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Homework Help: Standard Entropy Change and Spontaneity

  1. Oct 22, 2008 #1
    1. The problem statement, all variables and given/known data

    Quartz, SiO2(s), does not spontaneously decompose to silicon and oxygen at 25C in the reaction:

    SiO2(s) -> Si(s) + O2(g)

    even though the standard entropy change of the reaction is large and positive (delta S = +182.02 J K-1). Explain.


    So, how do I explain that this is the case? The book only describes that when delta S > 0 the reaction is spontaneous with no exceptions.

  2. jcsd
  3. Oct 22, 2008 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Hi jayred, welcome to PF. The [itex]\Delta S>0[/itex] criterion for spontaneity applies to systems at constant energy and volume. For systems at constant temperature and pressure, we use [itex]\Delta G<0[/itex] (where G is the Gibbs free energy) as the criterion for spontaneity.
  4. Oct 22, 2008 #3


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    Staff: Mentor

    Note, that

    [tex]\Delta G = \Delta H - T \Delta S[/tex]
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