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If a reaction is not spontaneous is there any of making that reaction occur?

  1. Mar 26, 2008 #1
    If a reaction is not spontaneous is there any way of making that reaction occur?

    Question Details: We know that
    del G=del H - T(del S)

    if del H is positive and del S is negative then del G is always positive. There is no way of making del G negative. My question is whether such a reaction take place at all (to any extent)?
    thx
     
    Last edited: Mar 27, 2008
  2. jcsd
  3. Mar 27, 2008 #2
    Not true! The changes in enthalpy and entropy refer to a single temperature and pressure. By changing the temperature or pressure, the free energy change will vary and may become negative. An example is the conversion of graphite to diamond. For this reaction (at room temperature and 1 atm.) the change in enthalpy is 450 cal. and the change in entropy is -0.79 cal/deg. But graphite and diamond have different densities, so that changing the pressure can effect a free energy change. Above a pressure of 22000 atm. diamond is more stable than graphite at room temperature.
     
  4. Mar 28, 2008 #3
    You could be right, but however seems strange to me that delta(H) of the reaction graphite --> diamond is positive; are you sure?
     
  5. Mar 28, 2008 #4
    If delta(G) of reaction is positive, the reaction can also happen, it only have a lower equilibrium constant:
    when delta(G) < 0 then K >1, when delta(G) = 0 then k = 1, when delta(G) > 0 then k < 1:
    delta(G) = -RTlnK
    Example:
    0.5N2 + O2 <--> NO2
    delta(G) = 3.32*10^(4) + 1.57*10^(2)*T
    at T = 1000K:
    delta(G) = +3.48*10^(4) J/mol
    but we all know how NO2 pollution by internal combustion engines (diesels) is a terrible world problem.
     
  6. Mar 28, 2008 #5
    Hi Lightarrow. I'm not sure of exact value, which I took from "Thermodynamics" by Lewis and Randall, but I am pretty sure that graphite is more stable than diamond at standard temperature and pressure. A graph of the free energy change can be found at

    www.physics.rutgers.edu/ugrad/351/Lecture 14.ppt

    Why do you find it strange? Perhaps it is the power of advertising: "A diamond is forever!"
     
  7. Mar 28, 2008 #6
    It finally eliminates my confusions.
    Thx pkleinod.
     
    Last edited: Mar 28, 2008
  8. Mar 29, 2008 #7
    Thanks for the interesting link.
    I found it strange just for the fact that in diamond there are more covalent bonds per atom than in graphite (in diamond every C is bound to other 4 C atoms, in graphite to other 3 atoms).
     
    Last edited: Mar 29, 2008
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