Why is Delta S purely dependent on gases?

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SUMMARY

The discussion centers on the relationship between the change in the number of moles of gas and the change in entropy (ΔS). It is established that an increase in the number of moles of gas, such as from 10 moles to 11 moles, will always result in an increase in entropy (ΔS > 0). The conversation highlights the importance of understanding mass conservation and the extensive nature of entropy, clarifying that the moles of gas rule is a fundamental principle in thermodynamics rather than a mere guideline.

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patrickbotros
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My teacher constantly stresses that 10 moles of a gas going to 11 moles of a gas will ALWAYS cause an increase in entropy (ΔS>0). But why? For example (I know that O2 shouldn't technically be a liquid, but just as an example):
2H2O(g)⇔H2O(g)+(3/2)O2(l)+2H2
It seems iffy to me. We go from 2mol(g)↔1mol(g), BUT we also have 2moles⇔(7/2)moles. Is the moles of gas rule just a "it usually works" kind of rule, or is it a tried-and-true physics fact?
Thanks so much! :smile:
 
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patrickbotros said:
10 moles of a gas going to 11 moles of a gas

10 moles of a gas don't change into 11 moles of a gas, that's mass conservation at wok. Apparently you think about some process in which number of moles of gas changes - but what process it is, and why does the number of moles of gas change? What are other things involved?

Or perhaps you just misunderstand the idea that entropy is an extensive property?
 

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