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Is Gibbs free energy just potential energy reduction?

  1. Jun 23, 2016 #1
    So I know that things in the Universe tend to move toward a state of least potential energy. This is why forces point in the direction of decreasing potential energy, as everything is trying to minimize its potential energy.

    So my main question is: does Gibb's free energy basically just describe this? Is the change in Gibbs energy negative, and therefore the process spontaneous, when the Universe/System minimizes its total potential energy?

    Main reason I was confused was that when there is a negative change in enthalpy, that means some form of potential energy was converted into heat. However, even when the enthalpy change is positive (and some subsequent gain in potential energy) , there can still be a change in entropy that makes the total Gibb's free energy negative.

    I know that systems always want to increase their entropy, but systems also want to decrease their potential energy. So is Gibb's free energy basically a battle between these two "desires" of the universe? Or is there somehow an associated potential energy decrease with an increase an entropy which makes the single "desire" of the universe to decrease its potential energy?


    Side question: Second Law of thermo states isolated system will always increase, or keep constant, its entropy over time. However there are spontaneous processes that decrease entropy while having a large decrease in enthalpy which makes total Gibbs free energy change negative and therefore the isolated system will have this process occur and entropy will increase. How is this explained in context of the "LAW"?


    I'm assuming this would all be a lot clearer if I look at a derivation of what Gibb's free energy actually represents right?
     
    Last edited: Jun 23, 2016
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  3. Jun 23, 2016 #2

    Ygggdrasil

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    The statement that ΔG ≤ 0 is basically just a restatement of the second law of thermodynamics (ΔSuniverse ≥ 0). So, minimizing free energy is equivalent to maximizing entropy.
     
  4. Jun 23, 2016 #3
    But is minimizing free energy equivalent to minimizing potential energy?
     
  5. Jun 23, 2016 #4

    Ygggdrasil

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    No. For example, placing all of the particles in the ground state of two-state system is the lowest potential energy case, but it is also a low entropy state. At any temperature above absolute zero, minimizing the free energy of the system will require some amount of particles in the excited state.
     
  6. Jun 23, 2016 #5
    So what's all this mess about the Universe tending to want to minimize potential energy? After all that's the direction that forces point in.

    Doesn't everything try to obtain the lowest potential energy state possible?
     
  7. Jun 23, 2016 #6

    Ygggdrasil

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    This is mostly true at the macroscopic scale (when energies are large), but at the microscopic scale (when the energies involved are similar to the thermal energy of the system), it's important to consider entropy.
     
  8. Jun 24, 2016 #7
    Minimizing the potential energy (or I would say the internal energy ) is true for isolated systems. If this system is in contact with an external reservoir then the system and the reservoir try to minimize the sum of their internal energies (Usys+Ur). If you only care about the equilibrium of the system and you do not want to go into analyzing the reservoir then the so-called Legendre transforms (such as Gibbs Free energy) are very convenient.

    Gibbs free energy is the optimal transform when the system is in contact with a pressure and temperature reservoirs.

    At any rate minimizing Usys+Ur at constant P and T is exactly equivalent to minimizing Gsys at the same constant T and P.
     
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