Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Entropy changes for reactions: Understanding Periodic Trends

  1. Feb 12, 2006 #1
    One question posed in a general chem book I have were the trends of a couple of reaction types. They don't answer the question of why, though. Maybe you can help...

    Identify periodic trends of these rxns:

    Set 1:

    H2 + X2 --> 2HX (X are halogens)

    Ex: H2(g) + Cl2(g) --> 2HCL (g)

    The trend is that the entropy changes become more positive for the heavier halogens.

    Set 2:

    2alkali + CL2 --> 2 alkaliCl

    Ex: 2Li(s) + Cl2(g) --> 2LiCl (s)

    The standard change in entropy for these rxns become increasingly negative with atomic mass of the alkali.

    I have some speculations, but i'm probably just missing something here. I don't mind a complicated explanation and would in fact welcome that with compare to a hand wave.

    Thank you,
  2. jcsd
  3. Feb 13, 2006 #2

    This is NOT a homework question!!! I am floored that a conceptual question, that arose from homework, would be moved.

    Like I said, the book asks if there is a trend, and there is. I am asking WHY there is a trend. That is not part of the homework and this question should be back where I posted it.


  4. Feb 13, 2006 #3


    User Avatar
    Science Advisor
    Homework Helper

    with all due respect, the "conceptual question that" arose from the homework....is in essence, the homework question. that's why the moderators moved it.

    But I don't think that such explanations to your questions are covered thoroughly in general chemistry, thus I somewhat believe you there, unless your teacher/professor is asking for a basic explanation in trend.

    From what I can think of, there are a couple of factors to consider for the first set: the entropy of mixing, which depends on the molar fractions involved, the actual positional entropies, the boltzman distribution of the energy levels.

    For the second, I'm not quite sure at the moment, but might want to consider the quantum aspects of it as well as the factors mentioned above.

    are you sure this question does not involve enthalpy and not entropy? In addition, I have not investigated the validity of the statements in the problem.
  5. Feb 13, 2006 #4
    Look, I don't want to step on anyones toes here. But, this is not a homework question. Questions that we pose arise from many different places. Is there an "I was talking to my friend and she said *blank* but I wondered about *something else*" section? No.

    And, you are right, these questions aren't answered in general chemistry books. Your "somewhat belief" of what i've said is odd, because it's obvious they don't answer these questions in Gen. Chem. I already answered the question they had in the book (which was, btw, "do you see any trends?" And, the answer in the back of the book was "with incr mol.wt. we see an incr/decr in ENTROPY."

    My question IS conceptual. I don't want an answer to a homework question, I want an explanation for what is going on at the atomic or quantum level. You have to understand that i've already taken Gen. Chem and I am retaking it at a second university. So, this time around, I am asking "why?", which as we all know, most of the "why" is not covered in these books.

    My thoughts on the decreasing entropy of the larger alkali-Cl --> rxns is that as they get larger, maybe they become more alike in electronegativity, and it becomes more orderly a solid (i'll have to take a look at bond lengths, and such, later on.)

    Can you expound on your answer? Do you see why the trend occurs? Or are you just throwing out factors that may be involved, but not actually looking for the trend? I think it is an interesting question, and I was hoping others would find it interesting as well. But, if it isn't worth your time to dive into it, that is ok.

    I truly think this would get more response in the chemistry forum. It is conceptual and therefore should have remained there.

    thank you,
  6. Feb 13, 2006 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This is going to start a "religious" war between the physicists (believers in microscopic explanations of thermo) and the chemists (believers in thermo as thermo). That is, as a chemist, I'm going to tell you that microscopic explanation of the trends you've noted is pure, unadulterated hand-waving, and that as such, it doesn't have a whole lot of predictive utility.

    In keeping with the spirit of the forum, let's see your ideas on HX before we get into the microscopic hand-waving. There are rationalizations for the behavior, and it's as instructive for you to play the silly game of "discovery" as it is for me to just hand them to you.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook