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Homework Help: Please explain entropy

  1. Dec 14, 2005 #1
    Can anyone please explain entropy!!
    its so confusing. i looked it up and i dont get it.
    How can entropy constantly be increaseing from every spontaneous occurence?
    for example, if here on earth, you g ointo a vacume, wouldnt you freeze? because it was absolute zero is it was a vacume....
    Last edited: Dec 14, 2005
  2. jcsd
  3. Dec 14, 2005 #2
    If you take a can filled with air, and heat it up 10 degrees more than it was 9lets say it started at 25 degrees celcius, the entropy is increased right...because the process was spontaneous.

    now what if you decrease the temperatue 10 degree, then the entropy is decreased right? is the entorpy decreased the same amount it was increased when it was heated up?

    Why is the reaction considered "spontaneou" when you were ading heat to it......?

  4. Dec 14, 2005 #3
    Maybe whast confusing me is what is considered to be more disordered.

    In order to determine if something is disordered, then there has to be a standard of what is considered ordered.....

    so what is ordered?
  5. Dec 15, 2005 #4


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    absolute zero pertains to temperature, temperature is related to the kinetic energy; in this sense how would you freeze in the vacuum void of particulate matter (the idea is a bit absurd to begin with).

    yes you increase the entropy by adding heat energy to the system of such gases, this process in and of itself has nothing to do with spontaneity.

    For your second question, and for a simplistic case, yes. [tex]( \frac{du}{dT} )_{p,n} = -Sm [/tex] that is the slope of molar free energy with respect to temperature for such a first order transition is equivalent to the negative of the molar entropy. With a cyclic process, you'll return to the same state entropy under particular conditions. Try finding a entropy v.s. temperature plot through google.

    You can try out a calculation, using dq(rev)=TdS, and knowing the dq=Cv(dT) at constant volume (relatively constant heat capacity over a small temperature range). Find out how much heat is required to heat the sample 10 degrees from a certain temperature and then find the entropy change, then the same for the cooling process.

    asyou mentioned, at lower temperatures, certain matter is more ordered, try researching up on the Nernst heat theorem. There are many perspectives of entropy, for one it has to do with the number of energy states being considered respectively.
  6. Dec 15, 2005 #5
    ok thanks.

    You mentioned free energy. I looked through my textbook, and it defines it as, G= H- TS, but never tells what gree energy (gibbs free energy) is, or what its good for.

    i was looking online, and it said that gibbs free energy ALWAYS increases, but can't gibbs energy be negative, and therefor decrease?

    can you help me with that please.

  7. Dec 15, 2005 #6


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    For a spontaneous reaction at constant temperature and pressure the change in Gibb's energy should be negative in value. Gibb's energy is derived from the clausius inequality and the criterion for spontaneity (dS+dSsurr> or equal to 0). One of the reasons it is useful is that it can be associated with the maximul non-expansion work for a system such as an electrochemical cell.
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