11. Oct 10, 2015 #10


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    I believe the term is "fuzzy", as in not clear.

    Biological or biochemical reactions would involved molecules and compounds found in biological systems, i.e., plants, animals, and various microorganisms.

    There are cases where biological organisms do not exist, but chemical reactions take place. In Nature, chemical reactions can be driven by pressure and geothermal activity. Consider subduction zones at the boundaries of the plates of the earth's lithosphere (mantle and upper crust).

    In man-made or engineered systems, corrosion is a good example of coupled reactions. Metal alloys tend to corrode over time if a protective layer is not applied or does not develop.

    In nuclear systems, radiation damage causes microstructural changes in materials. In addition to knocking about atoms and inducing dislocations in metal crystals, changes may include amorphization, dissolution and radiation induced segregation.
  12. Oct 11, 2015 #11
    Ah. Fuzzy. I can see that.

    Okay, maybe I can make the question more precise. Outside of biology, are there reactions in which a positive ΔG process is driven by a negative ΔG process? For example, suppose that the reaction that produces some compound AB out of its constitutents A and B is represents an increase in free energy.

    A + B ---> AB (ΔG > 0)

    If this reaction has to take place by itself, then the amount of product will be negligible. Equilibrium sets in with perhaps no more than a few molecules of AB present.

    But if the reaction is coupled in some way to another reaction that is strongly spontaneous, such as the decomposition of CD, then it is at least thermodynamically possible to produce a non-negligible amount of AB.

    CD ---> C + D (ΔG' < 0)

    A + B + CD ---> AB + C + D (ΔG" < 0)

    where ΔG" = ΔG' + ΔG

    Do the examples provided above by Dr Du and Astronuc fall into this category (ignoring the specific type of reaction, such as decomposition, displacement, etc. I just used composition and decomposition as an example.)
  13. Oct 12, 2015 #12


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    Of course I meant fuzzy, not fussy (just looked up its translation)!
    Thinking about it, I think there are never reactions with a positive Delta G, but only with a positive Delta G standard. The way a reaction with a positive Delta G standard is driven is by the reduction of the concentration of products or increase of the amount of reactants. Or, very often in biological systems, you are in fact considering other reactions where one of the reactants is replaced by some more reactive (and energetic) reactant like acetate by Acetyl-CoA.

    The first two mechanisms to drive a reaction are quite ubiquitous in multi component systems, so it is not limited to biologic systems.
  14. Oct 13, 2015 #13
    Thank you.

    Yes, my understanding is that total delta G is always non-positive at the particular temperature and pressure at which it occurs. That is a consequence of the 2nd Law of Thermodynamics. That is provided that you include all the relevant terms in the free energy, so that all sources of non-expansion work are taken as internal to the system. But it seems that one can usefully focus on a part of the system sometimes, and for that part of the system Delta G is positive because external non-expansion work is done on it. Like using an applied voltage to drive a reaction in the "wrong direction" --the direction of positive delta G, where G only includes chemical terms and not the charge x emf term.

    To me such processes seem kind of special. After all, it takes some effort to set up an electrochemical cell. A chemical reaction can do work against an applied voltage if it is set up to do so, but why should you expect the chemical reaction to be coupled to the circuitry necessary to achieve this unless someone set it up? That's what I mean by coupling. One system can be used to do work on another system, raising its free energy, but only if they happen to be coupled in the right way.

    Or am I wrong? Do voltaic cells or electrolytic cells occur naturally? Maybe that is what Astronuc was trying to tell me when he brought up corrosion.
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