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Enthalpy and Entropy

  1. Nov 10, 2003 #1
    In an exothermic rxn change in enthalpy is negative but change in entropy is positive (in respect to the surroudings)(increase in positional probability).

    In an endothermic rxn change in enthalpy is positive but change in entropy is negative (in respect to the surroundings)(decrease in positional probability).


    Plz say so
  2. jcsd
  3. Nov 10, 2003 #2


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    Wrong --- reread the assigned text, go back over the class notes, delete any mumbo-jumbo and hand-waving arguments that include the phrase "positional probability" in text and notes, also any references to "order/disorder," and try it again.
  4. Nov 10, 2003 #3
    I don't think so . . .

    I quote "In an endothermic process, heat flows from the surroudings into the system. In an exothermic process, heat flows in the surroundings from the system." -Chemistry 6th Edition, Zumdahl

    I quote "an exothermic process in the system causes heat to flow into the surroundings, increasing the random motions and thus the entropy of the surroundings. For this case, Change in Entropy of the surroundings is positive. The opposite is true for an endothermic process in a system at constant temperature (Change of Entropy of the surroundings is negative )." -Chemistry 6th Edition, Zumdahl

    This definitely supports my statements made previous.
    Last edited: Nov 12, 2003
  5. Nov 11, 2003 #4
    The terms endothermic and exothermic apply to the state function of enthalpy only, that is delta H.

    Nevertheless energy balance does not suffice to answer all questions about a chemical reaction including whether a reaction will take place at all.

    And thus we have entropy. And then delta G which is the succinct criterion btw reactions that can occur spontaneously.
  6. Nov 11, 2003 #5


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    Out of my head:

    Enthalpy is the measure of energy released by a reaction (can be positive or negative).

    Entropy is the measure of free energy of a system. A spontanious reaction would start from a large G to a small G, so you've got a -dG (delta G).
  7. Nov 11, 2003 #6


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  8. Nov 11, 2003 #7
    I am right . . . here is my support

    I entered school today immediately to ask my chemistry teacher and he says what I wrote above was correct. I also corresponded with another chemistry teacher to double-check. I also checked in another Chemistry book (Chemistry and Chemical Reactivity by Kotz and Purcell) and I quad-checked with my own by Zumdahl . . . a prominent authority in chemistry.

    Bystander . . . my "waving arguments" came from the book . . . it talked profusely about "positional probability", "entropy", and "Ethalpy" and their relations to each other. Positional probability increase with an increase in entropy and vice versa (under constant temperature and pressure).

    Thank you all for sending me for the "quest for the holy Grail (i.e. Chem answer)". (although it did waste A LOT OF MY TIME, I guess everything is relative )_

  9. Nov 11, 2003 #8


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    Well, so you came out better than huh?

    So could you explain the positional probability for me then? I don't quite understand what is meant with that The probability of finding a molecule at a certain position?? That would mean a low temperature, a reaction which lowers the temperature of the surroundings is an endothermic reaction, not exothermic..

    You are true that enthalphy in an exothermic reaction has a negative sign (heat is released).

    You also seem to be true that systems strive to increase entropy = disorder (and thus decrease free energy (G)?), an exothermic reaction releases energy in heat and thus increases entropy (and is generally speaking spontanious).

    Bystander, if you could comment more in detail what you think is wrong, since your previous post seemed to have caused some confusion as to what you meant to say :wink:
  10. Nov 11, 2003 #9
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    Last edited: Nov 11, 2003
  11. Nov 11, 2003 #10


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    Yes, I figured, thanks for pointing that out! :smile:
  12. Nov 11, 2003 #11
    positional probabilities

    I am sorry Monique that I cannot tell a full account about positional probability (my teachers enjoy giving me tons of hw - about 6-7 hours per night).

    Let me try to summarize what I know . . .

    Entropy is a thermodynamic function that describes the number of arrangements (positions and/or levels) that are available to a system existing in a given state. Nature spontaneously proceeds toward the states that have the highest probabilities of existing.

    Consider this example:

    Arrangement 1 has one microstate (a particular arrangement), Arrangement 2 has four microstates. Arrangement 3 has 6 microstates. According to positional probability Arrangement 3 is most likely or probable to occur.

    Here is another example ->

    A solid < liquid < gas in terms of positional probability, why?

    Because there is an increase in disorder from a solid state to a liquid state and so forth.

    Therefore, Positional Probability depends on the arrangement (state) and the number of ways (microstates) . . . these are correlated with an increase and decrease of Entropy (situational).

    I hope this helps!
  13. Nov 11, 2003 #12


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    Re: positional probabilities

    OK, now I know what you mean by the term. I wonder though, isn't that the strange way to describe nature? It proceeds to the state wherein there is the highest number of probabilities of existing.. that sounds really strange to me.. an increase in positional probability.. there must be a better term?
  14. Nov 12, 2003 #13
    The statistical definition of entropy is a lot easier to understand than the classical definition of entropy, dQ(reversible)/T.

    If a chemical reaction is spontaneous, that implies that the total entropy of the universe increases during the course of the reaction. The entropy of the reaction system may decrease if this is offset by a larger increase in entropy of the surroundings.

    Or put another way by Josiah Willard Gibbs, the greatest native born American physicist, if the change in Gibbs free energy is negative, the reaction is spontaneous.
  15. Nov 12, 2003 #14
    Richard Feynman was also born in the United States (see http://www.nobel.se/physics/laureates/1965/feynman-bio.html). Let's just say we can't compare the two b/c Gibbs got his degree in Mathematical physics while Feynman got his in Theoretical Physics.


    I.e. both were the "greatest" native born
  16. Nov 12, 2003 #15
    Yes, two great scientists. I think Gibbs is the darling of scientists because he had a typical reticent scientific personality--publish in obscure journals and let your work speak for you. Feynmann was more of a showman and big ego, thus far more familiar to the general public. I don't know how his colleagues took him--I imagine some disliked him for his ego.
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