# Standard Entropy Change and Spontaneity

1. Oct 22, 2008

### jayred

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.

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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.

Thanks!

2. Oct 22, 2008

### Mapes

Hi jayred, welcome to PF. The $\Delta S>0$ criterion for spontaneity applies to systems at constant energy and volume. For systems at constant temperature and pressure, we use $\Delta G<0$ (where G is the Gibbs free energy) as the criterion for spontaneity.

3. Oct 22, 2008

### Staff: Mentor

Note, that

$$\Delta G = \Delta H - T \Delta S$$