Non-spontaneous reaction and Gibbs Free Energy

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Quartz (SiO2) does not spontaneously decompose into silicon and oxygen at 25 degrees Celsius despite a large positive standard entropy change (ΔS = +182.02 J/K) because the enthalpy change (ΔH = +910 kJ/mole) is significantly positive. The Gibbs Free Energy equation (ΔG = ΔH - TΔS) indicates that a positive ΔH can outweigh the positive ΔS at this temperature, resulting in a non-spontaneous reaction. The confusion arises from the assumption that a positive ΔS alone guarantees spontaneity. Properly calculating ΔG with the provided values shows that the reaction is not favorable under standard conditions. Therefore, the reaction does not occur spontaneously due to the high energy requirement for the decomposition of quartz.
jmm5872
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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 \DeltaH is negative and \DeltaS is positive then the reaction is spontaneous at all temperatures.

I don't understand what concept I am missing.
 
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jmm5872 said:
I don't understand what concept I am missing.

Kinetics
 
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.

Kinetics
 
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