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Explain why quartz, SiO2(s), does not spontaneously decompose into silicon and oxygen at 25 °C in the reaction: SiO2 (s) → Si(s) + O2(g) even though the standard entropy change of the reaction is large and positive (i.e. AS° = 182.02 JK 'mol-'). + +

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Answer:

ΔH > O and ΔH > TΔS

Explanation:

When a reaction is non-spontaneous even though the standard entropy change of the reaction is large and positive the reason is the reaction is highly endothermic and dominates the term TΔS in the standard Gibbs free energy expression:

ΔG = ΔH-TΔS

Remember that for an endothermic  reaction to be spontaneous ΔG < 0 and this can occur only if the term TΔS is greater than ΔH. This is not the case for quartz which requires a good quantity of energy to overcome its crystaline 3D lattice in which  the oxygen are shared between 2 silicon atoms.

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