Non-spontaneous reaction and Gibbs Free Energy

  1. Quartz, SiO2(s), does not spontaneously decompose to silicon and oxygen at 25 degrees Celsius even though the standard entropy change of the reaction is large and positive (deltaS = +182.02 J/K). Explain why.

    I am confused by this one fundamentally because it seems like common sense that quartz doesn't spontaneously turn to solid silicon and oxygen gas at this temp.

    Looking at Gibbs Free Energy and the effects of temperature on this it says that if [tex]\Delta[/tex]H is negative and [tex]\Delta[/tex]S is positive then the reaction is spontaneous at all temperatures.

    I don't understand what concept I am missing.
  2. jcsd
  3. Ygggdrasil

    Ygggdrasil 1,693
    Science Advisor

  4. SiO2 --> Si + O2
    delta H= + 910 KJ/mole ( note kilo joules)
    delta S= + 182 J/K mole
    T= 298 K

    delta G= delta H -T delta S

    What you are missing is putting in the correct values and getting the answer.

    Last edited: Mar 11, 2011

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